The Effects of Cross & Self-Fertilisation in the Vegetable Kingdom by Charles Darwin, is part of the HackerNoon Books Series. You can jump to any chapter in this book here. OLANACEAE, PRIMULACEAE, POLYGONEAE, ETC.
SOLANACEAE, PRIMULACEAE, POLYGONEAE, ETC.
25. SOLANACEAE. Petunia violacea.
DINGY PURPLE VARIETY.
The flowers of this plant are so seldom visited during the day by insects in this country, that I have never seen an instance; but my gardener, on whom I can rely, once saw some humble-bees at work. Mr. Meehan says, that in the United States bees bore through the corolla for the nectar, and adds that their “fertilisation is carried on by night-moths.” (6/1. ‘Proceedings of the Academy of Natural Science of Philadelphia’ August 2, 1870 page 90.)
In France M. Naudin, after castrating a large number of flowers whilst in bud, left them exposed to the visits of insects, and about a quarter produced capsules (6/2. ‘Annales des Sc. Nat.’ 4th series Bot. Tome 9 cah. 5); but I am convinced that a much larger proportion of flowers in my garden are cross-fertilised by insects, for protected flowers with their own pollen placed on the stigma never yielded nearly a full complement of seed; whilst those left uncovered produced fine capsules, showing that pollen from other plants must have been brought to them, probably by moths. Plants growing vigorously and flowering in pots in the greenhouse, never yielded a single capsule; and this may be attributed, at least in chief part, to the exclusion of moths.
Six flowers on a plant covered by a net were crossed with pollen from a distinct plant and produced six capsules, containing by weight 4.44 grains of seed. Six other flowers were fertilised with their own pollen and produced only three capsules, containing only 1.49 grains weight of seed. From this it follows that an equal number of crossed and self-fertilised capsules would have contained seeds by weight as 100 to 67. I should not have thought the proportional contents of so few capsules worth giving, had not nearly the same result been confirmed by several subsequent trials.
Seeds of the two lots were placed on sand, and many of the self-fertilised seeds germinated before the crossed, and were rejected. Several pairs in an equal state of germination were planted on the opposite sides of Pots 1 and 2; but only the tallest plant on each side was measured. Seeds were also sown thickly on the two sides of a large pot (3), the seedlings being afterwards thinned, so that an equal number was left on each side; the three tallest on each side being measured. The pots were kept in the greenhouse, and the plants were trained up sticks. For some time the young crossed plants had no advantage in height over the self-fertilised; but their leaves were larger. When fully grown and in flower the plants were measured, as follows:—
TABLE 6/76. Petunia violacea (first generation).
Heights of plants measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 30 : 20 4/8.
Pot 2 : 34 4/8 : 27 4/8.
Pot 3 : 34 : 28 4/8. Pot 3 : 30 4/8 : 27 4/8. Pot 3 : 25 : 26.
Total : 154 : 130.
The five tallest crossed plants here average 30.8, and the five tallest self-fertilised 26 inches in height, or as 100 to 84.
Three capsules were obtained by crossing flowers on the above crossed plants, and three other capsules by again self-fertilising flowers on the self-fertilised plants. One of the latter capsules appeared as fine as any one of the crossed capsules; but the other two contained many imperfect seeds. From these two lots of seeds the plants of the following generation were raised.
CROSSED AND SELF-FERTILISED PLANTS OF THE SECOND GENERATION.
As in the last generation, many of the self-fertilised seeds germinated before the crossed.
Seeds in an equal state of germination were planted on the opposite sides of three pots. The crossed seedlings soon greatly exceeded in height the self-fertilised. In Pot 1, when the tallest crossed plant was 10 1/2 inches high, the tallest self-fertilised was only 3 1/2 inches; in Pot 2 the excess in height of the crossed was not quite so great. The plants were treated as in the last generation, and when fully grown measured as before. In Pot 3 both the crossed plants were killed at an early age by some animal, so that the self-fertilised had no competitors. Nevertheless these two self-fertilised plants were measured, and are included in Table 6/77. The crossed plants flowered long before their self-fertilised opponents in Pots 1 and 2, and before those growing separately in Pot 3.
TABLE 6/77. Petunia violacea (Second generation).
Heights of plants measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 57 2/8 : 13 4/8. Pot 1 : 36 2/8 : 8.
Pot 2 : 44 4/8 : 33 2/8. Pot 2 : 24 : 28.
Pot 3 : 0 : 46 2/8. Pot 3 : 0 : 28 4/8.
Total : 162.0 : 157.5.
The four crossed plants average 40.5, and the six self-fertilised 26.25 inches in height; or as 100 to 65. But this great inequality is in part accidental, owing to some of the self-fertilised plants being very short, and to one of the crossed being very tall.
Twelve flowers on these crossed plants were again crossed, and eleven capsules were produced; of these, five were poor and six good; the latter contained by weight 3.75 grains of seeds. Twelve flowers on the self-fertilised plants were again fertilised with their own pollen and produced no less than twelve capsules, and the six finest of these contained by weight 2.57 grains of seeds. It should however be observed that these latter capsules were produced by the plants in Pot 3, which were not exposed to any competition. The seeds in the six fine crossed capsules to those in the six finest self-fertilised capsules were in weight as 100 to 68. From these seeds the plants of the next generation were raised.
CROSSED AND SELF-FERTILISED PLANTS OF THE THIRD GENERATION.
TABLE 6/78. Petunia violacea (third generation; plants very young).
Heights of plants measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 1 4/8 : 5 6/8. Pot 1 : 1 : 4 4/8.
Pot 2 : 5 7/8 : 8 3/8. Pot 2 : 5 6/8 : 6 7/8.
Pot 3 : 4 : 5 5/8.
Pot 4 : 1 4/8 : 5 3/8.
Total : 19.63 : 36.50.
The above seeds were placed on sand, and after germinating were planted in pairs on the opposite sides of four pots; and all the remaining seeds were thickly sown on the two sides of a fifth large pot. The result was surprising, for the self-fertilised seedlings very early in life beat the crossed, and at one time were nearly double their height. At first the case appeared like that of Mimulus, in which after the third generation a tall and highly self-fertile variety appeared. But as in the two succeeding generations the crossed plants resumed their former superiority over the self-fertilised, the case must be looked at as an anomaly. The sole conjecture which I can form is that the crossed seeds had not been sufficiently ripened, and thus produced weakly plants, as occurred with Iberis. When the crossed plants were between 3 and 4 inches in height, the six finest in four of the pots were measured to the summits of their stems, and at the same time the six finest of the self-fertilised plants. The measurements are given in Table 6/78, and it may be here seen that all the self-fertilised plants exceed their opponents in height, whereas when subsequently measured the excess of the self-fertilised depended chiefly on the unusual tallness of two of the plants in Pot 2. The crossed plants here average 3.27, and the self-fertilised 6.08 inches in height; or as 100 to 186.
When fully grown they were again measured, as follows:—
TABLE 6/79. Petunia violacea (third generation; plants fully grown).
Heights of plants measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 41 4/8 : 40 6/8. Pot 1 : 48 : 39. Pot 1 : 36 : 48.
Pot 2 : 36 : 47. Pot 2 : 21 : 80 2/8. Pot 2 : 36 2/8 : 86 2/8.
Pot 3 : 52 : 46.
Pot 4 : 57 : 43 6/8.
Total : 327.75 : 431.00.
The eight crossed plants now averaged 40.96, and the eight self-fertilised plants 53.87 inches in height, or as 100 to 131; and this excess chiefly depended, as already stated, on the unusual tallness of two of the self-fertilised plants in Pot 2. The self-fertilised had therefore lost some of their former great superiority over the crossed plants. In three of the pots the self-fertilised plants flowered first; but in Pot 3 at the same time with the crossed.
The case is rendered the more strange, because the crossed plants in the fifth pot (not included in the two last tables), in which all the remaining seeds had been thickly sown, were from the first finer plants than the self-fertilised, and had larger leaves. At the period when the two tallest crossed plants in this pot were 6 4/8 and 4 5/8 inches high, the two tallest self-fertilised were only 4 inches. When the two crossed plants were 12 and 10 inches high, the two self-fertilised were only 8 inches. These latter plants, as well as many others on the same side of this pot never grew any higher, whereas several of the crossed plants grew to the height of two feet! On account of this great superiority of the crossed plants, the plants on neither side of this pot have been included in the two last tables.
Thirty flowers on the crossed plants in Pots 1 and 4 (Table 6/79) were again crossed, and produced seventeen capsules. Thirty flowers on the self-fertilised plants in the same two pots were again self-fertilised, but produced only seven capsules. The contents of each capsule of both lots were placed in separate watch-glasses, and the seeds from the crossed appeared to the eye to be at least double the number of those from the self-fertilised capsules.
In order to ascertain whether the fertility of the self-fertilised plants had been lessened by the plants having been self-fertilised for the three previous generations, thirty flowers on the crossed plants were fertilised with their own pollen. These yielded only five capsules, and their seeds being placed in separate watch-glasses did not seem more numerous than those from the capsules on the self-fertilised plants self-fertilised for the fourth time. So that as far as can be judged from so few capsules, the self-fertility of the self-fertilised plants had not decreased in comparison with that of the plants which had been intercrossed during the three previous generations. It should, however, be remembered that both lots of plants had been subjected in each generation to almost exactly similar conditions.
Seeds from the crossed plants again crossed, and from the self-fertilised again self-fertilised, produced by the plants in Pot 1 (Table 6/79), in which the three self-fertilised plants were on an average only a little taller than the crossed, were used in the following experiment. They were kept separate from two similar lots of seeds produced by the two plants in Pot 4 in the same table, in which the crossed plant was much taller than its self-fertilised opponent.
CROSSED AND SELF-FERTILISED PLANTS OF THE FOURTH GENERATION (RAISED FROM THE PLANTS IN POT 1, TABLE 6/79).
Crossed and self-fertilised seeds from plants of the last generation in Pot 1 in Table 6/79, were placed on sand, and after germinating, were planted in pairs on the opposite sides of four pots. The seedlings when in full flower were measured to the base of the calyx. The remaining seeds were sown crowded on the two sides of Pot 5; and the four tallest plants on each side of this pot were measured in the same manner.
TABLE 6/80. Petunia violacea (fourth generation; raised from plants of the third generation in Pot 1, table 6/79).
Heights of plants measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 29 2/8 : 30 2/8. Pot 1 : 36 2/8 : 34 6/8. Pot 1 : 49 : 31 3/8.
Pot 2 : 33 3/8 : 31 5/8. Pot 2 : 37 3/8 : 38 2/8. Pot 2 : 56 4/8 : 38 4/8.
Pot 3 : 46 : 45 1/8. Pot 3 : 67 2/8 : 45. Pot 3 : 54 3/8 : 23 2/8.
Pot 4 : 51 6/8 : 34. Pot 4 : 51 7/8 : 0.
Pot 5 : 49 4/8 : 22 3/8. Pot 5 : 46 3/8 : 24 2/8. Pot 5 : 40 : 24 6/8. Pot 5 : 53 : 30. Crowded plants.
Total : 701.88 : 453.50.
The fifteen crossed plants average 46.79, and the fourteen (one having died) self-fertilised plants 32.39 inches in height; or as 100 to 69. So that the crossed plants in this generation had recovered their wonted superiority over the self-fertilised plants; though the parents of the latter in Pot 1, Table 6/79, were a little taller than their crossed opponents.
CROSSED AND SELF-FERTILISED PLANTS OF THE FOURTH GENERATION (RAISED FROM THE PLANTS IN POT 4, TABLE 6/79).
Two similar lots of seeds, obtained from the plants in Pot 4 in Table 6/79, in which the single crossed plant was at first shorter, but ultimately much taller than its self-fertilised opponent, were treated in every way like their brethren of the same generation in the last experiment. We have in Table 6/81 the measurements of the present plants. Although the crossed plants greatly exceeded in height the self-fertilised; yet in three out of the five pots a self-fertilised plant flowered before any one of the crossed; in a fourth pot simultaneously; and in a fifth (namely Pot 2) a crossed plant flowered first.
TABLE 6/81. Petunia violacea (fourth generation; raised from plants of the third generation in Pot 4, Table 6/79).
Heights of plants measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 46 : 30 2/8. Pot 1 : 46 : 28.
Pot 2 : 50 6/8 : 25. Pot 2 : 40 2/8 : 31 3/8. Pot 2 : 37 3/8 : 22 4/8.
Pot 3 : 54 2/8 : 22 5/8. Pot 3 : 61 1/8 : 26 6/8. Pot 3 : 45 : 32.
Pot 4 : 30 : 24 4/8. Pot 4 : 29 1/8 : 26.
Pot 5 : 37 4/8 : 40 2/8. Pot 5 : 63 : 18 5/8. Pot 5 : 41 2/8 : 17 4/8. Crowded plants.
Total : 581.63 : 349.36.
The thirteen crossed plants here average 44.74, and the thirteen self-fertilised plants 26.87 inches in height; or as 100 to 60. The crossed parents of these were much taller, relatively to the self-fertilised parents, than in the last case; and apparently they transmitted some of this superiority to their crossed offspring. It is unfortunate that I did not turn these plants out of doors, so as to observe their relative fertility, for I compared the pollen from some of the crossed and self-fertilised plants in Pot 1, Table 6/81, and there was a marked difference in its state; that of the crossed plants contained hardly any bad and empty grains, whilst such abounded in the pollen of the self-fertilised plants.
THE EFFECTS OF A CROSS WITH A FRESH STOCK.
I procured from a garden in Westerham, whence my plants originally came, a fresh plant differing in no respect from mine except in the colour of the flowers, which was a fine purple. But this plant must have been exposed during at least four generations to very different conditions from those to which my plants had been subjected, as these had been grown in pots in the greenhouse. Eight flowers on the self-fertilised plants in Table 6/81, of the last or fourth self-fertilised generation, were fertilised with pollen from this fresh stock; all eight produced capsules containing together by weight 5.01 grains of seeds. The plants raised from these seeds may be called the Westerham-crossed.
Eight flowers on the crossed plants of the last or fourth generation in Table 6/81 were again crossed with pollen from one of the other crossed plants, and produced five capsules, containing by weight 2.07 grains of seeds. The plants raised from these seeds may be called the INTERCROSSED; and these form the fifth intercrossed generation.
Eight flowers on the self-fertilised plants of the same generation in Table 6/81 were again self-fertilised, and produced seven capsules, containing by weight 2.1 grains of seeds. The SELF-FERTILISED plants raised from these seeds form the fifth self-fertilised generation. These latter plants and the intercrossed are comparable in all respects with the crossed and self-fertilised plants of the four previous generations.
From the foregoing data it is easy to calculate that:
Ten Westerham-crossed capsules would have contained 6.26 grains weight of seed.
Ten intercrossed capsules would have contained 4.14 grains weight of seed.
Ten self-fertilised capsules would have contained 3.00 grains weight of seed.
We thus get the following ratios:—
Seeds from the Westerham-crossed capsules to those from the capsules of the fifth self-fertilised generation, in weight as 100 to 48.
Seeds from the Westerham-crossed capsules to those from the capsules of the fifth intercrossed generation, in weight as 100 to 66.
Seeds from the intercrossed capsules to those from the self-fertilised capsules, in weight as 100 to 72.
So that a cross with pollen from a fresh stock greatly increased the productiveness of the flowers on plants which had been self-fertilised for the four previous generations, in comparison not only with the flowers on the same plants self-fertilised for the fifth time, but with the flowers on the crossed plants crossed with pollen from another plant of the same old stock for the fifth time.
These three lots of seeds were placed on sand, and were planted in an equal state of germination in seven pots, each made tripartite by three superficial partitions. Some of the remaining seeds, whether or not in a state of germination, were thickly sown in an eighth pot. The pots were kept in the greenhouse, and the plants trained up sticks. They were first measured to the tops of their stems when coming into flower; and the twenty-two Westerham-crossed plants then averaged 25.51 inches; the twenty-three intercrossed plants 30.38; and the twenty-three self-fertilised plants 23.40 inches in height. We thus get the following ratios:—
The Westerham-crossed plants in height to the self-fertilised as 100 to 91.
The Westerham-crossed plants in height to the intercrossed as 100 to 119.
The intercrossed plants in height to the self-fertilised as 100 to 77.
These plants were again measured when their growth appeared on a casual inspection to be complete. But in this I was mistaken, for after cutting them down, I found that the summits of the stems of the Westerham-crossed plants were still growing vigorously; whilst the intercrossed had almost, and the self-fertilised had quite completed their growth. Therefore I do not doubt, if the three lots had been left to grow for another month, that the ratios would have been somewhat different from those deduced from the measurements in Table 6/82.
TABLE 6/82. Petunia violacea.
Heights of plants measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Westerham-Crossed Plants (from self-fertilised Plants of fourth generation crossed by a fresh stock).
Column 3: Intercrossed Plants (Plants of one and the same stock intercrossed for five generations).
Column 4: Self-fertilised Plants (self-fertilised for five generations).
Pot 1 : 64 5/8 : 57 2/8 : 43 6/8. Pot 1 : 24 : 64 : 56 3/8. Pot 1 : 51 4/8 : 58 6/8 : 31 5/8.
Pot 2 : 48 7/8 : 59 7/8 : 41 5/8. Pot 2 : 54 4/8 : 58 2/8 : 41 2/8. Pot 2 : 58 1/8 : 53 : 18 2/8.
Pot 3 : 62 : 52 2/8 : 46 6/8. Pot 3 : 53 2/8 : 54 6/8 : 45. Pot 3 : 62 7/8 : 61 6/8 : 19 4/8.
Pot 4 : 44 4/8 : 58 7/8 : 37 5/8. Pot 4 : 49 2/8 : 65 2/8 : 33 2/8. Pot 4 : .. : 59 6/8 : 32 2/8.
Pot 5 : 43 1/8 : 35 6/8 : 41 6/8. Pot 5 : 53 7/8 : 34 6/8 : 26 4/8. Pot 5 : 53 2/8 : 54 6/8 : 0.
Pot 6 : 37 4/8 : 56 : 46 4/8. Pot 6 : 61 : 63 5/8 : 29 6/8. Pot 6 : 0 : 57 7/8 : 14 4/8.
Pot 7 : 59 6/8 : 51 : 43. Pot 7 : 43 4/8 : 49 6/8 : 12 2/8. Pot 7 : 50 5/8 : 0 : 0.
Pot 8 : 37 7/8 : 38 5/8 : 21 6/8. Pot 8 : 37 2/8 : 44 5/8 : 14 5/8.
Total : 1051.25 : 1190.50 : 697.88.
The twenty-one Westerham-crossed plants now averaged 50.05 inches; the twenty-two intercrossed plants, 54.11 inches; and the twenty-one self-fertilised plants, 33.23 inches in height. We thus get the following ratios:—
The Westerham-crossed plants in height to the self-fertilised as 100 to 66.
The Westerham-crossed plants in height to the intercrossed as 100 to 108.
The intercrossed plants in height to the self-fertilised as 100 to 61.
We here see that the Westerham-crossed (the offspring of plants self-fertilised for four generations and then crossed with a fresh stock) have gained greatly in height, since they were first measured, relatively to the plants self-fertilised for five generations. They were then as 100 to 91, and now as 100 to 66 in height. The intercrossed plants (i.e., those which had been intercrossed for the last five generations) likewise exceed in height the self-fertilised plants, as occurred in all the previous generations with the exception of the abnormal plants of the third generation. On the other hand, the Westerham-crossed plants are exceeded in height by the intercrossed; and this is a surprising fact, judging from most of the other strictly analogous cases. But as the Westerham-crossed plants were still growing vigorously, while the intercrossed had almost ceased to grow, there can hardly be a doubt that if left to grow for another month they would have beaten the intercrossed in height. That they were gaining on them is clear, as when measured before they were as 100 to 119, and now as only 100 to 108 in height. The Westerham-crossed plants had also leaves of a darker green, and looked altogether more vigorous than the intercrossed; and what is much more important, they produced, as we shall presently see, much heavier seed-capsules. So that in fact the offspring from the self-fertilised plants of the fourth generation crossed by a fresh stock were superior to the intercrossed, as well as to the self-fertilised plants of the fifth generation—of which latter fact there could not be the least doubt.
These three lots of plants were cut down close to the ground and weighed. The twenty-one Westerham-crossed plants weighed 32 ounces; the twenty-two intercrossed plants, 34 ounces, and the twenty-one self-fertilised plants 7 1/4 ounces. The following ratios are calculated for an equal number of plants of each kind. But as the self-fertilised plants were just beginning to wither, their relative weight is here slightly too small; and as the Westerham-crossed were still growing vigorously, their relative weight with time allowed would no doubt have greatly increased.
The Westerham-crossed plants in weight to the self-fertilised as 100 to 22.
The Westerham-crossed plants in weight to the intercrossed as 100 to 101.
The intercrossed plants in weight to the self-fertilised as 100 to 22.3.
We here see, judging by weight instead of as before by height, that the Westerham-crossed and the intercrossed have an immense advantage over the self-fertilised. The Westerham-crossed are inferior to the intercrossed by a mere trifle; but it is almost certain that if they had been allowed to go on growing for another month, the former would have completely beaten the latter.
As I had an abundance of seeds of the same three lots, from which the foregoing plants had been raised, these were sown in three long parallel and adjoining rows in the open ground, so as to ascertain whether under these circumstances the results would be nearly the same as before. Late in the autumn (November 13) the ten tallest plants were carefully selected out of each row, and their heights measured, with the following result:—
TABLE 6/83. Petunia violacea (plants growing in the open ground).
Heights of plants measured in inches.
Column 1: Westerham-Crossed Plants (from self-fertilised Plants of the fourth generation crossed by a fresh stock).
Column 2: intercrossed Plants (Plants of one and the same stock intercrossed for five generations).
Column 3: self-fertilised Plants (self-fertilised for five generations).
The ten Westerham-crossed plants here average 36.67 inches in height; the ten intercrossed plants, 38.27 inches; and the ten self-fertilised, 23.31 inches. These three lots of plants were also weighed; the Westerham-crossed plants weighed 28 ounces; the intercrossed plants, 41 ounces; and the self-fertilised, 14.75 ounces. We thus get the following ratios:—
The Westerham-crossed plants in height to the self-fertilised as 100 to 63.
The Westerham-crossed plants in weight to the self-fertilised as 100 to 53.
The Westerham-crossed plants in height to the intercrossed as 100 to 104.
The Westerham-crossed plants in weight to the intercrossed as 100 to 146.
The intercrossed plants in height to the self-fertilised as 100 to 61.
The intercrossed plants in weight to the self-fertilised as 100 to 36.
Here the relative heights of the three lots are nearly the same (within three or four per cent) as with the plants in the pots. In weight there is a much greater difference: the Westerham-crossed exceed the self-fertilised by much less than they did before; but the self-fertilised plants in the pots had become slightly withered, as before stated, and were in consequence unfairly light. The Westerham-crossed plants are here inferior in weight to the intercrossed plants in a much higher degree than in the pots; and this appeared due to their being much less branched, owing to their having germinated in greater numbers and consequently being much crowded. Their leaves were of a brighter green than those of the intercrossed and self-fertilised plants.
RELATIVE FERTILITY OF THE THREE LOTS OF PLANTS.
None of the plants in pots in the greenhouse ever produced a capsule; and this may be attributed in chief part to the exclusion of moths. Therefore the fertility of the three lots could be judged of only by that of the plants growing out of doors, which from being left uncovered were probably cross-fertilised. The plants in the three rows were exactly of the same age and had been subjected to closely similar conditions, so that any difference in their fertility must be attributed to their different origin; namely, to the one lot being derived from plants self-fertilised for four generations and then crossed with a fresh stock; to the second lot being derived from plants of the same old stock intercrossed for five generations; and to the third lot being derived from plants self-fertilised for five generations. All the capsules, some nearly mature and some only half-grown, were gathered, counted, and weighed from the ten finest plants in each of the three rows, of which the measurements and weights have already been given. The intercrossed plants, as we have seen, were taller and considerably heavier than the plants of the other two lots, and they produced a greater number of capsules than did even the Westerham-crossed plants; and this may be attributed to the latter having grown more crowded and being in consequence less branched. Therefore the average weight of an equal number of capsules from each lot of plants seems to be the fairest standard of comparison, as their weights will have been determined chiefly by the number of the included seeds. As the intercrossed plants were taller and heavier than the plants of the other two lots, it might have been expected that they would have produced the finest or heaviest capsules; but this was very far from being the case.
The ten tallest Westerham-crossed plants produced 111 ripe and unripe capsules, weighing 121.2 grains. Therefore 100 of such capsules would have weighed 109.18 grains.
The ten tallest intercrossed plants produced 129 capsules, weighing 76.45 grains. Therefore 100 of these capsules would have weighed 59.26 grains.
The ten tallest self-fertilised plants produced only 44 capsules, weighing 22.35 grains. Therefore 100 of these capsules would have weighed 50.79 grains.
From these data we get the following ratios for the fertility of the three lots, as deduced from the relative weights of an equal number of capsules from the finest plants in each lot:—
Westerham-crossed plants to self-fertilised plants as 100 to 46.
Westerham-crossed plants to intercrossed plants as 100 to 54.
Intercrossed plants to self-fertilised plants as 100 to 86.
We here see how potent the influence of a cross with pollen from a fresh stock has been on the fertility of plants self-fertilised for four generations, in comparison with plants of the old stock when either intercrossed or self-fertilised for five generations; the flowers on all these plants having been left to be freely crossed by insects or to fertilise themselves. The Westerham-crossed plants were also much taller and heavier plants than the self-fertilised, both in the pots and open ground; but they were less tall and heavy than the intercrossed plants. This latter result, however, would almost certainly have been reversed, if the plants had been allowed to grow for another month, as the Westerham-crossed were still growing vigorously, whilst the intercrossed had almost ceased to grow. This case reminds us of the somewhat analogous one of Eschscholtzia, in which plants raised from a cross with a fresh stock did not grow higher than the self-fertilised or intercrossed plants, but produced a greater number of seed-capsules, which contained a far larger average number of seeds.
COLOUR OF THE FLOWERS ON THE ABOVE THREE LOTS OF PLANTS.
The original mother-plant, from which the five successive self-fertilised generations were raised, bore dingy purple flowers. At no time was any selection practised, and the plants were subjected in each generation to extremely uniform conditions. The result was, as in some previous cases, that the flowers on all the self-fertilised plants, both in the pots and open ground, were absolutely uniform in tint; this being a dull, rather peculiar flesh colour. This uniformity was very striking in the long row of plants growing in the open ground, and these first attracted my attention. I did not notice in which generation the original colour began to change and to become uniform, but I have every reason to believe that the change was gradual. The flowers on the intercrossed plants were mostly of the same tint, but not nearly so uniform as those on the self-fertilised plants, and many of them were pale, approaching almost to white. The flowers on the plants from the cross with the purple-flowered Westerham stock were, as might have been expected, much more purple and not nearly so uniform in tint. The self-fertilised plants were also remarkably uniform in height, as judged by the eye; the intercrossed less so, whilst the Westerham-crossed plants varied much in height.
Nicotiana tabacum.
This plant offers a curious case. Out of six trials with crossed and self-fertilised plants, belonging to three successive generations, in one alone did the crossed show any marked superiority in height over the self-fertilised; in four of the trials they were approximately equal; and in one (i.e., in the first generation) the self-fertilised plants were greatly superior to the crossed. In no case did the capsules from flowers fertilised with pollen from a distinct plant yield many more, and sometimes they yielded much fewer seeds than the capsules from self-fertilised flowers. But when the flowers of one variety were crossed with pollen from a slightly different variety, which had grown under somewhat different conditions,—that is, by a fresh stock,—the seedlings derived from this cross exceeded in height and weight those from the self-fertilised flowers in an extraordinary degree.
Twelve flowers on some plants of the common tobacco, raised from purchased seeds, were crossed with pollen from a distinct plant of the same lot, and these produced ten capsules. Twelve flowers on the same plants were fertilised with their own pollen, and produced eleven capsules. The seeds in the ten crossed capsules weighed 31.7 grains, whilst those in ten of the self-fertilised capsules weighed 47.67 grains; or as 100 to 150. The much greater productiveness of the self-fertilised than of the crossed capsules can hardly be attributed to chance, as all the capsules of both lots were very fine and healthy ones.
The seeds were placed on sand, and several pairs in an equal state of germination were planted on the opposite sides of three pots. The remaining seeds were thickly sown on the two sides of Pot 4, so that the plants in this pot were much crowded. The tallest plant on each side of each pot was measured. Whilst the plants were quite young the four tallest crossed plants averaged 7.87 inches, and the four tallest self-fertilised 14.87 inches in height; or as 100 to 189. The heights at this age are given in the two left columns of Table 6/84.
When in full flower the tallest plants on each side were again measured, see the two right hand columns in Table 6/84. But I should state that the pots were not large enough, and the plants never grew to their proper height. The four tallest crossed plants now averaged 18.5, and the four tallest self-fertilised plants 32.75 inches in height; or as 100 to 178. In all four pots a self-fertilised plant flowered before any one of the crossed.
In Pot 4, in which the plants were extremely crowded, the two lots were at first equal; and ultimately the tallest crossed plant exceeded by a trifle the tallest self-fertilised plant. This recalled to my mind an analogous case in the one generation of Petunia, in which the self-fertilised plants were throughout their growth taller than the crossed in all the pots except in the crowded one. Accordingly another trial was made, and some of the same crossed and self-fertilised seeds of tobacco were sown thickly on opposite sides of two additional pots; the plants being left to grow up much crowded. When they were between 13 and 14 inches in height there was no difference between the two sides, nor was there any marked difference when the plants had grown as tall as they could; for in one pot the tallest crossed plant was 26 1/2 inches in height, and exceeded by 2 inches the tallest self-fertilised plant, whilst in the other pot, the tallest crossed plant was shorter by 3 1/2 inches than the tallest self-fertilised plant, which was 22 inches in height.
TABLE 6/84. Nicotiana tabacum (first generation).
Heights of plants measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants, May 20, 1868.
Column 3: self-fertilised Plants, May 20, 1868.
Column 4: Crossed Plants, December 6, 1868.
Column 5: self-fertilised Plants, December 6, 1868.
Pot 1 : 15 4/8 : 26 : 40 : 44.
Pot 2 : 3 : 15 : 6 4/8 : 43.
Pot 3 : 8 : 13 4/8 : 16 : 33.
Pot 4 : 5 : 5 : 11 4/8 : 11.
Total : 31.5 : 59.5 : 74.0 : 131.0.
As the plants did not grow to their proper height in the above small pots in Table 6/84, four crossed and four self-fertilised plants were raised from the same seed, and were planted in pairs on the opposite sides of four very large pots containing rich soil; so that they were not exposed to at all severe mutual competition. When these plants were in flower I neglected to measure them, but record in my notes that all four self-fertilised plants exceeded in height the four crossed plants by 2 or 3 inches. We have seen that the flowers on the original or parent-plants which were crossed with pollen from a distinct plant yielded much fewer seeds than those fertilised with their own pollen; and the trial just given, as well as that in Table 6/84, show us clearly that the plants raised from the crossed seeds were inferior in height to those from the self-fertilised seeds; but only when not greatly crowded. When crowded and thus subjected to very severe competition, the crossed and self-fertilised plants were nearly equal in height.
CROSSED AND SELF-FERTILISED PLANTS OF THE SECOND GENERATION.
Twelve flowers on the crossed plants of the last generation growing in the four large pots just mentioned, were crossed with pollen from a crossed plant growing in one of the other pots; and twelve flowers on the self-fertilised plants were fertilised with their own pollen. All these flowers of both lots produced fine capsules. Ten of the crossed capsules contained by weight 38.92 grains of seeds, and ten of the self-fertilised capsules 37.74 grains; or as 100 to 97. Some of these seeds in an equal state of germination were planted in pairs on the opposite sides of five large pots. A good many of the crossed seeds germinated before the self-fertilised, and were of course rejected. The plants thus raised were measured when several of them were in full flower.
TABLE 6/85. Nicotiana tabacum (second generation).
Heights of plants measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 14 4/8 : 27 6/8. Pot 1 : 78 4/8 : 8 6/8. Pot 1 : 9 : 56.
Pot 2 : 60 4/8 : 16 6/8. Pot 2 : 44 6/8 : 7. Pot 2 : 10 : 50 4/8.
Pot 3 : 57 1/8 : 87 (A). Pot 3 : 1 2/8 : 81 2/8 (B).
Pot 4 : 6 6/8 : 19. Pot 4 : 31 : 43 2/8. Pot 4 : 69 4/8 : 4.
Pot 5 : 99 4/8 : 9 4/8. Pot 5 : 29 2/8 : 3.
Total : 511.63 : 413.75.
The thirteen crossed plants here average 39.35, and the thirteen self-fertilised plants 31.82 inches in height; or as 100 to 81. But it would be a very much fairer plan to exclude all the starved plants of only 10 inches and under in height; and in this case the nine remaining crossed plants average 53.84, and the seven remaining self-fertilised plants 51.78 inches in height, or as 100 to 96; and this difference is so small that the crossed and self-fertilised plants may be considered as of equal heights.
In addition to these plants, three crossed plants were planted separately in three large pots, and three self-fertilised plants in three other large pots, so that they were not exposed to any competition; and now the self-fertilised plants exceeded the crossed in height by a little, for the three crossed averaged 55.91, and the three self-fertilised 59.16 inches; or as 100 to 106.
CROSSED AND SELF-FERTILISED PLANTS OF THE THIRD GENERATION.
TABLE 6/86. Nicotiana tabacum (third generation). Seedlings from the self-fertilised plant A in pot 3, Table 6/85, of the last or second generation.
Heights of plants measured in inches.
Column 1: Number (Name) of Pot.
Column 2: From Self-fertilised Plant, crossed by a Crossed Plant.
Column 3: From Self-fertilised Plant again self-fertilised, forming the third Self-fertilised generation.
Pot 1 : 100 2/8 : 98. Pot 1 : 91 : 79.
Pot 2 : 110 2/8 : 59 1/8. Pot 2 : 100 4/8 : 66 6/8.
Pot 3 : 104 : 79 6/8.
Pot 4 : 84 2/8 : 110 4/8. Pot 4 : 76 4/8 : 64 1/8.
Total : 666.75 : 557.25.
As I wished to ascertain, firstly, whether those self-fertilised plants of the last generation, which greatly exceeded in height their crossed opponents, would transmit the same tendency to their offspring, and secondly, whether they possessed the same sexual constitution, I selected for experiment the two self-fertilised plants marked A and B in Pot 3 in Table 6/85, as these two were of nearly equal height, and were greatly superior to their crossed opponents. Four flowers on each plant were fertilised with their own pollen, and four others on the same plants were crossed with pollen from one of the crossed plants growing in another pot. This plan differs from that before followed, in which seedlings from crossed plants again crossed, have been compared with seedlings from self-fertilised plants again self-fertilised. The seeds from the crossed and self-fertilised capsules of the above two plants were placed in separate watch-glasses and compared, but were not weighed; and in both cases those from the crossed capsules seemed to be rather less numerous than those from the self-fertilised capsules. These seeds were planted in the usual manner, and the heights of the crossed and self-fertilised seedlings, when fully grown, are given in Tables 6/86 and 6/87.
The seven crossed plants in the first of these two tables average 95.25, and the seven self-fertilised 79.6 inches in height; or as 100 to 83. In half the pots a crossed plant, and in the other half a self-fertilised plant flowered first.
We now come to the seedlings raised from the other parent-plant B.
TABLE 6/87. Nicotiana tabacum (third generation). Seedlings from the self-fertilised plant B in pot 3, Table 6/85, of the last or second generation.
Heights of plants measured in inches.
Column 1: Number (Name) of Pot.
Column 2: From Self-fertilised Plant, crossed by a Crossed Plant.
Column 3: From Self-fertilised Plant again self-fertilised, forming the third Self-fertilised generation.
Pot 1 : 87 2/8 : 72 4/8. Pot 1 : 49 : 14 2/8.
Pot 2 : 98 4/8 : 73. Pot 2 : 0 : 110 4/8.
Pot 3 : 99 : 106 4/8. Pot 3 : 15 2/8 : 73 6/8.
Pot 4 : 97 6/8 : 48 6/8.
Pot 5 : 48 6/8 : 81 2/8. Pot 5 : 0 : 61 2/8.
Total : 495.50 : 641.75.
The seven crossed plants (for two of them died) here average 70.78 inches, and the nine self-fertilised plants 71.3 inches in height; or as 100 to barely 101. In four out of these five pots, a self-fertilised plant flowered before any one of the crossed plants. So that, differently from the last case, the self-fertilised plants are in some respects slightly superior to the crossed.
If we now consider the crossed and self-fertilised plants of the three generations, we find an extraordinary diversity in their relative heights. In the first generation, the crossed plants were inferior to the self-fertilised as 100 to 178; and the flowers on the original parent-plants which were crossed with pollen from a distinct plant yielded much fewer seeds than the self-fertilised flowers, in the proportion of 100 to 150. But it is a strange fact that the self-fertilised plants, which were subjected to very severe competition with the crossed, had on two occasions no advantage over them. The inferiority of the crossed plants of this first generation cannot be attributed to the immaturity of the seeds, for I carefully examined them; nor to the seeds being diseased or in any way injured in some one capsule, for the contents of the ten crossed capsules were mingled together and a few taken by chance for sowing. In the second generation the crossed and self-fertilised plants were nearly equal in height. In the third generation, crossed and self-fertilised seeds were obtained from two plants of the previous generation, and the seedlings raised from them differed remarkably in constitution; the crossed in the one case exceeded the self-fertilised in height in the ratio of 100 to 83, and in the other case were almost equal. This difference between the two lots, raised at the same time from two plants growing in the same pot, and treated in every respect alike, as well as the extraordinary superiority of the self-fertilised over the crossed plants in the first generation, considered together, make me believe that some individuals of the present species differ to a certain extent from others in their sexual affinities (to use the term employed by Gartner), like closely allied species of the same genus. Consequently if two plants which thus differ are crossed, the seedlings suffer and are beaten by those from the self-fertilised flowers, in which the sexual elements are of the same nature. It is known that with our domestic animals certain individuals are sexually incompatible, and will not produce offspring, although fertile with other individuals. (6/3. I have given evidence on this head in my ‘Variation of Animals and Plants under Domestication’ chapter 18 2nd edition volume 2 page 146.) But Kolreuter has recorded a case which bears more closely on our present one, as it shows that in the genus Nicotiana the varieties differ in their sexual affinities. (6/4. ‘Das Geschlecht der Pflanzen, Zweite Fortsetzung’ 1764 pages 55-60.) He experimented on five varieties of the common tobacco, and proved that they were varieties by showing that they were perfectly fertile when reciprocally crossed; but one of these varieties, if used either as the father or the mother, was more fertile than any of the others when crossed with a widely distinct species, N. glutinosa. As the different varieties thus differ in their sexual affinities, there is nothing surprising in the individuals of the same variety differing in a like manner to a slight degree.
Taking the plants of the three generations altogether, the crossed show no superiority over the self-fertilised, and I can account for this fact only by supposing that with this species, which is perfectly self-fertile without insect aid, most of the individuals are in the same condition, as those of the same variety of the common pea and of a few other exotic plants, which have been self-fertilised for many generations. In such cases a cross between two individuals does no good; nor does it in any case, unless the individuals differ in general constitution, either from so-called spontaneous variation, or from their progenitors having been subjected to different conditions. I believe that this is the true explanation in the present instance, because, as we shall immediately see, the offspring of plants, which did not profit at all by being crossed with a plant of the same stock, profited to an extraordinary degree by a cross with a slightly different sub-variety.
THE EFFECTS OF A CROSS WITH A FRESH STOCK.
I procured some seed of N. tabacum from Kew and raised some plants, which formed a slightly different sub-variety from my former plants; as the flowers were a shade pinker, the leaves a little more pointed, and the plants not quite so tall. Therefore the advantage in height which the seedlings gained by this cross cannot be attributed to direct inheritance. Two of the plants of the third self-fertilised generation, growing in Pots 2 and 5 in Table 6/87, which exceeded in height their crossed opponents (as did their parents in a still higher degree) were fertilised with pollen from the Kew plants, that is, by a fresh stock. The seedlings thus raised may be called the Kew-crossed. Some other flowers on the same two plants were fertilised with their own pollen, and the seedlings thus raised from the fourth self-fertilised generation. The crossed capsules produced by the plant in Pot 2, Table 6/87, were plainly less fine than the self-fertilised capsules on the same plant. In Pot 5 the one finest capsule was also a self-fertilised one; but the seeds produced by the two crossed capsules together exceeded in number those produced by the two self-fertilised capsules on the same plant. Therefore as far as the flowers on the parent-plants are concerned, a cross with pollen from a fresh stock did little or no good; and I did not expect that the offspring would have received any benefit, but in this I was completely mistaken.
The crossed and self-fertilised seeds from the two plants were placed on bare sand, and very many of the crossed seeds of both sets germinated before the self-fertilised seeds, and protruded their radicles at a quicker rate. Hence many of the crossed seeds had to be rejected, before pairs in an equal state of germination were obtained for planting on the opposite sides of sixteen large pots. The two series of seedlings raised from the parent-plants in the two Pots 2 and 5 were kept separate, and when fully grown were measured to the tips of their highest leaves, as shown in Table 6/88. But as there was no uniform difference in height between the crossed and self-fertilised seedlings raised from the two plants, their heights have been added together in calculating the averages. I should state that by the accidental fall of a large bush in the greenhouse, several plants in both the series were much injured. These were at once measured together with their opponents and afterwards thrown away. The others were left to grow to their full height, and were measured when in flower. This accident accounts for the small height of some of the pairs; but as all the pairs, whether only partly or fully grown, were measured at the same time, the measurements are fair.
The average height of the twenty-six crossed plants in the sixteen pots of the two series is 63.29, and that of the twenty-six self-fertilised plants is 41.67 inches; or as 100 to 66. The superiority of the crossed plants was shown in another way, for in every one of the sixteen pots a crossed plant flowered before a self-fertilised one, with the exception of Pot 6 of the second series, in which the plants on the two sides flowered simultaneously.
TABLE 6/88. Nicotiana tabacum. Plants raised from two plants of the third self-fertilised generation in Pots 2 and 5, in Table 6/87.
Heights of plants measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Kew-crossed Plants, pot 2, Table 6/87.
Column 3: Plants of the fourth Self-fertilised generation, pot 2, Table 6/87.
Column 4: Kew-crossed Plants, pot 5, Table 6/87.
Column 5: Plants of the fourth Self-fertilised generation, pot 5, Table 6/87.
Pot 1 : 84 6/8 : 68 4/8 : 77 6/8 : 56. Pot 1 : 31 : 5 : 7 2/8 : 5 3/8.
Pot 2 : 78 4/8 : 51 4/8 : 55 4/8 : 27 6/8. Pot 2 : 48 : 70 : 18 : 7.
Pot 3 : 77 3/8 : 12 6/8 : 76 2/8 : 60 6/8. Pot 3 : 77 1/8 : 6 6/8.
Pot 4 : 49 2/8 : 29 4/8 : 90 4/8 : 11 6/8. Pot 4 : 15 6/8 : 32 : 22 2/8 : 4 1/8.
Pot 5 : 89 : 85 : 94 2/8 : 28 4/8. Pot 5 : 17 : 5 3/8.
Pot 6 : 90 : 80 : 78 : 78 6/8.
Pot 7 : 84 4/8 : 48 6/8 : 85 4/8 : 61 4/8. Pot 7 : 76 4/8 : 56 4/8.
Pot 8 : 83 4/8 : 84 4/8 : 65 5/8 : 78 3/8. Pot 8 : : : 72 2/8 : 27 4/8.
Total : 902.63 : 636.13 : 743.13 : 447.38.
Some of the remaining seeds of both series, whether or not in a state of germination, were thickly sown on the opposite sides of two very large pots; and the six highest plants on each side of each pot were measured after they had grown to nearly their full height. But their heights were much less than in the former trials, owing to their extremely crowded condition. Even whilst quite young, the crossed seedlings manifestly had much broader and finer leaves than the self-fertilised seedlings.
TABLE 6/89. Nicotiana tabacum. Plants of the same parentage as those in Table 6/88, but grown extremely crowded in two large pots.
Heights of plants measured in inches.
Column 1: Kew-crossed Plants, from pot 2, Table 6/87.
Column 2: Plants of the fourth Self-fertilised generation, from pot 2, Table 6/87.
Column 3: Kew-crossed Plants, from pot 5, Table 6/87.
Column 4: Plants of the fourth Self-fertilised generation, from pot 5, Table 6/87.
175.63 : 101.50 : 202.75 : 105.13.
The twelve tallest crossed plants in the two pots belonging to the two series average here 31.53, and the twelve tallest self-fertilised plants 17.21 inches in height; or as 100 to 54. The plants on both sides, when fully grown, some time after they had been measured, were cut down close to the ground and weighed. The twelve crossed plants weighed 21.25 ounces; and the twelve self-fertilised plants only 7.83 ounces; or in weight as 100 to 37.
The rest of the crossed and self-fertilised seeds from the two parent-plants (the same as in the last experiment) was sown on the 1st of July in four long parallel and separate rows in good soil in the open ground; so that the seedlings were not subjected to any mutual competition. The summer was wet and unfavourable for their growth. Whilst the seedlings were very small the two crossed rows had a clear advantage over the two self-fertilised rows. When fully grown the twenty tallest crossed plants and the twenty tallest self-fertilised plants were selected and measured on the 11th of November to the extremities of their leaves, as shown in Table 6/90. Of the twenty crossed plants, twelve had flowered; whilst of the twenty self-fertilised plants one alone had flowered.
TABLE 6/90. Nicotiana tabacum. Plants raised from the same seeds as in the last two experiments, but sown separately in the open ground, so as not to compete together.
Heights of plants measured in inches.
Column 1: Kew-crossed Plants, from pot 2, Table 6/87.
Column 2: Plants of the fourth Self-fertilised generation, from pot 2, Table 6/87.
Column 3: Kew-crossed Plants, from pot 5, Table 6/87.
Column 4: Plants of the fourth Self-fertilised generation, from pot 5, Table 6/87.
478.75 : 286.86 : 496.13 : 417.25
The twenty tallest crossed plants here average 48.74, and the twenty tallest self-fertilised 35.2 inches in height; or as 100 to 72. These plants after being measured were cut down close to the ground, and the twenty crossed plants weighed 195.75 ounces, and the twenty self-fertilised plants 123.25 ounces; or as 100 to 63.
In Tables 6/88, 6/89 and 6/90, we have the measurements of fifty-six plants derived from two plants of the third self-fertilised generation crossed with pollen from a fresh stock, and of fifty-six plants of the fourth self-fertilised generation derived from the same two plants. These crossed and self-fertilised plants were treated in three different ways, having been put, firstly, into moderately close competition with one another in pots; secondly, having been subjected to unfavourable conditions and to very severe competition from being greatly crowded in two large pots; and thirdly, having been sown separately in open and good ground, so as not to suffer from any mutual competition. In all these cases the crossed plants in each lot were greatly superior to the self-fertilised. This was shown in several ways,—by the earlier germination of the crossed seeds, by the more rapid growth of the seedlings whilst quite young, by the earlier flowering of the mature plants, as well as by the greater height which they ultimately attained. The superiority of the crossed plants was shown still more plainly when the two lots were weighed; the weight of the crossed plants to that of the self-fertilised in the two crowded pots being as 100 to 37. Better evidence could hardly be desired of the immense advantage derived from a cross with a fresh stock.
26. PRIMULACEAE.—Cyclamen persicum. (6/5. Cyclamen repandum according to Lecoq ‘Geographie Botanique de l’Europe’ tome 8 1858 page 150, is proterandrous, and this I believe to be the case with Cyclamen persicum.)
Ten flowers crossed with pollen from plants known to be distinct seedlings, yielded nine capsules, containing on an average 34.2 seeds, with a maximum of seventy-seven in one. Ten flowers self-fertilised yielded eight capsules, containing on an average only 13.1 seeds, with a maximum of twenty-five in one. This gives a ratio of 100 to 38 for the average number of seeds per capsule for the crossed and self-fertilised flowers. The flowers hang downwards, and as the stigmas stand close beneath the anthers, it might have been expected that pollen would have fallen on them, and that they would have been spontaneously self-fertilised; but these covered-up plants did not produce a single capsule. On some other occasions uncovered plants in the same greenhouse produced plenty of capsules, and I suppose that the flowers had been visited by bees, which could hardly fail to carry pollen from plant to plant.
The seeds obtained in the manner just described were placed on sand, and after germinating were planted in pairs,—three crossed and three self-fertilised plants on the opposite sides of four pots. When the leaves were 2 or 3 inches in length, including the foot-stalks, the seedlings on both sides were equal. In the course of a month or two the crossed plants began to show a slight superiority over the self-fertilised, which steadily increased; and the crossed flowered in all four pots some weeks before, and much more profusely than the self-fertilised. The two tallest flower-stems on the crossed plants in each pot were now measured, and the average height of the eight stems was 9.49 inches. After a considerable interval of time the self-fertilised plants flowered, and several of their flower-stems (but I forgot to record how many) were roughly measured, and their average height was a little under 7.5 inches; so that the flower-stems on the crossed plants to those on the self-fertilised were at least as 100 to 79. The reason why I did not make more careful measurements of the self-fertilised plants was, that they looked such poor specimens that I determined to there them re-potted in larger pots and in the following year to measure them carefully; but we shall see that this was partly frustrated by so few flower-stems being then produced.
These plants were left uncovered in the greenhouse; and the twelve crossed plants produced forty capsules, whilst the twelve self-fertilised plants produced only five; or as 100 to 12. But this difference does not give a just idea of the relative fertility of the two lots. I counted the seeds in one of the finest capsules on the crossed plants, and it contained seventy-three; whilst the finest of the five capsules produced by the self-fertilised plants contained only thirty-five good seeds. In the other four capsules most of the seeds were barely half as large as those in the crossed capsules.
TABLE 6/91. Cyclamen persicum: 0 implies that no flower-stem was produced.
Heights of plants measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 10 : 0. Pot 1 : 9 2/8 : 0. Pot 1 : 10 2/8 : 0.
Pot 2 : 9 2/8 : 0. Pot 2 : 10 : 0. Pot 2 : 10 2/8 : 0.
Pot 3 : 9 1/8 : 8. Pot 3 : 9 5/8 : 6 7/8. Pot 3 : 9 5/8 : 6 6/8.
Pot 4 : 11 1/8 : 0. Pot 4 : 10 5/8 : 7 7/8. Pot 4 : 10 6/8 : 0.
Total : 119.88 : 29.50.
In the following year the crossed plants again bore many flowers before the self-fertilised bore a single one. The three tallest flower-stems on the crossed plants in each of the pots were measured, as shown in Table 6/91. In Pots 1 and 2 the self-fertilised plants did not produce a single flower-stem; in Pot 4 only one; and in Pot 3 six, of which the three tallest were measured.
The average height of the twelve flower-stems on the crossed plants is 9.99, and that of the four flower-stems on the self-fertilised plants 7.37 inches; or as 100 to 74. The self-fertilised plants were miserable specimens, whilst the crossed ones looked very vigorous.
ANAGALLIS.
Anagallis collina, var. grandiflora (pale red and blue-flowered sub-varieties).
Firstly, twenty-five flowers on some plants of the red variety were crossed with pollen from a distinct plant of the same variety, and produced ten capsules; thirty-one flowers were fertilised with their own pollen, and produced eighteen capsules. These plants, which were grown in pots in the greenhouse, were evidently in a very sterile condition, and the seeds in both sets of capsules, especially in the self-fertilised, although numerous, were of so poor a quality that it was very difficult to determine which were good and which bad. But as far as I could judge, the crossed capsules contained on an average 6.3 good seeds, with a maximum in one of thirteen; whilst the self-fertilised contained 6.05 such seeds, with a maximum in one of fourteen.
Secondly, eleven flowers on the red variety were castrated whilst young and fertilised with pollen from the blue variety, and this cross evidently much increased their fertility; for the eleven flowers yielded seven capsules, which contained on an average twice as many good seeds as before, namely, 12.7; with a maximum in two of the capsules of seventeen seeds. Therefore these crossed capsules yielded seeds compared with those in the foregoing self-fertilised capsules, as 100 to 48. These seeds were also conspicuously larger than those from the cross between two individuals of the same red variety, and germinated much more freely. The flowers on most of the plants produced by the cross between the two-coloured varieties (of which several were raised), took after their mother, and were red-coloured. But on two of the plants the flowers were plainly stained with blue, and to such a degree in one case as to be almost intermediate in tint.
The crossed seeds of the two foregoing kinds and the self-fertilised were sown on the opposite sides of two large pots, and the seedlings were measured when fully grown, as shown in Tables 6/92a and 6/92b.
TABLE 6/92a. Anagallis collina: Red variety crossed by a distinct plant of the red variety, and red variety self-fertilised.
Heights of plants measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 23 4/8 : 15 4/8. Pot 1 : 21 : 15 4/8. Pot 1 : 17 2/8 : 14.
Total : 61.75 : 45.00.
TABLE 6/92b. Anagallis collina: Red variety crossed by blue variety, and red variety self-fertilised.
Heights of plants measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 2 : 30 4/8 : 24 4/8. Pot 2 : 27 3/8 : 18 4/8. Pot 2 : 25 : 11 6/8.
Total : 82.88 : 54.75.
As the plants of the two lots are few in number, they may be run together for the general average; but I may first state that the height of the seedlings from the cross between two individuals of the red variety is to that of the self-fertilised plants of the red variety as 100 to 73; whereas the height of the crossed offspring from the two varieties to the self-fertilised plants of the red variety is as 100 to 66. So that the cross between the two varieties is here seen to be the most advantageous. The average height of all six crossed plants in the two lots taken together is 48.20, and that of the six self-fertilised plants 33.25; or as 100 to 69.
These six crossed plants produced spontaneously twenty-six capsules, whilst the six self-fertilised plants produced only two, or as 100 to 8. There is therefore the same extraordinary difference in fertility between the crossed and self-fertilised plants as in the last genus, Cyclamen, which belongs to the same family of the Primulaceae.
Primula veris. British flora. (var. officinalis, Linn.).
THE COWSLIP.
Most of the species in this genus are heterostyled or dimorphic; that is, they present two forms,—one long-styled with short stamens, and the other short-styled with long stamens. (6/6. See my paper ‘On the Two Forms or Dimorphic Condition in the Species of Primula’ in ‘Journal of the Proceedings of the Linnean Society’ volume 6 1862 page 77. A second paper, to which I presently refer ‘On the Hybrid-like Nature of the Offspring from the Illegitimate Unions of Dimorphic and Trimorphic Plants’ was published in volume 10 1867 page 393 of the same journal.) For complete fertilisation it is necessary that pollen from the one form should be applied to the stigma of the other form; and this is effected under nature by insects. Such unions, and the seedlings raised from them, I have called legitimate. If one form is fertilised with pollen from the same form, the full complement of seed is not produced; and in the case of some heterostyled genera no seed at all is produced. Such unions, and the seedlings raised from them, I have called illegitimate. These seedlings are often dwarfed and more or less sterile, like hybrids. I possessed some long-styled plants of Primula veris, which during four successive generations had been produced from illegitimate unions between long-styled plants; they were, moreover, in some degree inter-related, and had been subjected all the time to similar conditions in pots in the greenhouse. As long as they were cultivated in this manner, they grew well and were healthy and fertile. Their fertility even increased in the later generations, as if they were becoming habituated to illegitimate fertilisation. Plants of the first illegitimate generation when taken from the greenhouse and planted in moderately good soil out of doors grew well and were healthy; but when those of the two last illegitimate generations were thus treated they became excessively sterile and dwarfed, and remained so during the following year, by which time they ought to have become accustomed to growing out of doors, so that they must have possessed a weak constitution.
Under these circumstances, it seemed advisable to ascertain what would be the effect of legitimately crossing long-styled plants of the fourth illegitimate generation with pollen taken from non-related short-styled plants, growing under different conditions. Accordingly several flowers on plants of the fourth illegitimate generation (i.e., great-great-grandchildren of plants which had been legitimately fertilised), growing vigorously in pots in the greenhouse, were legitimately fertilised with pollen from an almost wild short-styled cowslip, and these flowers yielded some fine capsules. Thirty other flowers on the same illegitimate plants were fertilised with their own pollen, and these yielded seventeen capsules, containing on an average thirty-two seeds. This is a high degree of fertility; higher, I believe, than that which generally obtains with illegitimately fertilised long-styled plants growing out of doors, and higher than that of the previous illegitimate generations, although their flowers were fertilised with pollen taken from a distinct plant of the same form.
These two lots of seeds were sown (for they will not germinate well when placed on bare sand) on the opposite sides of four pots, and the seedlings were thinned, so that an equal number were left on the two sides. For some time there was no marked difference in height between the two lots; and in Pot 3, Table 6/93, the self-fertilised plants were rather the tallest. But by the time that they had thrown up young flower-stems, the legitimately crossed plants revealed much the finest, and had greener and larger leaves. The breadth of the largest leaf on each plant was measured, and those on the crossed plants were on an average a quarter of an inch (exactly .28 of an inch) broader than those on the self-fertilised plants. The plants, from being too much crowded, produced poor and short flower-stems. The two finest on each side were measured; the eight on the legitimately crossed plants averaged 4.08, and the eight on the illegitimately self-fertilised plants averaged 2.93 inches in height; or as 100 to 72.
These plants after they had flowered were turned out of their pots, and planted in fairly good soil in the open ground. In the following year (1870), when in full flower, the two tallest flower-stems on each side were again measured, as shown in Table 6/93, which likewise gives the number of flower-stems produced on both sides of all the pots.
TABLE 6/93. Primula veris.
Heights of plants measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Height: Legitimately crossed Plants.
Column 3: Number of Flower-stems produced: Legitimately crossed Plants.
Column 4: Height: Illegitimately crossed Plants.
Column 5: Number of Flower-stems produced: Illegitimately crossed Plants.
Pot 1 : 9 : 16 : 2 1/8 : 3. Pot 1 : 8 : : 3 4/8.
Pot 2 : 7 : 16 : 6 : 3. Pot 2 : 6 4/8 : : 5 4/8.
Pot 3 : 6 : 16 : 3 : 4. Pot 3 : 6 2/8 : : 0 4/8.
Pot 4 : 7 3/8 : 14 : 2 5/8 : 5. Pot 4 : 6 1/8 : : 2 4/8.
Total : 56.26 : 62 : 25.75 : 15.
The average height of the eight tallest flower-stems on the crossed plants is here 7.03 inches, and that of the eight tallest flower-stems on the self-fertilised plants 3.21 inches; or as 100 to 46. We see, also, that the crossed plants bore sixty-two flower-stems; that is, above four times as many as those (namely fifteen) borne by the self-fertilised plants. The flowers were left exposed to the visits of insects, and as many plants of both forms grew close by, they must have been legitimately and naturally fertilised. Under these circumstances the crossed plants produced 324 capsules, whilst the self-fertilised produced only 16; and these were all produced by a single plant in Pot 2, which was much finer than any other self-fertilised plant. Judging by the number of capsules produced, the fertility of an equal number of crossed and self-fertilised plants was as 100 to 5.
In the succeeding year (1871) I did not count all the flower-stems on these plants, but only those which produced capsules containing good seeds. The season was unfavourable, and the crossed plants produced only forty such flower-stems, bearing 168 good capsules, whilst the self-fertilised plants produced only two such flower-stems, bearing only 6 capsules, half of which were very poor ones. So that the fertility of the two lots, judging by the number of capsules, was as 100 to 3.5.
In considering the great difference in height and the wonderful difference in fertility between the two sets of plants, we should bear in mind that this is the result of two distinct agencies. The self-fertilised plants were the product of illegitimate fertilisation during five successive generations, in all of which, excepting the last, the plants had been fertilised with pollen taken from a distinct individual belonging to the same form, but which was more or less closely related. The plants had also been subjected in each generation to closely similar conditions. This treatment alone, as I know from other observations, would have greatly reduced the size and fertility of the offspring. On the other hand, the crossed plants were the offspring of long-styled plants of the fourth illegitimate generation legitimately crossed with pollen from a short-styled plant, which, as well as its progenitors, had been exposed to very different conditions; and this latter circumstance alone would have given great vigour to the offspring, as we may infer from the several analogous cases already given. How much proportional weight ought to be attributed to these two agencies,—the one tending to injure the self-fertilised offspring, and the other to benefit the crossed offspring,—cannot be determined. But we shall immediately see that the greater part of the benefit, as far as increased fertility is concerned, must be attributed to the cross having been made with a fresh stock.
Primula veris.
EQUAL-STYLED AND RED-FLOWERED VAR.
I have described in my paper ‘On the Illegitimate Unions of Dimorphic and Trimorphic Plants’ this remarkable variety, which was sent to me from Edinburgh by Mr. J. Scott. It possessed a pistil proper to the long-styled form, and stamens proper to the short-styled form; so that it had lost the heterostyled or dimorphic character common to most of the species of the genus, and may be compared with an hermaphrodite form of a bisexual animal. Consequently the pollen and stigma of the same flower are adapted for complete mutual fertilisation, instead of its being necessary that pollen should be brought from one form to another, as in the common cowslip. From the stigma and anthers standing nearly on the same level, the flowers are perfectly self-fertile when insects are excluded. Owing to the fortunate existence of this variety, it is possible to fertilise its flowers in a legitimate manner with their own pollen, and to cross other flowers in a legitimate manner with pollen from another variety or fresh stock. Thus the offspring from both unions can be compared quite fairly, free from any doubt from the injurious effects of an illegitimate union.
The plants on which I experimented had been raised during two successive generations from spontaneously self-fertilised seeds produced by plants under a net; and as the variety is highly self-fertile, its progenitors in Edinburgh may have been self-fertilised during some previous generations. Several flowers on two of my plants were legitimately crossed with pollen from a short-styled common cowslip growing almost wild in my orchard; so that the cross was between plants which had been subjected to considerably different conditions. Several other flowers on the same two plants were allowed to fertilise themselves under a net; and this union, as already explained, is a legitimate one.
The crossed and self-fertilised seeds thus obtained were sown thickly on the opposite sides of three pots, and the seedlings thinned, so that an equal number were left on the two sides. The seedlings during the first year were nearly equal in height, excepting in Pot 3, Table 6/94, in which the self-fertilised plants had a decided advantage. In the autumn the plants were bedded out, in their pots; owing to this circumstance, and to many plants growing in each pot, they did not flourish, and none were very productive in seeds. But the conditions were perfectly equal and fair for both sides. In the following spring I record in my notes that in two of the pots the crossed plants are “incomparably the finest in general appearance,” and in all three pots they flowered before the self-fertilised. When in full flower the tallest flower-stem on each side of each pot was measured, and the number of the flower-stems on both sides counted, as shown in Table 6/94. The plants were left uncovered, and as other plants were growing close by, the flowers no doubt were crossed by insects. When the capsules were ripe they were gathered and counted, and the result is likewise shown in Table 6/94.
TABLE 6/94. Primula veris (equal-styled, red-flowered variety).
Heights of plants measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Height of tallest flower-stem: crossed Plants.
Column 3: Number of Flower-stems: crossed Plants.
Column 4: Number of good capsules: crossed Plants.
Column 5: Height of tallest flower-stem: self-fertilised Plants.
Column 6: Number of Flower-stems: self-fertilised Plants.
Column 7: Number of good capsules: self-fertilised Plants.
Pot 1 : 10 : 14 : 163 : 6 4/8 : 6 : 6.
Pot 3 : 7 4/8 : 7 : 43 : 10 4/8 : 5 : 26.
Totals : 26.0 : 33 : 206 : 22.0 : 13 : 32.
The average height of the three tallest flower-stems on the crossed plants is 8.66 inches, and that of the three on the self-fertilised plants 7.33 inches; or as 100 to 85.
All the crossed plants together produced thirty-three flower-stems, whilst the self-fertilised bore only thirteen. The number of the capsules were counted only on the plants in Pots 1 and 3, for the self-fertilised plants in Pot 2 produced none; therefore those on the crossed plants on the opposite side were not counted. Capsules not containing any good seeds were rejected. The crossed plants in the above two pots produced 206, and the self-fertilised in the same pots only 32 capsules; or as 100 to 15. Judging from the previous generations, the extreme unproductiveness of the self-fertilised plants in this experiment was wholly due to their having been subjected to unfavourable conditions, and to severe competition with the crossed plants; for had they grown separately in good soil, it is almost certain that they would have produced a large number of capsules. The seeds were counted in twenty capsules from the crossed plants, and they averaged 24.75; whilst in twenty capsules from the self-fertilised plants the average was 17.65; or as 100 to 71. Moreover, the seeds from the self-fertilised plants were not nearly so fine as those from the crossed plants. If we consider together the number of capsules produced and the average number of contained seeds, the fertility of the crossed plants to the self-fertilised plants was as 100 to 11. We thus see what a great effect, as far as fertility is concerned, was produced by a cross between the two varieties, which had been long exposed to different conditions, in comparison with self-fertilisation; the fertilisation having been in both cases of the legitimate order.
Primula sinensis.
As the Chinese primrose is a heterostyled or dimorphic plant, like the common cowslip, it might have been expected that the flowers of both forms when illegitimately fertilised with their own pollen or with that from flowers on another plant of the same form, would have yielded less seed than the legitimately crossed flowers; and that the seedlings raised from illegitimately self-fertilised seeds would have been somewhat dwarfed and less fertile, in comparison with the seedlings from legitimately crossed seeds. This holds good in relation to the fertility of the flowers; but to my surprise there was no difference in growth between the offspring from a legitimate union between two distinct plants, and from an illegitimate union whether between the flowers on the same plant, or between distinct plants of the same form. But I have shown, in the paper before referred to, that in England this plant is in an abnormal condition, such as, judging from analogous cases, would tend to render a cross between two individuals of no benefit to the offspring. Our plants have been commonly raised from self-fertilised seeds; and the seedlings have generally been subjected to nearly uniform conditions in pots in greenhouses. Moreover, many of the plants are now varying and changing their character, so as to become in a greater or less degree equal-styled, and in consequence highly self-fertile. From the analogy of Primula veris there can hardly be a doubt that if a plant of Primula sinensis could have been procured direct from China, and if it had been crossed with one of our English varieties, the offspring would have shown wonderful superiority in height and fertility (though probably not in the beauty of their flowers) over our ordinary plants.
My first experiment consisted in fertilising many flowers on long-styled and short-styled plants with their own pollen, and other flowers on the same plants with pollen taken from distinct plants belonging to the same form; so that all the unions were illegitimate. There was no uniform and marked difference in the number of seeds obtained from these two modes of self-fertilisation, both of which were illegitimate. The two lots of seeds from both forms were sown thickly on opposite sides of four pots, and numerous plants thus raised. But there was no difference in their growth, excepting in one pot, in which the offspring from the illegitimate union of two long-styled plants exceeded in a decided manner in height the offspring of flowers on the same plants fertilised with their own pollen. But in all four pots the plants raised from the union of distinct plants belonging to the same form, flowered before the offspring from the self-fertilised flowers.
Some long-styled and short-styled plants were now raised from purchased seeds, and flowers on both forms were legitimately crossed with pollen from a distinct plant; and other flowers on both forms were illegitimately fertilised with pollen from the flowers on the same plant. The seeds were sown on opposite sides of Pots 1 to 4 in Table 6/95; a single plant being left on each side. Several flowers on the illegitimate long-styled and short-styled plants described in the last paragraph, were also legitimately and illegitimately fertilised in the manner just described, and their seeds were sown in Pots 5 to 8 in the same table. As the two sets of seedlings did not differ in any essential manner, their measurements are given in a single table. I should add that the legitimate unions in both cases yielded, as might have been expected, many more seeds than the illegitimate unions. The seedlings whilst half-grown presented no difference in height on the two sides of the several pots. When fully grown they were measured to the tips of their longest leaves, and the result is given in Table 6/95.
TABLE 6/95. Primula sinensis.
Heights of plants measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Plants from legitimately Crossed seeds.
Column 3: Plants from illegitimately Self-fertilised seeds.
Pot 1 : 8 2/8 : 8. From short-styled mother.
Pot 2 : 7 4/8 : 8 5/8. From short-styled mother.
Pot 3 : 9 5/8 : 9 3/8. From long-styled mother.
Pot 4 : 8 4/8 : 8 2/8. From long-styled mother.
Pot 5 : 9 3/8 : 9. From illegitimate short-styled mother.
Pot 6 : 9 7/8 : 9 4/8. From illegitimate short-styled mother.
Pot 7 : 8 4/8 : 9 4/8. From illegitimate long-styled mother.
Pot 8 : 10 4/8 : 10. From illegitimate long-styled mother.
Total : 72.13 : 72.25.
In six out of the eight pots the legitimately crossed plants exceeded in height by a trifle the illegitimately self-fertilised plants; but the latter exceeded the former in two of the pots in a more strongly marked manner. The average height of the eight legitimately crossed plants is 9.01, and that of the eight illegitimately self-fertilised 9.03 inches, or as 100 to 100.2. The plants on the opposite sides produced, as far as could be judged by the eye, an equal number of flowers. I did not count the capsules or the seeds produced by them; but undoubtedly, judging from many previous observations, the plants derived from the legitimately crossed seeds would have been considerably more fertile than those from the illegitimately self-fertilised seeds. The crossed plants, as in the previous case, flowered before the self-fertilised plants in all the pots except in Pot 2, in which the two sides flowered simultaneously; and this early flowering may, perhaps, be considered as an advantage.
27. POLYGONEAE.—Fagopyrum esculentum.
This plant was discovered by Hildebrand to be heterostyled, that is, to present, like the species of Primula, a long-styled and a short-styled form, which are adapted for reciprocal fertilisation. Therefore the following comparison of the growth of the crossed and self-fertilised seedlings is not fair, for we do not know whether the difference in their heights may not be wholly due to the illegitimate fertilisation of the self-fertilised flowers.
I obtained seeds by legitimately crossing flowers on long-styled and short-styled plants, and by fertilising other flowers on both forms with pollen from the same plant. Rather more seeds were obtained by the former than by the latter process; and the legitimately crossed seeds were heavier than an equal number of the illegitimately self-fertilised seeds, in the ratio of 100 to 82. Crossed and self-fertilised seeds from the short-styled parents, after germinating on sand, were planted in pairs on the opposite sides of a large pot; and two similar lots of seeds from long-styled parents were planted in a like manner on the opposite sides of two other pots. In all three pots the legitimately crossed seedlings, when a few inches in height, were taller than the self-fertilised; and in all three pots they flowered before them by one or two days. When fully grown they were all cut down close to the ground, and as I was pressed for time, they were placed in a long row, the cut end of one plant touching the tip of another, and the total length of the legitimately crossed plants was 47 feet 7 inches, and of the illegitimately self-fertilised plants 32 feet 8 inches. Therefore the average height of the fifteen crossed plants in all three pots was 38.06 inches, and that of the fifteen self-fertilised plants 26.13 inches; or as 100 to 69.
28. CHENOPODIACEAE.—Beta vulgaris.
A single plant, no others growing in the same garden, was left to fertilise itself, and the self-fertilised seeds were collected. Seeds were also collected from a plant growing in the midst of a large bed in another garden; and as the incoherent pollen is abundant, the seeds of this plant will almost certainly have been the product of a crossed between distinct plants by means of the wind. Some of the two lots of seeds were sown on the opposite sides of two very large pots; and the young seedlings were thinned, so that an equal but considerable number was left on the two sides. These plants were thus subjected to very severe competition, as well as to poor conditions. The remaining seeds were sown out of doors in good soil in two long and not closely adjoining rows, so that these seedlings were placed under favourable conditions, and were not subjected to any mutual competition. The self-fertilised seeds in the open ground came up very badly; and on removing the soil in two or three places, it was found that many had sprouted under ground and had then died. No such case had been observed before. Owing to the large number of seedlings which thus perished, the surviving self-fertilised plants grew thinly in the row, and thus had an advantage over the crossed plants, which grew very thickly in the other row. The young plants in the two rows were protected by a little straw during the winter, and those in the two large pots were placed in the greenhouse.
There was no difference between the two lots in the pots until the ensuing spring, when they had grown a little, and then some of the crossed plants were finer and taller than any of the self-fertilised. When in full flower their stems were measured, and the measurements are given in Table 6/96.
TABLE 6/96. Beta vulgaris.
Heights of flower stems measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 34 6/8 : 36. Pot 1 : 30 : 20 1/8. Pot 1 : 33 6/8 : 32 2/8. Pot 1 : 34 4/8 : 32.
Pot 2 : 42 3/8 : 42 1/8. Pot 2 : 33 1/8 : 26 4/8. Pot 2 : 31 2/8 : 29 2/8. Pot 2 : 33 : 20 2/8.
Total : 272.75 : 238.50.
The average height of the eight crossed plants is here 34.09, and that of the eight self-fertilised plants 29.81; or as 100 to 87.
With respect to the plants in the open ground, each long row was divided into half, so as to diminish the chance of any accidental advantage in one part of either row; and the four tallest plants in the two halves of the two rows were carefully selected and measured. The eight tallest crossed plants averaged 30.92, and the eight tallest self-fertilised 30.7 inches in height, or as 100 to 99; so that they were practically equal. But we should bear in mind that the trial was not quite fair, as the self-fertilised plants had a great advantage over the crossed in being much less crowded in their own row, owing to the large number of seeds which had perished under ground after sprouting. Nor were the lots in the two rows subjected to any mutual competition.
29. CANNACEAE.—Canna warscewiczi.
In most or all the species belonging to this genus, the pollen is shed before the flower expands, and adheres in a mass to the foliaceous pistil close beneath the stigmatic surface. As the edge of this mass generally touches the edge of the stigma, and as it was ascertained by trials purposely made that a very few pollen-grains suffice for fertilisation, the present species and probably all the others of the genus are highly self-fertile. Exceptions occasionally occur in which, from the stamen being slightly shorter than usual, the pollen is deposited a little beneath the stigmatic surface, and such flowers drop off unimpregnated unless they are artificially fertilised. Sometimes, though rarely, the stamen is a little longer than usual, and then the whole stigmatic surface gets thickly covered with pollen. As some pollen is generally deposited in contact with the edge of the stigma, certain authors have concluded that the flowers are invariably self-fertilised. This is an extraordinary conclusion, for it implies that a great amount of pollen is produced for no purpose. On this view, also, the large size of the stigmatic surface is an unintelligible feature in the structure of the flower, as well as the relative position of all the parts, which is such that when insects visit the flowers to suck the copious nectar, they cannot fail to carry pollen from one flower to another. (6/7. Delpino has described ‘Bot. Zeitung’ 1867 page 277 and ‘Scientific Opinion’ 1870 page 135, the structure of the flowers in this genus, but he was mistaken in thinking that self-fertilisation is impossible, at least in the case of the present species. Dr. Dickie and Professor Faivre state that the flowers are fertilised in the bud, and that self-fertilisation is inevitable. I presume that they were misled by the pollen being deposited at a very early period on the pistil: see ‘Journal of Linnean Society Botany’ volume 10 page 55 and ‘Variabilité des Espèces’ 1868 page 158.)
According to Delpino, bees eagerly visit the flowers in North Italy, but I have never seen any insect visiting the flowers of the present species in my hothouse, although many plants grew there during several years. Nevertheless these plants produced plenty of seed, as they likewise did when covered by a net; they are therefore fully capable of self-fertilisation, and have probably been self-fertilised in this country for many generations. As they are cultivated in pots, and are not exposed to competition with surrounding plants, they have also been subjected for a considerable time to somewhat uniform conditions. This, therefore, is a case exactly parallel with that of the common pea, in which we have no right to expect much or any good from intercrossing plants thus descended and thus treated; and no good did follow, excepting that the cross-fertilised flowers yielded rather more seeds than the self-fertilised. This species was one of the earlier ones on which I experimented, and as I had not then raised any self-fertilised plants for several successive generations under uniform conditions, I did not know or even suspect that such treatment would interfere with the advantages to be gained from a cross. I was therefore much surprised at the crossed plants not growing more vigorously than the self-fertilised, and a large number of plants were raised, notwithstanding that the present species is an extremely troublesome one to experiment on. The seeds, even those which have been long soaked in water, will not germinate well on bare sand; and those that were sown in pots (which plan I was forced to follow) germinated at very unequal intervals of time; so that it was difficult to get pairs of the same exact age, and many seedlings had to be pulled up and thrown away. My experiments were continued during three successive generations; and in each generation the self-fertilised plants were again self-fertilised, their early progenitors in this country having probably been self-fertilised for many previous generations. In each generation, also, the crossed plants were fertilised with pollen from another crossed plant.
Of the flowers which were crossed in the three generations, taken together, a rather larger proportion yielded capsules than did those which were self-fertilised. The seeds were counted in forty-seven capsules from the crossed flowers, and they contained on an average 9.95 seeds; whereas forty-eight capsules from the self-fertilised flowers contained on an average 8.45 seeds; or as 100 to 85. The seeds from the crossed flowers were not heavier, on the contrary a little lighter, than those from the self-fertilised flowers, as was thrice ascertained. On one occasion I weighed 200 of the crossed and 106 of the self-fertilised seeds, and the relative weight of an equal number was as 100 for the crossed to 101.5 for the self-fertilised. With other plants, when the seeds from the self-fertilised flowers were heavier than those from the crossed flowers, this appeared to be due generally to fewer having been produced by the self-fertilised flowers, and to their having been in consequence better nourished. But in the present instance the seeds from the crossed capsules were separated into two lots,—namely, those from the capsules containing over fourteen seeds, and those from the capsules containing under fourteen seeds, and the seeds from the more productive capsules were the heavier of the two; so that the above explanation here fails.
As pollen is deposited at a very early age on the pistil, generally in contact with the stigma, some flowers whilst still in bud were castrated for my first experiment, and were afterwards fertilised with pollen from a distinct plant. Other flowers were fertilised with their own pollen. From the seeds thus obtained, I succeeded in rearing only three pairs of plants of equal age. The three crossed plants averaged 32.79 inches, and the three self-fertilised 32.08 inches in height; so that they were nearly equal, the crossed having a slight advantage. As the same result followed in all three generations, it would be superfluous to give the heights of all the plants, and I will give only the averages.
In order to raise crossed and self-fertilised plants of the second generation, some flowers on the above crossed plants were crossed within twenty-four hours after they had expanded with pollen from a distinct plant; and this interval would probably not be too great to allow of cross-fertilisation being effectual. Some flowers on the self-fertilised plants of the last generation were also self-fertilised. From these two lots of seeds, ten crossed and twelve self-fertilised plants of equal ages were raised; and these were measured when fully grown. The crossed averaged 36.98, and the self-fertilised averaged 37.42 inches in height; so that here again the two lots were nearly equal; but the self-fertilised had a slight advantage.
In order to raise plants of the third generation, a better plan was followed, and flowers on the crossed plants of the second generation were selected in which the stamens were too short to reach the stigmas, so that they could not possibly have been self-fertilised. These flowers were crossed with pollen from a distinct plant. Flowers on the self-fertilised plants of the second generation were again self-fertilised. From the two lots of seeds thus obtained, twenty-one crossed and nineteen self-fertilised plants of equal age, and forming the third generation, were raised in fourteen large pots. They were measured when fully grown, and by an odd chance the average height of the two lots was exactly the same, namely, 35.96 inches; so that neither side had the least advantage over the other. To test this result, all the plants on both sides in ten out of the above fourteen pots were cut down after they had flowered, and in the ensuing year the stems were again measured; and now the crossed plants exceeded by a little (namely, 1.7 inches) the self-fertilised. They were again cut down, and on their flowering for the third time, the self-fertilised plants had a slight advantage (namely, 1.54 inches) over the crossed. Hence the result arrived at with these plants during the previous trials was confirmed, namely, that neither lot had any decided advantage over the other. It may, however, be worth mentioning that the self-fertilised plants showed some tendency to flower before the crossed plants: this occurred with all three pairs of the first generation; and with the cut down plants of the third generation, a self-fertilised plant flowered first in nine out of the twelve pots, whilst in the remaining three pots a crossed plant flowered first.
If we consider all the plants of the three generations taken together, the thirty-four crossed plants average 35.98, and the thirty-four self-fertilised plants 36.39 inches in height; or as 100 to 101. We may therefore conclude that the two lots possessed equal powers of growth; and this I believe to be the result of long-continued self-fertilisation, together with exposure to similar conditions in each generation, so that all the individuals had acquired a closely similar constitution.
30. GRAMINACEAE.—Zea mays.
This plant is monoecious, and was selected for trial on this account, no other such plant having been experimented on. (6/8. Hildebrand remarks that this species seems at first sight adapted to be fertilised by pollen from the same plant, owing to the male flowers standing above the female flowers; but practically it must generally be fertilised by pollen from another plant, as the male flowers usually shed their pollen before the female flowers are mature: ‘Monatsbericht der K. Akad.’ Berlin October 1872 page 743.) It is also anemophilous, or is fertilised by the wind; and of such plants only the common beet had been tried. Some plants were raised in the greenhouse, and were crossed with pollen taken from a distinct plant; and a single plant, growing quite separately in a different part of the house, was allowed to fertilise itself spontaneously. The seeds thus obtained were placed on damp sand, and as they germinated in pairs of equal age were planted on the opposite sides of four very large pots; nevertheless they were considerably crowded. The pots were kept in the hothouse. The plants were first measured to the tips of their leaves when only between 1 and 2 feet in height, as shown in Table 6/97.
TABLE 6/97. Zea mays.
Heights of plants measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 23 4/8 : 17 3/8. Pot 1 : 12 : 20 3/8. Pot 1 : 21 : 20.
Pot 2 : 22 : 20. Pot 2 : 19 1/8 : 18 3/8. Pot 2 : 21 4/8 : 18 5/8.
Pot 3 : 22 1/8 : 18 5/8. Pot 3 : 20 3/8 : 15 2/8. Pot 3 : 18 2/8 : 16 4/8. Pot 3 : 21 5/8 : 18. Pot 3 : 23 2/8 : 16 2/8.
Pot 4 : 21 : 18. Pot 4 : 22 1/8 : 12 6/8. Pot 4 : 23 : 15 4/8. Pot 4 : 12 : 18.
Total : 302.88 : 263.63.
The fifteen crossed plants here average 20.19, and the fifteen self-fertilised plants 17.57 inches in height; or as 100 to 87. Mr. Galton made a graphical representation, in accordance with the method described in the introductory chapter, of the above measurements, and adds the words “very good” to the curves thus formed.
Shortly afterwards one of the crossed plants in Pot 1 died; another became much diseased and stunted; and the third never grew to its full height. They seemed to have been all injured, probably by some larva gnawing their roots. Therefore all the plants on both sides of this pot were rejected in the subsequent measurements. When the plants were fully grown they were again measured to the tips of the highest leaves, and the eleven crossed plants now averaged 68.1, and the eleven self-fertilised plants 62.34 inches in height; or as 100 to 91. In all four pots a crossed plant flowered before any one of the self-fertilised; but three of the plants did not flower at all. Those that flowered were also measured to the summits of the male flowers: the ten crossed plants averaged 66.51, and the nine self-fertilised plants 61.59 inches in height; or as 100 to 93.
A large number of the same crossed and self-fertilised seeds were sown in the middle of the summer in the open ground in two long rows. Very much fewer of the self-fertilised than of the crossed plants produced flowers; but those that did flower, flowered almost simultaneously. When fully grown the ten tallest plants in each row were selected and measured to the tips of their highest leaves, as well as to the summits of their male flowers. The crossed averaged to the tips of their leaves 54 inches in height, and the self-fertilised 44.65, or as 100 to 83; and to the summits of their male flowers, 53.96 and 43.45 inches; or as 100 to 80.
Phalaris canariensis.
Hildebrand has shown in the paper referred to under the last species, that this hermaphrodite grass is better adapted for cross-fertilisation than for self-fertilisation. Several plants were raised in the greenhouse close together, and their flowers were mutually intercrossed. Pollen from a single plant growing quite separately was collected and placed on the stigmas of the same plant. The seeds thus produced were self-fertilised, for they were fertilised with pollen from the same plant, but it will have been a mere chance whether with pollen from the same flowers. Both lots of seeds, after germinating on sand, were planted in pairs on the opposite sides of four pots, which were kept in the greenhouse. When the plants were a little over a foot in height they were measured, and the crossed plants averaged 13.38, and the self-fertilised 12.29 inches in height; or as 100 to 92.
When in full flower they were again measured to the extremities of their culms, as shown in Table 6/98.
TABLE 6/98. Phalaris canariensis.
Heights of plants measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 42 2/8 : 41 2/8. Pot 1 : 39 6/8 : 45 4/8.
Pot 2 : 37 : 31 6/8. Pot 2 : 49 4/8 : 37 2/8. Pot 4 : 29 : 42 3/8. Pot 2 : 37 : 34 7/8.
Pot 3 : 37 6/8 : 28. Pot 3 : 35 4/8 : 28. Pot 3 : 43 : 34.
Pot 4 : 40 2/8 : 35 1/8. Pot 4 : 37 : 34 4/8.
Total : 428.00 : 392.63.
The eleven crossed plants now averaged 38.9, and the eleven self-fertilised plants 35.69 inches in height; or as 100 to 92, which is the same ratio as before. Differently to what occurred with the maize, the crossed plants did not flower before the self-fertilised; and though both lots flowered very poorly from having been kept in pots in the greenhouse, yet the self-fertilised plants produced twenty-eight flower-heads, whilst the crossed produced only twenty!
Two long rows of the same seeds were sown out of doors, and care was taken that they were sown in nearly equal number; but a far greater number of the crossed than of the self-fertilised seeds yielded plants. The self-fertilised plants were in consequence not so much crowded as the crossed, and thus had an advantage over them. When in full flower, the twelve tallest plants were carefully selected from both rows and measured, as shown in Table 6/99.
TABLE 6/99. Phalaris canariensis (growing in the open ground).
Heights of plants measured in inches.
Column 1: Crossed Plants, twelve tallest.
Column 2: Self-fertilised Plants, twelve tallest.
Total : 429.5 : 402.0.
The twelve crossed plants here average 35.78, and the twelve self-fertilised 33.5 inches in height; or as 100 to 93. In this case the crossed plants flowered rather before the self-fertilised, and thus differed from those growing in the pots.]
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