CONVOLVULACEAE

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. CONVOLVULACEAE

CONVOLVULACEAE.

A plant of Ipomoea purpurea, or as it is often called in England the convolvulus major, a native of South America, grew in my greenhouse. Ten flowers on this plant were fertilised with pollen from the same flower; and ten other flowers on the same plant were crossed with pollen from a distinct plant. The fertilisation of the flowers with their own pollen was superfluous, as this convolvulus is highly self-fertile; but I acted in this manner to make the experiments correspond in all respects. Whilst the flowers are young the stigma projects beyond the anthers; and it might have been thought that it could not be fertilised without the aid of humble-bees, which often visit the flowers; but as the flower grows older the stamens increase in length, and their anthers brush against the stigma, which thus receives some pollen. The number of seeds produced by the crossed and self-fertilised flowers differed very little.

[Crossed and self-fertilised seeds obtained in the above manner were allowed to germinate on damp sand, and as often as pairs germinated at the same time they were planted in the manner described in the Introduction (Chapter 1), on the opposite sides of two pots. Five pairs were thus planted; and all the remaining seeds, whether or not in a state of germination, were planted on the opposite sides of a third pot, so that the young plants on both sides were here greatly crowded and exposed to very severe competition. Rods of iron or wood of equal diameter were given to all the plants to twine up; and as soon as one of each pair reached the summit both were measured. A single rod was placed on each side of the crowded pot, Number 3, and only the tallest plant on each side was measured.

TABLE 2/1. Ipomoea purpurea (First Generation.).

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Seedlings from Crossed Plants.

Column 3: Seedlings from Self-fertilised Plants.

Pot 1 : 87 4/8 : 69. Pot 1 : 87 4/8 : 66. Pot 1 : 89 : 73.

Pot 2 : 88 : 68 4/8. Pot 2 : 87 : 60 4/8.

Pot 3 : 77 : 57. Plants crowded; the tallest one measured on each side.

Total : 516 : 394.

The average height of the six crossed plants is here 86 inches, whilst that of the six self-fertilised plants is only 65.66 inches, so that the crossed plants are to the self-fertilised in height as 100 to 76. It should be observed that this difference is not due to a few of the crossed plants being extremely tall, or to a few of the self-fertilised being extremely short, but to all the crossed plants attaining a greater height than their antagonists. The three pairs in Pot 1 were measured at two earlier periods, and the difference was sometimes greater and sometimes less than that at the final measuring. But it is an interesting fact, of which I have seen several other instances, that one of the self-fertilised plants, when nearly a foot in height, was half an inch taller than the crossed plant; and again, when two feet high, it was 1 3/8 of an inch taller, but during the ten subsequent days the crossed plant began to gain on its antagonist, and ever afterward asserted its supremacy, until it exceeded its self-fertilised opponent by 16 inches.

The five crossed plants in Pots 1 and 2 were covered with a net, and produced 121 capsules; the five self-fertilised plants produced eighty-four capsules, so that the numbers of capsules were as 100 to 69. Of the 121 capsules on the crossed plants sixty-five were the product of flowers crossed with pollen from a distinct plant, and these contained on an average 5.23 seeds per capsule; the remaining fifty-six capsules were spontaneously self-fertilised. Of the eighty-four capsules on the self-fertilised plants, all the product of renewed self-fertilisation, fifty-five (which were alone examined) contained on an average 4.85 seeds per capsule. Therefore the cross-fertilised capsules, compared with the self-fertilised capsules, yielded seeds in the proportion of 100 to 93. The crossed seeds were relatively heavier than the self-fertilised seeds. Combining the above data (i.e., number of capsules and average number of contained seeds), the crossed plants, compared with the self-fertilised, yielded seeds in the ratio of 100 to 64.

These crossed plants produced, as already stated, fifty-six spontaneously self-fertilised capsules, and the self-fertilised plants produced twenty-nine such capsules. The former contained on an average, in comparison with the latter, seeds in the proportion of 100 to 99.

In Pot 3, on the opposite sides of which a large number of crossed and self-fertilised seeds had been sown and the seedlings allowed to struggle together, the crossed plants had at first no great advantage. At one time the tallest crossed was 25 1/8 inches high, and the tallest self-fertilised plants 21 3/8. But the difference afterwards became much greater. The plants on both sides, from being so crowded, were poor specimens. The flowers were allowed to fertilise themselves spontaneously under a net; the crossed plants produced thirty-seven capsules, the self-fertilised plants only eighteen, or as 100 to 47. The former contained on an average 3.62 seeds per capsule; and the latter 3.38 seeds, or as 100 to 93. Combining these data (i.e., number of capsules and average number of seeds), the crowded crossed plants produced seeds compared with the self-fertilised as 100 to 45. These latter seeds, however, were decidedly heavier, a hundred weighing 41.64 grains, than those from the capsules on the crossed plants, of which a hundred weighed 36.79 grains; and this probably was due to the fewer capsules borne by the self-fertilised plants having been better nourished. We thus see that the crossed plants in this the first generation, when grown under favourable conditions, and when grown under unfavourable conditions from being much crowded, greatly exceeded in height, and in the number of capsules produced, and slightly in the number of seeds per capsule, the self-fertilised plants.

CROSSED AND SELF-FERTILISED PLANTS OF THE SECOND GENERATION.

Flowers on the crossed plants of the last generation (Table 2/1) were crossed by pollen from distinct plants of the same generation; and flowers on the self-fertilised plants were fertilised by pollen from the same flower. The seeds thus produced were treated in every respect as before, and we have in Table 2/2 the result.

TABLE 2/2. Ipomoea purpurea (Second Generation.).

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 87 : 67 4/8. Pot 1 : 83 : 68 4/8. Pot 1 : 83 : 80 4/8.

Pot 2 : 85 4/8 : 61 4/8. Pot 2 : 89 : 79. Pot 2 : 77 4/8 : 41.

Total : 505 : 398.

Here again every single crossed plant is taller than its antagonist. The self-fertilised plant in Pot 1, which ultimately reached the unusual height of 80 4/8 inches, was for a long time taller than the opposed crossed plant, though at last beaten by it. The average height of the six crossed plants is 84.16 inches, whilst that of the six self-fertilised plants is 66.33 inches, or as 100 to 79.

CROSSED AND SELF-FERTILISED PLANTS OF THE THIRD GENERATION.

Seeds from the crossed plants of the last generation (Table 2/2) again crossed, and from the self-fertilised plants again self-fertilised, were treated in all respects exactly as before, with the following result:—

TABLE 2/3. Ipomoea purpurea (Third Generation.).

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 74 : 56 4/8. Pot 1 : 72 : 51 4/8. Pot 1 : 73 4/8 : 54.

Pot 2 : 82 : 59. Pot 2 : 81 : 30. Pot 2 : 82 : 66.

Total : 464.5 : 317.

Again all the crossed plants are higher than their antagonists: their average height is 77.41 inches, whereas that of the self-fertilised is 52.83 inches, or as 100 to 68.

I attended closely to the fertility of the plants of this third generation. Thirty flowers on the crossed plants were crossed with pollen from other crossed plants of the same generation, and the twenty-six capsules thus produced contained, on an average, 4.73 seeds; whilst thirty flowers on the self-fertilised plants, fertilised with the pollen from the same flower, produced twenty-three capsules, each containing 4.43 seeds. Thus the average number of seeds in the crossed capsules was to that in the self-fertilised capsules as 100 to 94. A hundred of the crossed seeds weighed 43.27 grains, whilst a hundred of the self-fertilised seeds weighed only 37.63 grains. Many of these lighter self-fertilised seeds placed on damp sand germinated before the crossed; thus thirty-six of the former germinated whilst only thirteen of the latter or crossed seeds germinated. In Pot 1 the three crossed plants produced spontaneously under the net (besides the twenty-six artificially cross-fertilised capsules) seventy-seven self-fertilised capsules containing on an average 4.41 seeds; whilst the three self-fertilised plants produced spontaneously (besides the twenty-three artificially self-fertilised capsules) only twenty-nine self-fertilised capsules, containing on an average 4.14 seeds. Therefore the average number of seeds in the two lots of spontaneously self-fertilised capsules was as 100 to 94. Taking into consideration the number of capsules together with the average number of seeds, the crossed plants (spontaneously self-fertilised) produced seeds in comparison with the self-fertilised plants (spontaneously self-fertilised) in the proportion of 100 to 35. By whatever method the fertility of these plants is compared, the crossed are more fertile than the self-fertilised plants.

I tried in several ways the comparative vigour and powers of growth of the crossed and self-fertilised plants of this third generation. Thus, four self-fertilised seeds which had just germinated were planted on one side of a pot, and after an interval of forty-eight hours, four crossed seeds in the same state of germination were planted on the opposite side; and the pot was kept in the hothouse. I thought that the advantage thus given to the self-fertilised seedlings would have been so great that they would never have been beaten by the crossed ones. They were not beaten until all had grown to a height of 18 inches; and the degree to which they were finally beaten is shown in Table 2/4. We here see that the average height of the four crossed plants is 76.62, and of the four self-fertilised plants 65.87 inches, or as 100 to 86; therefore less than when both sides started fair.

TABLE 2/4. Ipomoea purpurea (Third Generation, the self-fertilised plants having had a start of forty-eight hours).

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 3 : 78 4/8 : 73 4/8. Pot 3 : 77 4/8 : 53. Pot 3 : 73 : 61 4/8. Pot 3 : 77 4/8 : 75 4/8.

Total : 306.5 : 263.5.

Crossed and self-fertilised seeds of the third generation were also sown out of doors late in the summer, and therefore under unfavourable conditions, and a single stick was given to each lot of plants to twine up. The two lots were sufficiently separate so as not to interfere with each other’s growth, and the ground was clear of weeds. As soon as they were killed by the first frost (and there was no difference in their hardiness), the two tallest crossed plants were found to be 24.5 and 22.5 inches, whilst the two tallest self-fertilised plants were only 15 and 12.5 inches in height, or as 100 to 59.

I likewise sowed at the same time two lots of the same seeds in a part of the garden which was shady and covered with weeds. The crossed seedlings from the first looked the most healthy, but they twined up a stick only to a height of 7 1/4 inches; whilst the self-fertilised were not able to twine at all; and the tallest of them was only 3 1/2 inches in height.

Lastly, two lots of the same seeds were sown in the midst of a bed of candy-tuft (Iberis) growing vigorously. The seedlings came up, but all the self-fertilised ones soon died excepting one, which never twined and grew to a height of only 4 inches. Many of the crossed seedlings, on the other hand, survived; and some twined up the stems of the Iberis to the height of 11 inches. These cases prove that the crossed seedlings have an immense advantage over the self-fertilised, both when growing isolated under very unfavourable conditions, and when put into competition with each other or with other plants, as would happen in a state of nature.

CROSSED AND SELF-FERTILISED PLANTS OF THE FOURTH GENERATION.

Seedlings raised as before from the crossed and self-fertilised plants of the third generation in Table 2/3, gave results as follows:—

TABLE 2/5. Ipomoea purpurea (Fourth Generation).

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 84 : 80. Pot 1 : 47 : 44 1/2.

Pot 2 : 83 : 73 1/2. Pot 2 : 59 : 51 1/2.

Pot 3 : 82 : 56 1/2. Pot 3 : 65 1/2 : 63. Pot 3 : 68 : 52.

Total : 488.5 : 421.0.

Here the average height of the seven crossed plants is 69.78 inches, and that of the seven self-fertilised plants 60.14; or as 100 to 86. This smaller difference relatively to that in the former generations, may be attributed to the plants having been raised during the depth of winter, and consequently to their not having grown vigorously, as was shown by their general appearance and from several of them never reaching the summits of the rods. In Pot 2, one of the self-fertilised plants was for a long time taller by two inches than its opponent, but was ultimately beaten by it, so that all the crossed plants exceeded their opponents in height. Of twenty-eight capsules produced by the crossed plants fertilised by pollen from a distinct plant, each contained on an average 4.75 seeds; of twenty-seven self-fertilised capsules on the self-fertilised plants, each contained on an average 4.47 seeds; so that the proportion of seeds in the crossed and self-fertilised capsules was as 100 to 94.

Some of the same seeds, from which the plants in Table 2/5 had been raised, were planted, after they had germinated on damp sand, in a square tub, in which a large Brugmansia had long been growing. The soil was extremely poor and full of roots; six crossed seeds were planted in one corner, and six self-fertilised seeds in the opposite corner. All the seedlings from the latter soon died excepting one, and this grew to the height of only 1 1/2 inches. Of the crossed plants three survived, and they grew to the height of 2 1/2 inches, but were not able to twine round a stick; nevertheless, to my surprise, they produced some small miserable flowers. The crossed plants thus had a decided advantage over the self-fertilised plants under this extremity of bad conditions.

CROSSED AND SELF-FERTILISED PLANTS OF THE FIFTH GENERATION.

These were raised in the same manner as before, and when measured gave the following results:—

TABLE 2/6. Ipomoea purpurea (Fifth Generation).

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 96 : 73. Pot 1 : 86 : 78. Pot 1 : 69 : 29.

Pot 2 : 84 : 51. Pot 2 : 84 : 84. Pot 2 : 76 1/4 : 59.

Total : 495.25 : 374.00.

The average height of the six crossed plants is 82.54 inches, and that of the six self-fertilised plants 62.33 inches, or as 100 to 75. Every crossed plant exceeded its antagonist in height. In Pot 1 the middle plant on the crossed side was slightly injured whilst young by a blow, and was for a time beaten by its opponent, but ultimately recovered the usual superiority. The crossed plants produced spontaneously a vast number more capsules than did the self-fertilised plants; and the capsules of the former contained on an average 3.37 seeds, whilst those of the latter contained only 3.0 per capsule, or as 100 to 89. But looking only to the artificially fertilised capsules, those on the crossed plants again crossed contained on an average 4.46 seeds, whilst those on the self-fertilised plants again self-fertilised contained 4.77 seeds; so that the self-fertilised capsules were the more fertile of the two, and of this unusual fact I can offer no explanation.

CROSSED AND SELF-FERTILISED PLANTS OF THE SIXTH GENERATION.

These were raised in the usual manner, with the following result. I should state that there were originally eight plants on each side; but as two of the self-fertilised became extremely unhealthy and never grew to near their full height, these as well as their opponents have been struck out of the list. If they had been retained, they would have made the average height of the crossed plants unfairly greater than that of the self-fertilised. I have acted in the same manner in a few other instances, when one of a pair plainly became very unhealthy.

TABLE 2/7. Ipomoea purpurea (Sixth Generation).

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 93 : 50 1/2. Pot 1 : 91 : 65.

Pot 2 : 79 : 50. Pot 2 : 86 1/2 : 87. Pot 2 : 88 : 62.

Pot 3 : 87 1/2 : 64 1/2.

Total : 525 : 379.

The average height of the six crossed plants is here 87.5, and of the six self-fertilised plants 63.16, or as 100 to 72. This large difference was chiefly due to most of the plants, especially the self-fertilised ones, having become unhealthy towards the close of their growth, and they were severely attacked by aphides. From this cause nothing can be inferred with respect to their relative fertility. In this generation we have the first instance of a self-fertilised plant in Pot 2 exceeding (though only by half an inch) its crossed opponent. This victory was fairly won after a long struggle. At first the self-fertilised plant was several inches taller than its opponent, but when the latter was 4 1/2 feet high it had grown equal; it then grew a little taller than the self-fertilised plant, but was ultimately beaten by it to the extent of half an inch, as shown in Table 2/7. I was so much surprised at this case that I saved the self-fertilised seeds of this plant, which I will call the “Hero,” and experimented on its descendants, as will hereafter be described.

Besides the plants included in Table 2/7, nine crossed and nine self-fertilised plants of the same lot were raised in two other pots, 4 and 5. These pots had been kept in the hothouse, but from want of room were, whilst the plants were young, suddenly moved during very cold weather into the coldest part of the greenhouse. They all suffered greatly, and never quite recovered. After a fortnight only two of the nine self-fertilised seedlings were alive, whilst seven of the crossed survived. The tallest of these latter plants when measured was 47 inches in height, whilst the tallest of the two surviving self-fertilised plants was only 32 inches. Here again we see how much more vigorous the crossed plants are than the self-fertilised.

CROSSED AND SELF-FERTILISED PLANTS OF THE SEVENTH GENERATION.

These were raised as heretofore with the following result:—

TABLE 2/8. Ipomoea purpurea (Seventh Generation).

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 84 4/8 : 74 6/8. Pot 1 : 84 6/8 : 84. Pot 1 : 76 2/8 : 55 4/8.

Pot 2 : 84 4/8 : 65. Pot 2 : 90 : 51 2/8. Pot 2 : 82 2/8 : 80 4/8.

Pot 3 : 83 : 67 6/8. Pot 3 : 86 : 60 2/8.

Pot 4 : 84 2/8 : 75 2/8.

Total : 755.50 : 614.25.

Each of these nine crossed plants is higher than its opponent, though in one case only by three-quarters of an inch. Their average height is 83.94 inches, and that of the self-fertilised plants 68.25, or as 100 to 81. These plants, after growing to their full height, became very unhealthy and infested with aphides, just when the seeds were setting, so that many of the capsules failed, and nothing can be said on their relative fertility.

CROSSED AND SELF-FERTILISED PLANTS OF THE EIGHTH GENERATION.

As just stated, the plants of the last generation, from which the present ones were raised, were very unhealthy and their seeds of unusually small size; and this probably accounts for the two lots behaving differently to what they did in any of the previous or succeeding generations. Many of the self-fertilised seeds germinated before the crossed ones, and these were of course rejected. When the crossed seedlings in Table 2/9 had grown to a height of between 1 and 2 feet, they were all, or almost all, shorter than their self-fertilised opponents, but were not then measured. When they had acquired an average height of 32.28 inches, that of the self-fertilised plants was 40.68, or as 100 to 122. Moreover, every one of the self-fertilised plants, with a single exception, exceeded its crossed opponent. When, however, the crossed plants had grown to an average height of 77.56 inches, they just exceeded (namely, by .7 of an inch) the average height of the self-fertilised plants; but two of the latter were still taller than their crossed opponents. I was so much astonished at this whole case, that I tied string to the summits of the rods; the plants being thus allowed to continue climbing upwards. When their growth was complete they were untwined, stretched straight, and measured. The crossed plants had now almost regained their accustomed superiority, as may be seen in Table 2/9.

The average height of the eight crossed plants is here 113.25 inches, and that of the self-fertilised plants 96.65, or as 100 to 85. Nevertheless two of the self-fertilised plants, as may be seen in Table 2/9, were still higher than their crossed opponents. The latter manifestly had much thicker stems and many more lateral branches, and looked altogether more vigorous than the self-fertilised plants, and generally flowered before them. The earlier flowers produced by these self-fertilised plants did not set any capsules, and their anthers contained only a small amount of pollen; but to this subject I shall return. Nevertheless capsules produced by two other self-fertilised plants of the same lot, not included in Table 2/9, which had been highly favoured by being grown in separate pots, contained the large average number of 5.1 seeds per capsule.

TABLE 2/9. Ipomoea purpurea (Eighth Generation).

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 111 6/8 : 96. Pot 1 : 127 : 54. Pot 1 : 130 6/8 : 93 4/8.

Pot 2 : 97 2/8 : 94. Pot 2 : 89 4/8 : 125 6/8.

Pot 3 : 103 6/8 : 115 4/8. Pot 3 : 100 6/8 : 84 6/8. Pot 3 : 147 4/8 : 109 6/8.

Total : 908.25 : 773.25.

CROSSED AND SELF-FERTILISED PLANTS OF THE NINTH GENERATION.

The plants of this generation were raised in the same manner as before, with the result shown in Table 2/10.

The fourteen crossed plants average in height 81.39 inches and the fourteen self-fertilised plants 64.07, or as 100 to 79. One self-fertilised plant in Pot 3 exceeded, and one in Pot 4 equalled in height, its opponent. The self-fertilised plants showed no sign of inheriting the precocious growth of their parents; this having been due, as it would appear, to the abnormal state of the seeds from the unhealthiness of their parents. The fourteen self-fertilised plants yielded only forty spontaneously self-fertilised capsules, to which must be added seven, the product of ten flowers artificially self-fertilised. On the other hand, the fourteen crossed plants yielded 152 spontaneously self-fertilised capsules; but thirty-six flowers on these plants were crossed (yielding thirty-three capsules), and these flowers would probably have produced about thirty spontaneously self-fertilised capsules. Therefore an equal number of the crossed and self-fertilised plants would have produced capsules in the proportion of about 182 to 47, or as 100 to 26. Another phenomenon was well pronounced in this generation, but I believe had occurred previously to a slight extent; namely, that most of the flowers on the self-fertilised plants were somewhat monstrous. The monstrosity consisted in the corolla being irregularly split so that it did not open properly, with one or two of the stamens slightly foliaceous, coloured, and firmly coherent to the corolla. I observed this monstrosity in only one flower on the crossed plants. The self-fertilised plants, if well nourished, would almost certainly, in a few more generations, have produced double flowers, for they had already become in some degree sterile. (2/1. See on this subject ‘Variation of Animals and Plants under Domestication’ chapter 18 2nd edition volume 2 page 152.)

TABLE 2/10. Ipomoea purpurea (Ninth Generation).

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 83 4/8 : 57. Pot 1 : 85 4/8 : 71. Pot 1 : 83 4/8 : 48 3/8.

Pot 2 : 83 2/8 : 45. Pot 2 : 64 2/8 : 43 6/8. Pot 2 : 64 3/8 : 38 4/8.

Pot 3 : 79 : 63. Pot 3 : 88 1/8 : 71. Pot 3 : 61 : 89 4/8.

Pot 4 : 82 4/8 : 82 4/8. Pot 4 : 90 : 76 1/8.

Pot 5 : 89 4/8 : 67. Pot 5 : 92 4/8 : 74 2/8. Pot 5 : 92 4/8 : 70. Crowded plants.

Total : 1139.5 : 897.0.

CROSSED AND SELF-FERTILISED PLANTS OF THE TENTH GENERATION.

Six plants were raised in the usual manner from the crossed plants of the last generation (Table 2/10) again intercrossed, and from the self-fertilised again self-fertilised. As one of the crossed plants in Pot 1 in Table 2/11 became much diseased, having crumpled leaves, and producing hardly any capsules, it and its opponent have been struck out of the table.

TABLE 2/11. Ipomoea purpurea (Tenth Generation).

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 92 3/8 : 47 2/8. Pot 1 : 94 4/8 : 34 6/8.

Pot 2 : 87 : 54 4/8. Pot 2 : 89 5/8 : 49 2/8. Pot 2 : 105 : 66 2/8.

Total : 468.5 : 252.0.

The five crossed plants average 93.7 inches, and the five self-fertilised only 50.4, or as 100 to 54. This difference, however, is so great that it must be looked at as in part accidental. The six crossed plants (the diseased one here included) yielded spontaneously 101 capsules, and the six self-fertilised plants 88, the latter being chiefly produced by one of the plants. But as the diseased plant, which yielded hardly any seed, is here included, the ratio of 101 to 88 does not fairly give the relative fertility of the two lots. The stems of the six crossed plants looked so much finer than those of the six self-fertilised plants, that after the capsules had been gathered and most of the leaves had fallen off, they were weighed. Those of the crossed plants weighed 2,693 grains, whilst those of the self-fertilised plants weighed only 1,173 grains, or as 100 to 44; but as the diseased and dwarfed crossed plant is here included, the superiority of the former in weight was really greater.]

THE EFFECTS ON THE OFFSPRING OF CROSSING DIFFERENT FLOWERS ON THE SAME PLANT, INSTEAD OF CROSSING DISTINCT INDIVIDUALS.

In all the foregoing experiments, seedlings from flowers crossed by pollen from a distinct plant (though in the later generations more or less closely related) were put into competition with, and almost invariably proved markedly superior in height to the offspring from self-fertilised flowers. I wished, therefore, to ascertain whether a cross between two flowers on the same plant would give to the offspring any superiority over the offspring from flowers fertilised with their own pollen. I procured some fresh seed and raised two plants, which were covered with a net; and several of their flowers were crossed with pollen from a distinct flower on the same plant. Twenty-nine capsules thus produced contained on an average 4.86 seeds per capsule; and 100 of these seeds weighed 36.77 grains. Several other flowers were fertilised with their own pollen, and twenty-six capsules thus produced contained on an average 4.42 seeds per capsule; 100 of which weighed 42.61 grains. So that a cross of this kind appears to have increased slightly the number of seeds per capsule, in the ratio of 100 to 91; but these crossed seeds were lighter than the self-fertilised in the ratio of 86 to 100. I doubt, however, from other observations, whether these results are fully trustworthy. The two lots of seeds, after germinating on sand, were planted in pairs on the opposite sides of nine pots, and were treated in every respect like the plants in the previous experiments. The remaining seeds, some in a state of germination and some not so, were sown on the opposite sides of a large pot (Number 10); and the four tallest plants on each side of this pot were measured. The result is shown in Table 2/12.

TABLE 2/12. Ipomoea purpurea.

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 82 : 77 4/8. Pot 1 : 75 : 87. Pot 1 : 65 : 64. Pot 1 : 76 : 87 2/8.

Pot 2 : 78 4/8 : 84. Pot 2 : 43 : 86 4/8. Pot 2 : 65 4/8 : 90 4/8.

Pot 3 : 61 2/8 : 86. Pot 3 : 85 : 69 4/8. Pot 3 : 89 : 87 4/8.

Pot 4 : 83 : 80 4/8. Pot 4 : 73 4/8 : 88 4/8. Pot 4 : 67 : 84 4/8.

Pot 5 : 78 : 66 4/8. Pot 5 : 76 6/8 : 77 4/8. Pot 5 : 57 : 81 4/8.

Pot 6 : 70 4/8 : 80. Pot 6 : 79 : 82 4/8. Pot 6 : 79 6/8 : 55 4/8.

Pot 7 : 76 : 77. Pot 7 : 84 4/8 : 83 4/8. Pot 7 : 79 : 73 4/8.

Pot 8 : 73 : 76 4/8. Pot 8 : 67 : 82. Pot 8 : 83 : 80 4/8.

Pot 9 : 73 2/8 : 78 4/8. Pot 9 : 78 : 67 4/8.

Pot 10 : 34 : 82 4/8. Pot 10 : 82 : 36 6/8. Pot 10 : 84 6/8 : 69 4/8. Pot 10 : 71 : 75 2/8. Crowded plants.

Total : 2270.25 : 2399.75.

The average height of the thirty-one crossed plants is 73.23 inches, and that of the thirty-one self-fertilised plants 77.41 inches; or as 100 to 106. Looking to each pair, it may be seen that only thirteen of the crossed plants, whilst eighteen of the self-fertilised plants exceed their opponents. A record was kept with respect to the plant which flowered first in each pot; and only two of the crossed flowered before one of the self-fertilised in the same pot; whilst eight of the self-fertilised flowered first. It thus appears that the crossed plants are slightly inferior in height and in earliness of flowering to the self-fertilised. But the inferiority in height is so small, namely as 100 to 106, that I should have felt very doubtful on this head, had I not cut down all the plants (except those in the crowded pot Number 10) close to the ground and weighed them. The twenty-seven crossed plants weighed 16 1/2 ounces, and the twenty-seven self-fertilised plants 20 1/2 ounces; and this gives a ratio of 100 to 124.

A self-fertilised plant of the same parentage as those in Table 2/12 had been raised in a separate pot for a distinct purpose; and it proved partially sterile, the anthers containing very little pollen. Several flowers on this plant were crossed with the little pollen which could be obtained from the other flowers on the same plant; and other flowers were self-fertilised. From the seeds thus produced four crossed and four self-fertilised plants were raised, which were planted in the usual manner on the opposite sides of two pots. All these four crossed plants were inferior in height to their opponents; they averaged 78.18 inches, whilst the four self-fertilised plants averaged 84.8 inches; or as 100 to 108. (2/2. From one of these self-fertilised plants, spontaneously self-fertilised, I gathered twenty-four capsules, and they contained on an average only 3.2 seeds per capsule; so that this plant had apparently inherited some of the sterility of its parent.) This case, therefore, confirms the last. Taking all the evidence together, we must conclude that these strictly self-fertilised plants grew a little taller, were heavier, and generally flowered before those derived from a cross between two flowers on the same plant. These latter plants thus present a wonderful contrast with those derived from a cross between two distinct individuals.

THE EFFECTS ON THE OFFSPRING OF A CROSS WITH A DISTINCT OR FRESH STOCK BELONGING TO THE SAME VARIETY.

From the two foregoing series of experiments we see, firstly, the good effects during several successive generations of a cross between distinct plants, although these were in some degree inter-related and had been grown under nearly the same conditions; and, secondly, the absence of all such good effects from a cross between flowers on the same plant; the comparison in both cases being made with the offspring of flowers fertilised with their own pollen. The experiments now to be given show how powerfully and beneficially plants, which have been intercrossed during many successive generations, having been kept all the time under nearly uniform conditions, are affected by a cross with another plant belonging to the same variety, but to a distinct family or stock, which had grown under different conditions.

[Several flowers on the crossed plants of the ninth generation in Table 2/10, were crossed with pollen from another crossed plant of the same lot. The seedlings thus raised formed the tenth intercrossed generation, and I will call them the “INTERCROSSED PLANTS.” Several other flowers on the same crossed plants of the ninth generation were fertilised (not having been castrated) with pollen taken from plants of the same variety, but belonging to a distinct family, which had been grown in a distant garden at Colchester, and therefore under somewhat different conditions. The capsules produced by this cross contained, to my surprise, fewer and lighter seeds than did the capsules of the intercrossed plants; but this, I think, must have been accidental. The seedlings raised from them I will call the “COLCHESTER-CROSSED.” The two lots of seeds, after germinating on sand, were planted in the usual manner on the opposite sides of five pots, and the remaining seeds, whether or not in a state of germination, were thickly sown on the opposite sides of a very large pot, Number 6 in Table 2/13. In three of the six pots, after the young plants had twined a short way up their sticks, one of the Colchester-crossed plants was much taller than any one of the intercrossed plants on the opposite side of the same pot; and in the three other pots somewhat taller. I should state that two of the Colchester-crossed plants in Pot 4, when about two-thirds grown, became much diseased, and were, together with their intercrossed opponents, rejected. The remaining nineteen plants, when almost fully grown, were measured, with the following result:

TABLE 2/13. Ipomoea purpurea.

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Colchester-Crossed Plants.

Column 3: Intercrossed Plants of the Tenth Generation.

Pot 1 : 87 : 78. Pot 1 : 87 4/8 : 68 4/8. Pot 1 : 85 1/8 : 94 4/8.

Pot 2 : 93 6/8 : 60. Pot 2 : 85 4/8 : 87 2/8. Pot 2 : 90 5/8 : 45 4/8.

Pot 3 : 84 2/8 : 70 1/8. Pot 3 : 92 4/8 : 81 6/8. Pot 3 : 85 : 86 2/8.

Pot 4 : 95 6/8 : 65 1/8.

Pot 5 : 90 4/8 : 85 6/8. Pot 5 : 86 6/8 : 63. Pot 5 : 84 : 62 6/8.

Pot 6 : 90 4/8 : 43 4/8. Pot 6 : 75 : 39 6/8. Pot 6 : 71 : 30 2/8. Pot 6 : 83 6/8 : 86. Pot 6 : 63 : 53. Pot 6 : 65 : 48 6/8. Crowded plants in a very large pot.

Total : 1596.50 : 1249.75.

In sixteen out of these nineteen pairs, the Colchester-crossed plant exceeded in height its intercrossed opponent. The average height of the Colchester-crossed is 84.03 inches, and that of the intercrossed 65.78 inches; or as 100 to 78. With respect to the fertility of the two lots, it was too troublesome to collect and count the capsules on all the plants; so I selected two of the best pots, 5 and 6, and in these the Colchester-crossed produced 269 mature and half-mature capsules, whilst an equal number of the intercrossed plants produced only 154 capsules; or as 100 to 57. By weight the capsules from the Colchester-crossed plants were to those from the intercrossed plants as 100 to 51; so that the former probably contained a somewhat larger average number of seeds.]

We learn from this important experiment that plants in some degree related, which had been intercrossed during the nine previous generations, when they were fertilised with pollen from a fresh stock, yielded seedlings as superior to the seedlings of the tenth intercrossed generation, as these latter were to the self-fertilised plants of the corresponding generation. For if we look to the plants of the ninth generation in Table 2/10 (and these offer in most respects the fairest standard of comparison) we find that the intercrossed plants were in height to the self-fertilised as 100 to 79, and in fertility as 100 to 26; whilst the Colchester-crossed plants are in height to the intercrossed as 100 to 78, and in fertility as 100 to 51.

[THE DESCENDANTS OF THE SELF-FERTILISED PLANT, NAMED HERO, WHICH APPEARED IN THE SIXTH SELF-FERTILISED GENERATION.

In the five generations before the sixth, the crossed plant of each pair was taller than its self-fertilised opponent; but in the sixth generation (Table 2/7, Pot 2) the Hero appeared, which after a long and dubious struggle conquered its crossed opponent, though by only half an inch. I was so much surprised at this fact, that I resolved to ascertain whether this plant would transmit its powers of growth to its seedlings. Several flowers on Hero were therefore fertilised with their own pollen, and the seedlings thus raised were put into competition with self-fertilised and intercrossed plants of the corresponding generation. The three lots of seedlings thus all belong to the seventh generation. Their relative heights are shown in Tables 2/14 and 2/15.

TABLE 2/14. Ipomoea purpurea.

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Self-fertilised Plants of the Seventh Generation, Children of Hero.

Column 3: Self-fertilised Plants of the Seventh Generation.

Pot 1 : 74 : 89 4/8. Pot 1 : 60 : 61. Pot 1 : 55 2/8 : 49.

Pot 2 : 92 : 82. Pot 2 : 91 6/8 : 56. Pot 2 : 74 2/8 : 38.

Total : 447.25 : 375.50.

The average height of the six self-fertilised children of Hero is 74.54 inches, whilst that of the ordinary self-fertilised plants of the corresponding generation is only 62.58 inches, or as 100 to 84.

TABLE 2/15. Ipomoea purpurea.

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Self-fertilised Plants of the Seventh Generation, Children of Hero.

Column 3: Intercrossed Plants of the Seventh Generation.

Pot 3 : 92 : 76 6/8.

Pot 4 : 87 : 89. Pot 4 : 87 6/8 : 86 6/8.

Total : 266.75 : 252.50.

Here the average height of the three self-fertilised children of Hero is 88.91 inches, whilst that of the intercrossed plants is 84.16; or as 100 to 95. We thus see that the self-fertilised children of Hero certainly inherit the powers of growth of their parents; for they greatly exceed in height the self-fertilised offspring of the other self-fertilised plants, and even exceed by a trifle the intercrossed plants,—all of the corresponding generation.

Several flowers on the self-fertilised children of Hero in Table 2/14 were fertilised with pollen from the same flower; and from the seeds thus produced, self-fertilised plants of the eighth generation (grandchildren of Hero) were raised. Several other flowers on the same plants were crossed with pollen from the other children of Hero. The seedlings raised from this cross may be considered as the offspring of the union of brothers and sisters. The result of the competition between these two sets of seedlings (namely self-fertilised and the offspring of brothers and sisters) is given in Table 2/16.

TABLE 2/16. Ipomoea purpurea.

Heights of Plants in inches:

Column 1: Number (Name) of Pot.

Column 2: Self-fertilised Grandchildren of Hero, from the Self-fertilised Children. Eighth Generation.

Column 3: Grandchildren from a cross between the self-fertilised children of Hero. Eighth Generation.

Pot 1 : 86 6/8 : 95 6/8. Pot 1 : 90 3/8 : 95 3/8.

Pot 2 : 96 : 85. Pot 2 : 77 2/8 : 93.

Pot 3 : 73 : 86 2/8. Pot 3 : 66 : 82 2/8. Pot 3 : 84 4/8 : 70 6/8.

Pot 4 : 88 1/8 : 66 3/8. Pot 4 : 84 : 15 4/8. Pot 4 : 36 2/8 : 38. Pot 4 : 74 : 78 3/8.

Pot 5 : 90 1/8 : 82 6/8. Pot 5 : 90 5/8 : 83 6/8.

Total : 1037.00 : 973.16.

The average height of the thirteen self-fertilised grandchildren of Hero is 79.76 inches, and that of the grandchildren from a cross between the self-fertilised children is 74.85; or as 100 to 94. But in Pot 4 one of the crossed plants grew only to a height of 15 1/2 inches; and if this plant and its opponent are struck out, as would be the fairest plan, the average height of the crossed plants exceeds only by a fraction of an inch that of the self-fertilised plants. It is therefore clear that a cross between the self-fertilised children of Hero did not produce any beneficial effect worth notice; and it is very doubtful whether this negative result can be attributed merely to the fact of brothers and sisters having been united, for the ordinary intercrossed plants of the several successive generations must often have been derived from the union of brothers and sisters (as shown in Chapter 1), and yet all of them were greatly superior to the self-fertilised plants. We are therefore driven to the suspicion, which we shall soon see strengthened, that Hero transmitted to its offspring a peculiar constitution adapted for self-fertilisation.

It would appear that the self-fertilised descendants of Hero have not only inherited from Hero a power of growth equal to that of the ordinary intercrossed plants, but have become more fertile when self-fertilised than is usual with the plants of the present species. The flowers on the self-fertilised grandchildren of Hero in Table 2.16 (the eighth generation of self-fertilised plants) were fertilised with their own pollen and produced plenty of capsules, ten of which (though this is too few a number for a safe average) contained 5.2 seeds per capsule,—a higher average than was observed in any other case with the self-fertilised plants. The anthers produced by these self-fertilised grandchildren were also as well developed and contained as much pollen as those on the intercrossed plants of the corresponding generation; whereas this was not the case with the ordinary self-fertilised plants of the later generations. Nevertheless some few of the flowers produced by the grandchildren of Hero were slightly monstrous, like those of the ordinary self-fertilised plants of the later generations. In order not to recur to the subject of fertility, I may add that twenty-one self-fertilised capsules, spontaneously produced by the great-grandchildren of Hero (forming the ninth generation of self-fertilised plants), contained on an average 4.47 seeds; and this is as high an average as the self-fertilised flowers of any generation usually yielded.

Several flowers on the self-fertilised grandchildren of Hero in Table 2/16 were fertilised with pollen from the same flower; and the seedlings raised from them (great-grandchildren of Hero) formed the ninth self-fertilised generation. Several other flowers were crossed with pollen from another grandchild, so that they may be considered as the offspring of brothers and sisters, and the seedlings thus raised may be called the INTERCROSSED great-grandchildren. And lastly, other flowers were fertilised with pollen from a distinct stock, and the seedlings thus raised may be called the COLCHESTER-CROSSED great-grandchildren. In my anxiety to see what the result would be, I unfortunately planted the three lots of seeds (after they had germinated on sand) in the hothouse in the middle of winter, and in consequence of this the seedlings (twenty in number of each kind) became very unhealthy, some growing only a few inches in height, and very few to their proper height. The result, therefore, cannot be fully trusted; and it would be useless to give the measurements in detail. In order to strike as fair an average as possible, I first excluded all the plants under 50 inches in height, thus rejecting all the most unhealthy plants. The six self-fertilised thus left were on an average 66.86 inches high; the eight intercrossed plants 63.2 high; and the seven Colchester-crossed 65.37 high; so that there was not much difference between the three sets, the self-fertilised plants having a slight advantage. Nor was there any great difference when only the plants under 36 inches in height were excluded. Nor again when all the plants, however much dwarfed and unhealthy, were included. In this latter case the Colchester-crossed gave the lowest average of all; and if these plants had been in any marked manner superior to the other two lots, as from my former experience I fully expected they would have been, I cannot but think that some vestige of such superiority would have been evident, notwithstanding the very unhealthy condition of most of the plants. No advantage, as far as we can judge, was derived from intercrossing two of the grandchildren of Hero, any more than when two of the children were crossed. It appears therefore that Hero and its descendants have varied from the common type, not only in acquiring great power of growth, and increased fertility when subjected to self-fertilisation, but in not profiting from a cross with a distinct stock; and this latter fact, if trustworthy, is a unique case, as far as I have observed in all my experiments.]

A SUMMARY ON THE GROWTH, VIGOUR, AND FERTILITY OF THE SUCCESSIVE GENERATIONS OF THE CROSSED AND SELF-FERTILISED PLANTS OF Ipomoea purpurea, TOGETHER WITH SOME MISCELLANEOUS OBSERVATIONS.

In Table 2/17, we see the average or mean heights of the ten successive generations of the intercrossed and self-fertilised plants, grown in competition with each other; and in the right hand column we have the ratios of the one to the other, the height of the intercrossed plants being taken at 100. In the bottom line the mean height of the seventy-three intercrossed plants is shown to be 85.84 inches, and that of the seventy-three self-fertilised plants 66.02 inches, or as 100 to 77.

TABLE 2/17. Ipomoea purpurea. Summary of measurements of the ten generations.

Heights of Plants in inches:

Column 1: Name of Generation.

Column 2: Number of Crossed Plants.

Column 3: Average Height of Crossed Plants.

Column 4: Number of Self-fertilised Plants.

Column 5: Average Height of Self-fertilised Plants.

Column 6: n in Ratio between Average Heights of Crossed and Self-fertilised Plants, expressed as 100 to n.

First generation Table 2/1 : 6 : 86.00 : 6 : 65.66 : 76.

Second generation Table 2/2 : 6 : 84.16 : 6 : 66.33 : 79.

Third generation Table 2/3 : 6 : 77.41 : 6 : 52.83 : 68.

Fourth generation Table 2/5 : 7 : 69.78 : 7 : 60.14 : 86.

Fifth generation Table 2/6 : 6 : 82.54 : 6 : 62.33 : 75.

Sixth generation Table 2/7 : 6 : 87.50 : 6 : 63.16 : 72.

Seventh generation Table 2/8 : 9 : 83.94 : 9 : 68.25 : 81.

Eighth generation Table 2/9 : 8 : 113.25 : 8 : 96.65 : 85.

Ninth generation Table 2/10 : 14 : 81.39 : 14 : 64.07 : 79.

Tenth generation Table 2/11 : 5 : 93.70 : 5 : 50.40 : 54.

All ten generations together : 73 : 85.84 : 73 : 66.02 : 77.

(DIAGRAM 2/1. Diagram showing the mean heights of the crossed and self-fertilised plants of Ipomoea purpurea in the ten generations; the mean height of the crossed plants being taken as 100. On the right hand, the mean heights of the crossed and self-fertilised plants of all the generations taken together are shown (as eleven pairs of unequal vertical lines.))

The mean height of the self-fertilised plants in each of the ten generations is also shown in the diagram 2/1, that of the intercrossed plants being taken at 100, and on the right side we see the relative heights of the seventy-three intercrossed plants, and of the seventy-three self-fertilised plants. The difference in height between the crossed and self-fertilised plants will perhaps be best appreciated by an illustration: If all the men in a country were on an average 6 feet high, and there were some families which had been long and closely interbred, these would be almost dwarfs, their average height during ten generations being only 4 feet 8 1/4 inches.

It should be especially observed that the average difference between the crossed and self-fertilised plants is not due to a few of the former having grown to an extraordinary height, or to a few of the self-fertilised being extremely short, but to all the crossed plants having surpassed their self-fertilised opponents, with the few following exceptions. The first occurred in the sixth generation, in which the plant named “Hero” appeared; two in the eighth generation, but the self-fertilised plants in this generation were in an anomalous condition, as they grew at first at an unusual rate and conquered for a time the opposed crossed plants; and two exceptions in the ninth generation, though one of these plants only equalled its crossed opponent. Therefore, of the seventy-three crossed plants, sixty-eight grew to a greater height than the self-fertilised plants, to which they were opposed.

In the right-hand column of figures, the difference in height between the crossed and self-fertilised plants in the successive generations is seen to fluctuate much, as might indeed have been expected from the small number of plants measured in each generation being insufficient to give a fair average. It should be remembered that the absolute height of the plants goes for nothing, as each pair was measured as soon as one of them had twined up to the summit of its rod. The great difference in the tenth generation, namely, 100 to 54, no doubt was partly accidental, though, when these plants were weighed, the difference was even greater, namely, 100 to 44. The smallest amount of difference occurred in the fourth and the eighth generations, and this was apparently due to both the crossed and self-fertilised plants having become unhealthy, which prevented the former attaining their usual degree of superiority. This was an unfortunate circumstance, but my experiments were not thus vitiated, as both lots of plants were exposed to the same conditions, whether favourable or unfavourable.

There is reason to believe that the flowers of this Ipomoea, when growing out of doors, are habitually crossed by insects, so that the first seedlings which I raised from purchased seeds were probably the offspring of a cross. I infer that this is the case, firstly from humble-bees often visiting the flowers, and from the quantity of pollen left by them on the stigmas of such flowers; and, secondly, from the plants raised from the same lot of seed varying greatly in the colour of their flowers, for as we shall hereafter see, this indicates much intercrossing. (2/3. Verlot says ‘Sur la Production des Variétés’ 1865 page 66, that certain varieties of a closely allied plant, the Convolvulus tricolor, cannot be kept pure unless grown at a distance from all other varieties.) It is, therefore, remarkable that the plants raised by me from flowers which were, in all probability, self-fertilised for the first time after many generations of crossing, should have been so markedly inferior in height to the intercrossed plants as they were, namely, as 76 to 100. As the plants which were self-fertilised in each succeeding generation necessarily became much more closely interbred in the later than in the earlier generations, it might have been expected that the difference in height between them and the crossed plants would have gone on increasing; but, so far is this from being the case, that the difference between the two sets of plants in the seventh, eighth, and ninth generations taken together is less than in the first and second generations together. When, however, we remember that the self-fertilised and crossed plants are all descended from the same mother-plant, that many of the crossed plants in each generation were related, often closely related, and that all were exposed to the same conditions, which, as we shall hereafter find, is a very important circumstance, it is not at all surprising that the difference between them should have somewhat decreased in the later generations. It is, on the contrary, an astonishing fact, that the crossed plants should have been victorious, even to a slight degree, over the self-fertilised plants of the later generations.

The much greater constitutional vigour of the crossed than of the self-fertilised plants, was proved on five occasions in various ways; namely, by exposing them, while young, to a low temperature or to a sudden change of temperature, or by growing them, under very unfavourable conditions, in competition with full-grown plants of other kinds.

With respect to the productiveness of the crossed and self-fertilised plants of the successive generations, my observations unfortunately were not made on any uniform plan, partly from the want of time, and partly from not having at first intended to observe more than a single generation. A summary of the results is here given in a tabulated form, the fertility of the crossed plants being taken as 100.

TABLE 2/18. Ratio of productiveness of crossed and self-fertilised plants. Ipomoea purpurea.

FIRST GENERATION OF CROSSED AND SELF-FERTILISED PLANTS GROWING IN COMPETITION WITH ONE ANOTHER.

Sixty-five capsules produced from flowers on five crossed plants fertilised by pollen from a distinct plant, and fifty-five capsules produced from flowers on five self-fertilised plants fertilised by their own pollen, contained seeds in the proportion of : 100 to 93.

Fifty-six spontaneously self-fertilised capsules on the above five crossed plants, and twenty-five spontaneously self-fertilised capsules on the above five self-fertilised plants, yielded seeds in the proportion of : 100 to 99.

Combining the total number of capsules produced by these plants, and the average number of seeds in each, the above crossed and self-fertilised plants yielded seeds in the proportion of : 100 to 64.

Other plants of this first generation grown under unfavourable conditions and spontaneously self-fertilised, yielded seeds in the proportion of : 100 to 45.

THIRD GENERATION OF CROSSED AND SELF-FERTILISED PLANTS.

Crossed capsules compared with self-fertilised capsules contained seeds in the ratio of : 100 to 94.

An equal number of crossed and self-fertilised plants, both spontaneously self-fertilised, produced capsules in the ratio of : 100 to 38.

And these capsules contained seeds in the ratio of : 100 to 94.

Combining these data, the productiveness of the crossed to the self-fertilised plants, both spontaneously self-fertilised, was as : 100 to 35.

FOURTH GENERATION OF CROSSED AND SELF-FERTILISED PLANTS.

Capsules from flowers on the crossed plants fertilised by pollen from another plant, and capsules from flowers on the self-fertilised plants fertilised with their own pollen, contained seeds in the proportion of : 100 to 94.

FIFTH GENERATION OF CROSSED AND SELF-FERTILISED PLANTS.

The crossed plants produced spontaneously a vast number more pods (not actually counted) than the self-fertilised, and these contained seeds in the proportion of : 100 to 89.

NINTH GENERATION OF CROSSED AND SELF-FERTILISED PLANTS.

Fourteen crossed plants, spontaneously self-fertilised, and fourteen self-fertilised plants spontaneously self-fertilised, yielded capsules (the average number of seeds per capsule not having been ascertained) in the proportion of : 100 to 26.

PLANTS DERIVED FROM A CROSSED WITH A FRESH STOCK COMPARED WITH INTERCROSSED PLANTS.

The offspring of intercrossed plants of the ninth generation, crossed by a fresh stock, compared with plants of the same stock intercrossed during ten generations, both sets of plants left uncovered and naturally fertilised, produced capsules by weight as : 100 to 51.

We see in this table that the crossed plants are always in some degree more productive than the self-fertilised plants, by whatever standard they are compared. The degree differs greatly; but this depends chiefly on whether an average was taken of the seeds alone, or of the capsules alone, or of both combined. The relative superiority of the crossed plants is chiefly due to their producing a much greater number of capsules, and not to each capsule containing a larger average number of seeds. For instance, in the third generation the crossed and self-fertilised plants produced capsules in the ratio of 100 to 38, whilst the seeds in the capsules on the crossed plants were to those on the self-fertilised plants only as 100 to 94. In the eighth generation the capsules on two self-fertilised plants (not included in table 2/18), grown in separate pots and thus not subjected to any competition, yielded the large average of 5.1 seeds. The smaller number of capsules produced by the self-fertilised plants may be in part, but not altogether, attributed to their lessened size or height; this being chiefly due to their lessened constitutional vigour, so that they were not able to compete with the crossed plants growing in the same pots. The seeds produced by the crossed flowers on the crossed plants were not always heavier than the self-fertilised seeds on the self-fertilised plants. The lighter seeds, whether produced from crossed or self-fertilised flowers, generally germinated before the heavier seeds. I may add that the crossed plants, with very few exceptions, flowered before their self-fertilised opponents, as might have been expected from their greater height and vigour.

The impaired fertility of the self-fertilised plants was shown in another way, namely, by their anthers being smaller than those in the flowers on the crossed plants. This was first observed in the seventh generation, but may have occurred earlier. Several anthers from flowers on the crossed and self-fertilised plants of the eighth generation were compared under the microscope; and those from the former were generally longer and plainly broader than the anthers of the self-fertilised plants. The quantity of pollen contained in one of the latter was, as far as could be judged by the eye, about half of that contained in one from a crossed plant. The impaired fertility of the self-fertilised plants of the eighth generation was also shown in another manner, which may often be observed in hybrids—namely, by the first-formed flowers being sterile. For instance, the fifteen first flowers on a self-fertilised plant of one of the later generations were carefully fertilised with their own pollen, and eight of them dropped off; at the same time fifteen flowers on a crossed plant growing in the same pot were self-fertilised, and only one dropped off. On two other crossed plants of the same generation, several of the earliest flowers were observed to fertilise themselves and to produce capsules. In the plants of the ninth, and I believe of some previous generations, very many of the flowers, as already stated, were slightly monstrous; and this probably was connected with their lessened fertility.

All the self-fertilised plants of the seventh generation, and I believe of one or two previous generations, produced flowers of exactly the same tint, namely, of a rich dark purple. So did all the plants, without any exception, in the three succeeding generations of self-fertilised plants; and very many were raised on account of other experiments in progress not here recorded. My attention was first called to this fact by my gardener remarking that there was no occasion to label the self-fertilised plants, as they could always be known by their colour. The flowers were as uniform in tint as those of a wild species growing in a state of nature; whether the same tint occurred, as is probable, in the earlier generations, neither my gardener nor self could recollect. The flowers on the plants which were first raised from purchased seed, as well as during the first few generations, varied much in the depth of the purple tint; many were more or less pink, and occasionally a white variety appeared. The crossed plants continued to the tenth generation to vary in the same manner as before, but to a much less degree, owing, probably, to their having become more or less closely inter-related. We must therefore attribute the extraordinary uniformity of colour in the flowers on the plants of the seventh and succeeding self-fertilised generations, to inheritance not having been interfered with by crosses during several preceding generations, in combination with the conditions of life having been very uniform.

A plant appeared in the sixth self-fertilised generation, named the Hero, which exceeded by a little in height its crossed antagonist, and which transmitted its powers of growth and increased self-fertility to its children and grandchildren. A cross between the children of Hero did not give to the grandchildren any advantage over the self-fertilised grandchildren raised from the self-fertilised children. And as far as my observations can be trusted, which were made on very unhealthy plants, the great-grandchildren raised from intercrossing the grandchildren had no advantage over the seedlings from the grandchildren the product of continued self-fertilisation; and what is far more remarkable, the great-grandchildren raised by crossing the grandchildren with a fresh stock, had no advantage over either the intercrossed or self-fertilised great-grandchildren. It thus appears that Hero and its descendants differed in constitution in an extraordinary manner from ordinary plants of the present species.

Although the plants raised during ten successive generations from crosses between distinct yet inter-related plants almost invariably exceeded in height, constitutional vigour, and fertility their self-fertilised opponents, it has been proved that seedlings raised by intercrossing flowers on the same plant are by no means superior, on the contrary are somewhat inferior in height and weight, to seedlings raised from flowers fertilised with their own pollen. This is a remarkable fact, which seems to indicate that self-fertilisation is in some manner more advantageous than crossing, unless the cross brings with it, as is generally the case, some decided and preponderant advantage; but to this subject I shall recur in a future chapter.

The benefits which so generally follow from a cross between two plants apparently depend on the two differing somewhat in constitution or character. This is shown by the seedlings from the intercrossed plants of the ninth generation, when crossed with pollen from a fresh stock, being as superior in height and almost as superior in fertility to the again intercrossed plants, as these latter were to seedlings from self-fertilised plants of the corresponding generation. We thus learn the important fact that the mere act of crossing two distinct plants, which are in some degree inter-related and which have been long subjected to nearly the same conditions, does little good as compared with that from a cross between plants belonging to different stocks or families, and which have been subjected to somewhat different conditions. We may attribute the good derived from the crossing of the intercrossed plants during the ten successive generations to their still differing somewhat in constitution or character, as was indeed proved by their flowers still differing somewhat in colour. But the several conclusions which may be deduced from the experiments on Ipomoea will be more fully considered in the final chapters, after all my other observations have been given.

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