DIFFERENCE BETWEEN CROSSED AND SELF-FERTILISED PLANTS IN CONSTITUTIONAL VIGOUR AND IN OTHER RESPECTS

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 . DIFFERENCE BETWEEN CROSSED AND SELF-FERTILISED PLANTS IN CONSTITUTIONAL VIGOUR AND IN OTHER RESPECTS DIFFERENCE BETWEEN CROSSED AND SELF-FERTILISED PLANTS IN CONSTITUTIONAL VIGOUR AND IN OTHER RESPECTS GREATER CONSTITUTIONAL VIGOUR OF CROSSED PLANTS. As in almost all my experiments an equal number of crossed and self-fertilised seeds, or more commonly seedlings just beginning to sprout, were planted on the opposite sides of the same pots, they had to compete with one another; and the greater height, weight, and fertility of the crossed plants may be attributed to their possessing greater innate constitutional vigour. Generally the plants of the two lots whilst very young were of equal height; but afterwards the crossed gained insensibly on their opponents, and this shows that they possessed some inherent superiority, though not displayed at a very early period in life. There were, however, some conspicuous exceptions to the rule of the two lots being at first equal in height; thus the crossed seedlings of the broom (Sarothamnus scoparius) when under three inches in height were more than twice as tall as the self-fertilised plants. After the crossed or the self-fertilised plants had once grown decidedly taller than their opponents, a still increasing advantage would tend to follow from the stronger plants robbing the weaker ones of nourishment and overshadowing them. This was evidently the case with the crossed plants of Viola tricolor, which ultimately quite overwhelmed the self-fertilised. But that the crossed plants have an inherent superiority, independently of competition, was sometimes well shown when both lots were planted separately, not far distant from one another, in good soil in the open ground. This was likewise shown in several cases, even with plants growing in close competition with one another, by one of the self-fertilised plants exceeding for a time its crossed opponent, which had been injured by some accident or was at first sickly, but being ultimately conquered by it. The plants of the eighth generation of Ipomoea were raised from small seeds produced by unhealthy parents, and the self-fertilised plants grew at first very rapidly, so that when the plants of both lots were about three feet in height, the mean height of the crossed to that of the self-fertilised was as 100 to 122; when they were about six feet high the two lots were very nearly equal, but ultimately when between eight and nine feet in height, the crossed plants asserted their usually superiority, and were to the self-fertilised in height as 100 to 85. The constitutional superiority of the crossed over the self-fertilised plants was proved in another way in the third generation of Mimulus, by self-fertilised seeds being sown on one side of a pot, and after a certain interval of time crossed seeds on the opposite side. The self-fertilised seedlings thus had (for I ascertained that the seeds germinated simultaneously) a clear advantage over the crossed in the start for the race. Nevertheless they were easily beaten (as may be seen under the head of Mimulus) when the crossed seeds were sown two whole days after the self-fertilised. But when the interval was four days, the two lots were nearly equal throughout life. Even in this latter case the crossed plants still possessed an inherent advantage, for after both lots had grown to their full height they were cut down, and without being disturbed were transferred to a larger pot, and when in the ensuing year they had again grown to their full height they were measured; and now the tallest crossed plants were to the tallest self-fertilised plants in height as 100 to 75, and in fertility (i.e., by weight of seeds produced by an equal number of capsules from both lots) as 100 to 34. My usual method of proceeding, namely, to plant several pairs of crossed and self-fertilised seeds in an equal state of germination on the opposite sides of the same pots, so that the plants were subjected to moderately severe mutual competition, was I think the best that could have been followed, and was a fair test of what occurs in a state of nature. For plants sown by nature generally come up crowded, and are almost always exposed to very severe competition with one another and with other kinds of plants. This latter consideration led me to make some trials, chiefly but not exclusively with Ipomoea and Mimulus, by sowing crossed and self-fertilised seeds on the opposite sides of large pots in which other plants had long been growing, or in the midst of other plants out of doors. The seedlings were thus subjected to very severe competition with plants of other kinds; and in all such cases, the crossed seedlings exhibited a great superiority in their power of growth over the self-fertilised. After the germinating seedlings had been planted in pairs on the opposite sides of several pots, the remaining seeds, whether or not in a state of germination, were in most cases sown very thickly on the two sides of an additional large pot; so that the seedlings came up extremely crowded, and were subjected to extremely severe competition and unfavourable conditions. In such cases the crossed plants almost invariably showed a greater superiority over the self-fertilised, than did the plants which grew in pairs in the pots. Sometimes crossed and self-fertilised seeds were sown in separate rows in the open ground, which was kept clear of weeds; so that the seedlings were not subjected to any competition with other kinds of plants. Those however in each row had to struggle with the adjoining ones in the same row. When fully grown, several of the tallest plants in each row were selected, measured, and compared. The result was in several cases (but not so invariably as might have been expected) that the crossed plants did not exceed in height the self-fertilised in nearly so great a degree as when grown in pairs in the pots. Thus with the plants of Digitalis, which competed together in pots, the crossed were to the self-fertilised in height as 100 to 70; whilst those which were grown separately were only as 100 to 85. Nearly the same result was observed with Brassica. With Nicotiana the crossed were to the self-fertilised plants in height, when grown extremely crowded together in pots, as 100 to 54; when grown much less crowded in pots as 100 to 66, and when grow in the open ground, so as to be subjected to but little competition, as 100 to 72. On the other hand with Zea, there was a greater difference in height between the crossed and self-fertilised plants growing out of doors, than between the pairs which grew in pots in the hothouse; but this may be attributed to the self-fertilised plants being more tender, so that they suffered more than the crossed, when both lots were exposed to a cold and wet summer. Lastly, with one out of two series of Reseda odorata, grown out of doors in rows, as well as with Beta vulgaris, the crossed plants did not at all exceed the self-fertilised in height, or exceeded them by a mere trifle. The innate power of the crossed plants to resist unfavourable conditions far better than did the self-fertilised plants, was shown on two occasions in a curious manner, namely, with Iberis and in the third generation of Petunia, by the great superiority in height of the crossed over the self-fertilised seedlings, when both sets were grown under extremely unfavourable conditions; whereas owing to special circumstances exactly the reverse occurred with the plants raised from the same seeds and grown in pairs in pots. A nearly analogous case was observed on two other occasions with plants of the first generation of Nicotiana. The crossed plants always withstood the injurious effects of being suddenly removed into the open air after having been kept in the greenhouse better than did the self-fertilised. On several occasions they also resisted much better cold and intemperate weather. This was manifestly the case with some crossed and self-fertilised plants of Ipomoea, which were suddenly moved from the hothouse to the coldest part of a cool greenhouse. The offspring of plants of the eighth self-fertilised generation of Mimulus crossed by a fresh stock, survived a frost which killed every single self-fertilised and intercrossed plant of the same old stock. Nearly the same result followed with some crossed and self-fertilised plants of Viola tricolor. Even the tips of the shoots of the crossed plants of Sarothamnus scoparius were not touched by a very severe winter; whereas all the self-fertilised plants were killed halfway down to the ground, so that they were not able to flower during the next summer. Young crossed seedlings of Nicotiana withstood a cold and wet summer much better than the self-fertilised seedlings. I have met with only one exception to the rule of crossed plants being hardier than the self-fertilised: three long rows of Eschscholtzia plants, consisting of crossed seedlings from a fresh stock, of intercrossed seedlings of the same stock, and of self-fertilised ones, were left unprotected during a severe winter, and all perished except two of the self-fertilised. But this case is not so anomalous as it at first appears, for it should be remembered that the self-fertilised plants of Eschscholtzia always grow taller and are heavier than the crossed; the whole benefit of a cross with this species being confined to increased fertility. Independently of any external cause which could be detected, the self-fertilised plants were more liable to premature death than were the crossed; and this seems to me a curious fact. Whilst the seedlings were very young, if one died its antagonist was pulled up and thrown away, and I believe that many more of the self-fertilised died at this early age than of the crossed; but I neglected to keep any record. With Beta vulgaris, however, it is certain that a large number of the self-fertilised seeds perished after germinating beneath the ground, whereas the crossed seeds sown at the same time did not thus suffer. When a plant died at a somewhat more advanced age the fact was recorded; and I find in my notes that out of several hundred plants, only seven of the crossed died, whilst of the self-fertilised at least twenty-nine were thus lost, that is more than four times as many. Mr. Galton, after examining some of my tables, remarks: “It is very evident that the columns with the self-fertilised plants include the larger number of exceptionally small plants;” and the frequent presence of such puny plants no doubt stands in close relation with their liability to premature death. The self-fertilised plants of Petunia completed their growth and began to wither sooner than did the intercrossed plants; and these latter considerably before the offspring from a cross with a fresh stock. PERIOD OF FLOWERING. In some cases, as with Digitalis, Dianthus, and Reseda, a larger number of the crossed than of the self-fertilised plants threw up flower-stems; but this probably was merely the result of their greater power of growth; for in the first generation of Lobelia fulgens, in which the self-fertilised plants greatly exceeded in height the crossed plants, some of the latter failed to throw up flower-stems. With a large number of species, the crossed plants exhibited a well-marked tendency to flower before the self-fertilised ones growing in the same pots. It should however be remarked that no record was kept of the flowering of many of the species; and when a record was kept, the flowering of the first plant in each pot was alone observed, although two or more pairs grew in the same pot. I will now give three lists,—one of the species in which the first plant that flowered was a crossed one,—a second in which the first that flowered was a self-fertilised plant,—and a third of those which flowered at the same time. [SPECIES, OF WHICH THE FIRST PLANTS THAT FLOWERED WERE OF CROSSED PARENTAGE. Ipomoea purpurea. I record in my notes that in all ten generations many of the crossed plants flowered before the self-fertilised; but no details were kept. Mimulus luteus (First Generation). Ten flowers on the crossed plants were fully expanded before one on the self-fertilised. Mimulus luteus (Second and Third Generation). In both these generations a crossed plant flowered before one of the self-fertilised in all three pots. Mimulus luteus (Fifth Generation). In all three pots a crossed plant flowered first; yet the self-fertilised plants, which belonged to the new tall variety, were in height to the crossed as 126 to 100. Mimulus luteus. Plants derived from a cross with a fresh stock as well as the intercrossed plants of the old stock, flowered before the self-fertilised plants in nine out of the ten pots. Salvia coccinea. A crossed plant flowered before any one of the self-fertilised in all three pots. Origanum vulgare. During two successive seasons several crossed plants flowered before the self-fertilised. Brassica oleracea (First Generation). All the crossed plants growing in pots and in the open ground flowered first. Brassica oleracea (Second Generation). A crossed plant in three out of the four pots flowered before any one of the self-fertilised. Iberis umbellata. In both pots a crossed plant flowered first. Eschscholtzia californica. Plants derived from the Brazilian stock crossed by the English stock flowered in five out of the nine pots first; in four of them a self-fertilised plant flowered first; and not in one pot did an intercrossed plant of the old stock flower first. Viola tricolor. A crossed plant in five out of the six pots flowered before any one of the self-fertilised. Dianthus caryophyllus (First Generation). In two large beds of plants, four of the crossed plants flowered before any one of the self-fertilised. Dianthus caryophyllus (Second Generation). In both pots a crossed plant flowered first. Dianthus caryophyllus (Third Generation). In three out of the four pots a crossed plant flowered first; yet the crossed were to the self-fertilised in height only as 100 to 99, but in weight as 100 to 49. Dianthus caryophyllus. Plants derived from a cross with a fresh stock, and the intercrossed plants of the old stock, both flowered before the self-fertilised in nine out of the ten pots. Hibiscus africanus. In three out of the four pots a crossed plant flowered before any one of the self-fertilised; yet the latter were to the crossed in height as 109 to 100. Tropaeolum minus. A crossed plant flowered before any one of the self-fertilised in three out of the four pots, and simultaneously in the fourth pot. Limnanthes douglasii. A crossed plant flowered before any one of the self-fertilised in four out of the five pots. Phaseolus multiflorus. In both pots a crossed plant flowered first. Specularia speculum. In all four pots a crossed plant flowered first. Lobelia ramosa (First Generation). In all four pots a crossed plant flowered before any one of the self-fertilised. Lobelia ramosa (Second Generation). In all four pots a crossed plant flowered some days before any one of the self-fertilised. Nemophila insignis. In four out of the five pots a crossed plant flowered first. Borago officinalis. In both pots a crossed plant flowered first. Petunia violacea (Second Generation). In all three pots a crossed plant flowered first. Nicotiana tabacum. A plant derived from a cross with a fresh stock flowered before any one of the self-fertilised plants of the fourth generation, in fifteen out of the sixteen pots. Cyclamen persicum. During two successive seasons a crossed plant flowered some weeks before any one of the self-fertilised in all four pots. Primula veris (equal-styled var.) In all three pots a crossed plant flowered first. Primula sinensis. In all four pots plants derived from an illegitimate cross between distinct plants flowered before any one of the self-fertilised plants. Primula sinensis. A legitimately crossed plant flowered before any one of the self-fertilised plants in seven out of the eight pots. Fagopyrum esculentum. A legitimately crossed plant flowered from one to two days before any one of the self-fertilised plants in all three pots. Zea mays. In all four pots a crossed plant flowered first. Phalaris canariensis. The crossed plants flowered before the self-fertilised in the open ground, but simultaneously in the pots. SPECIES OF WHICH THE FIRST PLANTS THAT FLOWERED WERE OF SELF-FERTILISED PARENTAGE. Eschscholtzia californica (First Generation). The crossed plants were at first taller than the self-fertilised, but on their second growth during the following year the self-fertilised exceeded the crossed in height, and now they flowered first in three out of the four pots. Lupinus luteus. Although the crossed plants were to the self-fertilised in height as 100 to 82; yet in all three pots the self-fertilised plants flowered first. Clarkia elegans. Although the crossed plants were, as in the last case, to the self-fertilised in height as 100 to 82, yet in the two pots the self-fertilised flowered first. Lobelia fulgens (First Generation). The crossed plants were to the self-fertilised in height only as 100 to 127, and the latter flowered much before the crossed. Petunia violacea (Third Generation). The crossed plants were to the self-fertilised in height as 100 to 131, and in three out of the four pots a self-fertilised plant flowered first; in the fourth pot simultaneously. Petunia violacea (Fourth generation). Although the crossed plants were to the self-fertilised in height as 100 to 69, yet in three out of the five pots a self-fertilised plant flowered first; in the fourth pot simultaneously, and only in the fifth did a crossed plant flower first. Nicotiana tabacum (First Generation). The crossed plants were to the self-fertilised in height only as 100 to 178, and a self-fertilised plant flowered first in all four pots. Nicotiana tabacum (Third Generation). The crossed plants were to the self-fertilised in height as 100 to 101, and in four out of the five pots a self-fertilised plant flowered first. Canna warscewiczi. In the three generations taken together the crossed were to the self-fertilised in height as 100 to 101; in the first generation the self-fertilised plants showed some tendency to flower first, and in the third generation they flowered first in nine out of the twelve pots. SPECIES IN WHICH THE CROSSED AND SELF-FERTILISED PLANTS FLOWERED ALMOST SIMULTANEOUSLY. Mimulus luteus (Sixth Generation). The crossed plants were inferior in height and vigour to the self-fertilised plants, which all belonged to the new white-flowered tall variety, yet in only half the pots did the self-fertilised plants flower first, and in the other half the crossed plants. Viscaria oculata. The crossed plants were only a little taller than the self-fertilised (namely, as 100 to 97), but considerably more fertile, yet both lots flowered almost simultaneously. Lathyrus odoratus (Second Generation). Although the crossed plants were to the self-fertilised in height as 100 to 88, yet there was no marked difference in their period of flowering. Lobelia fulgens (Second Generation). Although the crossed plants were to the self-fertilised in height as 100 to 91, yet they flowered simultaneously. Nicotiana tabacum (Third Generation). Although the crossed plants were to the self-fertilised in height as 100 to 83, yet in half the pots a self-fertilised plant flowered first, and in the other half a crossed plant.] These three lists include fifty-eight cases, in which the period of flowering of the crossed and self-fertilised plants was recorded. In forty-four of them a crossed plant flowered first either in a majority of the pots or in all; in nine instances a self-fertilised plant flowered first, and in five the two lots flowered simultaneously. One of the most striking cases is that of Cyclamen, in which the crossed plants flowered some weeks before the self-fertilised in all four pots during two seasons. In the second generation of Lobelia ramosa, a crossed plant flowered in all four pots some days before any one of the self-fertilised. Plants derived from a cross with a fresh stock generally showed a very strongly marked tendency to flower before the self-fertilised and the intercrossed plants of the old stock; all three lots growing in the same pots. Thus with Mimulus and Dianthus, in only one pot out of ten, and in Nicotiana in only one pot out of sixteen, did a self-fertilised plant flower before the plants of the two crossed kinds,—these latter flowering almost simultaneously. A consideration of the two first lists, especially of the second one, shows that a tendency to flower first is generally connected with greater power of growth, that is, with greater height. But there are some remarkable exceptions to this rule, proving that some other cause comes into play. Thus the crossed plants both of Lupinus luteus and Clarkia elegans were to the self-fertilised plants in height as 100 to 82, and yet the latter flowered first. In the third generation of Nicotiana, and in all three generations of Canna, the crossed and self-fertilised plants were of nearly equal height, yet the self-fertilised tended to flower first. On the other hand, with Primula sinensis, plants raised from a cross between two distinct individuals, whether these were legitimately or illegitimately crossed, flowered before the illegitimately self-fertilised plants, although all the plants were of nearly equal height in both cases. So it was with respect to height and flowering with Phaseolus, Specularia, and Borago. The crossed plants of Hibiscus were inferior in height to the self-fertilised, in the ratio of 100 to 109, and yet they flowered before the self-fertilised in three out of the four pots. On the whole, there can be no doubt that the crossed plants exhibit a tendency to flower before the self-fertilised, almost though not quite so strongly marked as to grow to a greater height, to weigh more, and to be more fertile. A few other cases not included in the above three lists deserve notice. In all three pots of Viola tricolor, naturally crossed plants the offspring of crossed plants flowered before naturally crossed plants the offspring of self-fertilised plants. Flowers on two plants, both of self-fertilised parentage, of the sixth generation of Mimulus luteus were intercrossed, and other flowers on the same plants were fertilised with their own pollen; intercrossed seedlings and seedlings of the seventh self-fertilised generation were thus raised, and the latter flowered before the intercrossed in three out of the five pots. Flowers on a plant both of Mimulus luteus and of Ipomoea purpurea were crossed with pollen from other flowers on the same plant, and other flowers were fertilised with their own pollen; intercrossed seedlings of this peculiar kind, and others strictly self-fertilised being thus raised. In the case of the Mimulus the self-fertilised plants flowered first in seven out of the eight pots, and in the case of the Ipomoea in eight out of the ten pots; so that an intercross between the flowers on the same plant was very far from giving to the offspring thus raised, any advantage over the strictly self-fertilised plants in their period of flowering. EFFECTS OF CROSSING FLOWERS ON THE SAME PLANT. In the discussion on the results of a cross with a fresh stock, given under Table 7/C in the last chapter, it was shown that the mere act of crossing by itself does no good; but that the advantages thus derived depend on the plants which are crossed, either consisting of distinct varieties which will almost certainly differ somewhat in constitution, or on the progenitors of the plants which are crossed, though identical in every external character, having been subjected to somewhat different conditions and having thus acquired some slight difference in constitution. All the flowers produced by the same plant have been developed from the same seed; those which expand at the same time have been exposed to exactly the same climatic influences; and the stems have all been nourished by the same roots. Therefore in accordance with the conclusion just referred to, no good ought to result from crossing flowers on the same plant. (8/1. It is, however, possible that the stamens which differ in length or construction in the same flower may produce pollen differing in nature, and in this manner a cross might be made effective between the several flowers on the same plant. Mr. Macnab states in a communication to M. Verlot ‘La Production des Varietes’ 1865 page 42, that seedlings raised from the shorter and longer stamens of rhododendron differ in character; but the shorter stamens apparently are becoming rudimentary, and the seedlings are dwarfs, so that the result may be simply due to a want of fertilising power in the pollen, as in the case of the dwarfed plants of Mirabilis raised by Naudin by the use of too few pollen-grains. Analogous statements have been made with respect to the stamens of Pelargonium. With some of the Melastomaceae, seedlings raised by me from flowers fertilised by pollen from the shorter stamens, certainly differed in appearance from those raised from the longer stamens, with differently coloured anthers; but here, again, there is some reason for believing that the shorter stamens are tending towards abortion. In the very different case of trimorphic heterostyled plants, the two sets of stamens in the same flower have widely different fertilising powers.) In opposition to this conclusion is the fact that a bud is in one sense a distinct individual, and is capable of occasionally or even not rarely assuming new external characters, as well as new constitutional peculiarities. Plants raised from buds which have thus varied may be propagated for a great length of time by grafts, cuttings, etc., and sometimes even by seminal generation. (8/2. I have given numerous cases of such bud-variations in my ‘Variation of Animals and Plants under Domestication’ chapter 11 2nd edition volume 1 page 448.) There exist also numerous species in which the flowers on the same plant differ from one another,—as in the sexual organs of monoecious and polygamous plants,—in the structure of the circumferential flowers in many Compositae, Umbelliferae, etc.,—in the structure of the central flower in some plants,—in the two kinds of flowers produced by cleistogene species,—and in several other such cases. These instances clearly prove that the flowers on the same plant have often varied independently of one another in many important respects, such variations having been fixed, like those on distinct plants during the development of species. It was therefore necessary to ascertain by experiment what would be the effect of intercrossing flowers on the same plant, in comparison with fertilising them with their own pollen or crossing them with pollen from a distinct plant. Trials were carefully made on five genera belonging to four families; and in only one case, namely, Digitalis, did the offspring from a cross between the flowers on the same plant receive any benefit, and the benefit here was small compared with that derived from a cross between distinct plants. In the chapter on Fertility, when we consider the effects of cross-fertilisation and self-fertilisation on the productiveness of the parent-plants we shall arrive at nearly the same result, namely, that a cross between the flowers on the same plant does not at all increase the number of the seeds, or only occasionally and to a slight degree. I will now give an abstract of the results of the five trials which were made. 1. Digitalis purpurea. Seedlings raised from intercrossed flowers on the same plant, and others from flowers fertilised with their own pollen, were grown in the usual manner in competition with one another on the opposite sides of ten pots. In this and the four following cases, the details may be found under the head of each species. In eight pots, in which the plants did not grow much crowded, the flower-stems on sixteen intercrossed plants were in height to those on sixteen self-fertilised plants, as 100 to 94. In the two other pots on which the plants grew much crowded, the flower-stems on nine intercrossed plants were in height to those on nine self-fertilised plants, as 100 to 90. That the intercrossed plants in these two latter pots had a real advantage over their self-fertilised opponents, was well shown by their relative weights when cut down, which was as 100 to 78. The mean height of the flower-stems on the twenty-five intercrossed plants in the ten pots taken together, was to that of the flower-stems on the twenty-five self-fertilised plants, as 100 to 92. Thus the intercrossed plants were certainly superior to the self-fertilised in some degree; but their superiority was small compared with that of the offspring from a cross between distinct plants over the self-fertilised, this being in the ratio of 100 to 70 in height. Nor does this latter ratio show at all fairly the great superiority of the plants derived from a cross between distinct individuals over the self-fertilised, as the former produced more than twice as many flower-stems as the latter, and were much less liable to premature death. 2. Ipomoea purpurea. Thirty-one intercrossed plants raised from a cross between flowers on the same plants were grown in ten pots in competition with the same number of self-fertilised plants, and the former were to the latter in height as 100 to 105. So that the self-fertilised plants were a little taller than the intercrossed; and in eight out of the ten pots a self-fertilised plant flowered before any one of the crossed plants in the same pots. The plants which were not greatly crowded in nine of the pots (and these offer the fairest standard of comparison) were cut down and weighed; and the weight of the twenty-seven intercrossed plants was to that of the twenty-seven self-fertilised as 100 to 124; so that by this test the superiority of the self-fertilised was strongly marked. To this subject of the superiority of the self-fertilised plants in certain cases, I shall have to recur in a future chapter. If we now turn to the offspring from a cross between distinct plants when put into competition with self-fertilised plants, we find that the mean height of seventy-three such crossed plants, in the course of ten generations, was to that of the same number of self-fertilised plants as 100 to 77; and in the case of the plants of the tenth generation in weight as 100 to 44. Thus the contrast between the effects of crossing flowers on the same plant, and of crossing flowers on distinct plants, is wonderfully great. 3. Mimulus luteus. Twenty-two plants raised by crossing flowers on the same plant were grown in competition with the same number of self-fertilised plants; and the former were to the latter in height as 100 to 105, and in weight as 100 to 103. Moreover, in seven out of the eight pots a self-fertilised plant flowered before any of the intercrossed plants. So that here again the self-fertilised exhibit a slight superiority over the intercrossed plants. For the sake of comparison, I may add that seedlings raised during three generations from a cross between distinct plants were to the self-fertilised plants in height as 100 to 65. 4. Pelargonium zonale. Two plants growing in separate pots, which had been propagated by cuttings from the same plant, and therefore formed in fact parts of the same individual, were intercrossed, and other flowers on one of these plants were self-fertilised; but the seedlings obtained by the two processes did not differ in height. When, on the other hand, flowers on one of the above plants were crossed with pollen taken from a distinct seedling, and other flowers were self-fertilised, the crossed offspring thus obtained were to the self-fertilised in height as 100 to 74. 5. Origanum vulgare. A plant which had been long cultivated in my kitchen garden, had spread by stolons so as to form a large bed or clump. Seedlings raised by intercrossing flowers on these plants, which strictly consisted of the same plant, and other seedlings raised from self-fertilised flowers, were carefully compared from their earliest youth to maturity; and they did not differ at all in height or in constitutional vigour. Some flowers on these seedlings were then crossed with pollen taken from a distinct seedling, and other flowers were self-fertilised; two fresh lots of seedlings being thus raised, which were the grandchildren of the plant that had spread by stolons and formed a large clump in my garden. These differed much in height, the crossed plants being to the self-fertilised as 100 to 86. They differed, also, to a wonderful degree in constitutional vigour. The crossed plants flowered first, and produced exactly twice as many flower-stems; and they afterwards increased by stolons to such an extent as almost to overwhelm the self-fertilised plants. Reviewing these five cases, we see that in four of them, the effect of a cross between flowers on the same plant (even on offsets of the same plant growing on separate roots, as with the Pelargonium and Origanum) does not differ from that of the strictest self-fertilisation. Indeed, in two of the cases the self-fertilised plants were superior to such intercrossed plants. With Digitalis a cross between the flowers on the same plant certainly did do some good, yet very slight compared with that from a cross between distinct plants. On the whole the results here arrived at, if we bear in mind that the flower-buds are to a certain extent distinct individuals and occasionally vary independently of one another, agree well with our general conclusion, that the advantages of a cross depend on the progenitors of the crossed plants possessing somewhat different constitutions, either from having been exposed to different conditions, or to their having varied from unknown causes in a manner which we in our ignorance are forced to speak of as spontaneous. Hereafter I shall have to recur to this subject of the inefficiency of a cross between the flowers on the same plant, when we consider the part which insects play in the cross-fertilisation of flowers. ON THE TRANSMISSION OF THE GOOD EFFECTS FROM A CROSS AND OF THE EVIL EFFECTS FROM SELF-FERTILISATION. We have seen that seedlings from a cross between distinct plants almost always exceed their self-fertilised opponents in height, weight, and constitutional vigour, and, as will hereafter be shown, often in fertility. To ascertain whether this superiority would be transmitted beyond the first generation, seedlings were raised on three occasions from crossed and self-fertilised plants, both sets being fertilised in the same manner, and therefore not as in the many cases given in Tables 7/A, 7/B, 7/C, in which the crossed plants were again crossed and the self-fertilised again self-fertilised. Firstly, seedlings were raised from self-fertilised seeds produced under a net by crossed and self-fertilised plants of Nemophila insignis; and the latter were to the former in height as 133 to 100. But these seedlings became very unhealthy early in life, and grew so unequally that some of them in both lots were five times as tall as the others. Therefore this experiment was quite worthless; but I have felt bound to give it, as opposed to my general conclusion. I should state that in this and the two following trials, both sets of plants were grown on the opposite sides of the same pots, and treated in all respects alike. The details of the experiments may be found under the head of each species. Secondly, a crossed and a self-fertilised plant of Heartsease (Viola tricolor) grew near together in the open ground and near to other plants of heartsease; and as both produced an abundance of very fine capsules, the flowers on both were certainly cross-fertilised by insects. Seeds were collected from both plants, and seedlings raised from them. Those from the crossed plants flowered in all three pots before those from the self-fertilised plants; and when fully grown the former were to the latter in height as 100 to 82. As both sets of plants were the product of cross-fertilisation, the difference in their growth and period of flowering was clearly due to their parents having been of crossed and self-fertilised parentage; and it is equally clear that they transmitted different constitutional powers to their offspring, the grandchildren of the plants which were originally crossed and self-fertilised. Thirdly, the Sweet Pea (Lathyrus odoratus) habitually fertilises itself in this country. As I possessed plants, the parents and grandparents of which had been artificially crossed and other plants descended from the same parents which had been self-fertilised for many previous generations, these two lots of plants were allowed to fertilise themselves under a net, and their self-fertilised seeds saved. The seedlings thus raised were grown in competition with each other in the usual manner, and differed in their powers of growth. Those from the self-fertilised plants which had been crossed during the two previous generations were to those from the plants self-fertilised during many previous generations in height as 100 to 90. These two lots of seeds were likewise tried by being sown under very unfavourable conditions in poor exhausted soil, and the plants whose grandparents and great-grandparents had been crossed showed in an unmistakable manner their superior constitutional vigour. In this case, as in that of the heartsease, there could be no doubt that the advantage derived from a cross between two plants was not confined to the offspring of the first generation. That constitutional vigour due to cross-parentage is transmitted for many generations may also be inferred as highly probable, from some of Andrew Knight’s varieties of the common pea, which were raised by crossing distinct varieties, after which time they no doubt fertilised themselves in each succeeding generation. These varieties lasted for upwards of sixty years, “but their glory is now departed.” (8/3. See the evidence on this head in my ‘Variation under Domestication’ chapter 9 volume 1 2nd edition page 397.) On the other hand, most of the varieties of the common pea, which there is no reason to suppose owe their origin to a cross, have had a much shorter existence. Some also of Mr. Laxton’s varieties produced by artificial crosses have retained their astonishing vigour and luxuriance for a considerable number of generations; but as Mr. Laxton informs me, his experience does not extend beyond twelve generations, within which period he has never perceived any diminution of vigour in his plants. An allied point may be here noticed. As the force of inheritance is strong with plants (of which abundant evidence could be given), it is almost certain that seedlings from the same capsule or from the same plant would tend to inherit nearly the same constitution; and as the advantage from a cross depends on the plants which are crossed differing somewhat in constitution, it may be inferred as probable that under similar conditions a cross between the nearest relations would not benefit the offspring so much as one between non-related plants. In support of this conclusion we have some evidence, as Fritz Muller has shown by his valuable experiments on hybrid Abutilons, that the union of brothers and sisters, parents and children, and of other near relations is highly injurious to the fertility of the offspring. In one case, moreover, seedlings from such near relations possessed very weak constitutions. (8/4. ‘Jenaische Zeitschrift fur Naturw.’ B. 7 pages 22 and 45 1872 and 1873 pages 441-450.) This same observer also found three plants of a Bignonia growing near together. (8/5. ‘Botanische Zeitung’ 1868 page 626.) He fertilised twenty-nine flowers on one of them with their own pollen, and they did not set a single capsule. Thirty flowers were then fertilised with pollen from a distinct plant, one of the three growing together, and they yielded only two capsules. Lastly, five flowers were fertilised with pollen from a fourth plant growing at a distance, and all five produced capsules. It seems therefore probable, as Fritz Muller suggests, that the three plants growing near together were seedlings from the same parent, and that from being closely related they had little power of fertilising one another. (8/6. Some remarkable cases are given in my ‘Variation under Domestication’ chapter 17 2nd edition volume 2 page 121, of hybrids of Gladiolus and Cistus, any one of which could be fertilised by pollen from any other, but not by its own pollen.) Lastly, the fact of the intercrossed plants in Table 7/A not exceeding in height the self-fertilised plants in a greater and greater degree in the later generations, is probably the result of their having become more and more closely inter-related. UNIFORM COLOUR OF THE FLOWERS ON PLANTS, SELF-FERTILISED AND GROWN UNDER SIMILAR CONDITIONS FOR SEVERAL GENERATIONS. At the commencement of my experiments, the parent-plants of Mimulus luteus, Ipomoea purpurea, Dianthus caryophyllus, and Petunia violacea, raised from purchased seeds, varied greatly in the colour of their flowers. This occurs with many plants which have been long cultivated as an ornament for the flower-garden, and which have been propagated by seeds. The colour of the flowers was a point to which I did not at first in the least attend, and no selection whatever was practised. Nevertheless, the flowers produced by the self-fertilised plants of the above four species became absolutely uniform in tint, or very nearly so, after they had been grown for some generations under closely similar conditions. The intercrossed plants, which were more or less closely inter-related in the later generations, and which had been likewise cultivated all the time under similar conditions, became more uniform in the colour of their flowers than were the original parent-plants, but much less so than the self-fertilised plants. When self-fertilised plants of one of the later generations were crossed with a fresh stock, and seedlings thus raised, these presented a wonderful contrast in the diversified tints of their flowers compared with those of the self-fertilised seedlings. As such cases of flowers becoming uniformly coloured without any aid from selection seem to me curious, I will give a full abstract of my observations. Mimulus luteus. A tall variety, bearing large, almost white flowers blotched with crimson, appeared amongst the intercrossed and self-fertilised plants of the third and fourth generations. This variety increased so rapidly, that in the sixth generation of self-fertilised plants every single one consisted of it. So it was with all the many plants which were raised, up to the last or ninth self-fertilised generation. Although this variety first appeared amongst the intercrossed plants, yet from their offspring being intercrossed in each succeeding generation, it never prevailed amongst them; and the flowers on the several intercrossed plants of the ninth generation differed considerably in colour. On the other hand, the uniformity in colour of the flowers on the plants of all the later self-fertilised generations was quite surprising; on a casual inspection they might have been said to be quite alike, but the crimson blotches were not of exactly the same shape, or in exactly the same position. Both my gardener and myself believe that this variety did not appear amongst the parent-plants, raised from purchased seeds, but from its appearance amongst both the crossed and self-fertilised plants of the third and fourth generations; and from what I have seen of the variation of this species on other occasions, it is probable that it would occasionally appear under any circumstances. We learn, however, from the present case that under the peculiar conditions to which my plants were subjected, this particular variety, remarkable for its colouring, largeness of the corolla, and increased height of the whole plant, prevailed in the sixth and all the succeeding self-fertilised generations to the complete exclusion of every other variety. Ipomoea purpurea. My attention was first drawn to the present subject by observing that the flowers on all the plants of the seventh self-fertilised generation were of a uniform, remarkably rich, dark purple tint. The many plants which were raised during the three succeeding generations, up to the last or tenth, all produced flowers coloured in the same manner. They were absolutely uniform in tint, like those of a constant species living in a state of nature; and the self-fertilised plants might have been distinguished with certainty, as my gardener remarked, without the aid of labels, from the intercrossed plants of the later generations. These, however, had more uniformly coloured flowers than those which were first raised from the purchased seeds. This dark purple variety did not appear, as far as my gardener and myself could recollect, before the fifth or sixth self-fertilised generation. However this may have been, it became, through continued self-fertilisation and the cultivation of the plants under uniform conditions, perfectly constant, to the exclusion of every other variety. Dianthus caryophyllus. The self-fertilised plants of the third generation all bore flowers of exactly the same pale rose-colour; and in this respect they differed quite remarkably from the plants growing in a large bed close by and raised from seeds purchased from the same nursery garden. In this case it is not improbable that some of the parent-plants which were first self-fertilised may have borne flowers thus coloured; but as several plants were self-fertilised in the first generation, it is extremely improbable that all bore flowers of exactly the same tint as those of the self-fertilised plants of the third generation. The intercrossed plants of the third generation likewise produced flowers almost, though not quite so uniform in tint as those of the self-fertilised plants. Petunia violacea. In this case I happened to record in my notes that the flowers on the parent-plant which was first self-fertilised were of a “dingy purple colour.” In the fifth self-fertilised generation, every one of the twenty-one self-fertilised plants growing in pots, and all the many plants in a long row out of doors, produced flowers of absolutely the same tint, namely, of a dull, rather peculiar and ugly flesh colour; therefore, considerably unlike those on the parent-plant. I believe that this change of colour supervened quite gradually; but I kept no record, as the point did not interest me until I was struck with the uniform tint of the flowers on the self-fertilised plants of the fifth generation. The flowers on the intercrossed plants of the corresponding generation were mostly of the same dull flesh colour, but not nearly so uniform as those on the self-fertilised plants, some few being very pale, almost white. The self-fertilised plants which grew in a long row in the open ground were also remarkable for their uniformity in height, as were the intercrossed plants in a less degree, both lots being compared with a large number of plants raised at the same time under similar conditions from the self-fertilised plants of the fourth generation crossed by a fresh stock. I regret that I did not attend to the uniformity in height of the self-fertilised seedlings in the later generations of the other species. These few cases seem to me to possess much interest. We learn from them that new and slight shades of colour may be quickly and firmly fixed, independently of any selection, if the conditions are kept as nearly uniform as is possible, and no intercrossing be permitted. With Mimulus, not only a grotesque style of colouring, but a larger corolla and increased height of the whole plant were thus fixed; whereas with most plants which have been long cultivated for the flower-garden, no character is more variable than that of colour, excepting perhaps that of height. From the consideration of these cases we may infer that the variability of cultivated plants in the above respects is due, firstly, to their being subjected to somewhat diversified conditions, and, secondly, to their being often intercrossed, as would follow from the free access of insects. I do not see how this inference can be avoided, as when the above plants were cultivated for several generations under closely similar conditions, and were intercrossed in each generation, the colour of their flowers tended in some degree to change and to become uniform. When no intercrossing with other plants of the same stock was allowed,—that is, when the flowers were fertilised with their own pollen in each generation—their colour in the later generations became as uniform as that of plants growing in a state of nature, accompanied at least in one instance by much uniformity in the height of the plants. But in saying that the diversified tints of the flowers on cultivated plants treated in the ordinary manner are due to differences in the soil, climate, etc., to which they are exposed, I do not wish to imply that such variations are caused by these agencies in any more direct manner than that in which the most diversified illnesses, as colds, inflammation of the lungs or pleura, rheumatism, etc., may be said to be caused by exposure to cold. In both cases the constitution of the being which is acted on is of preponderant importance. About HackerNoon Book Series: We bring you the most important technical, scientific, and insightful public domain books. This book is part of the public domain. Charles Darwin (2003). The Effects of Cross & Self-Fertilisation in the Vegetable Kingdom. Urbana, Illinois: Project Gutenberg. Retrieved October 2022, https://www.gutenberg.org/cache/epub/4346/pg4346-images.html This eBook is for the use of anyone anywhere at no cost and with almost no restrictions whatsoever. You may copy it, give it away or re-use it under the terms of the Project Gutenberg License included with this eBook or online at www.gutenberg.org , located at https://www.gutenberg.org/policy/license.html .

DIFFERENCE BETWEEN CROSSED AND SELF-FERTILISED PLANTS IN CONSTITUTIONAL VIGOUR AND IN OTHER RESPECTS

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Charles Darwin

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DIFFERENCE BETWEEN CROSSED AND SELF-FERTILISED PLANTS IN CONSTITUTIONAL VIGOUR AND IN OTHER RESPECTS

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