The Effects of Cross & Self-Fertilisation in the Vegetable Kingdom · Charles Darwin
EQUAL-STYLED AND RED-FLOWERED VAR.
I have described in my paper ‘On the Illegitimate Unions of Dimorphic and Trimorphic Plants’ this remarkable variety, which was sent to me from Edinburgh by Mr. J. Scott. It possessed a pistil proper to the long-styled form, and stamens proper to the short-styled form; so that it had lost the heterostyled or dimorphic character common to most of the species of the genus, and may be compared with an hermaphrodite form of a bisexual animal. Consequently the pollen and stigma of the same flower are adapted for complete mutual fertilisation, instead of its being necessary that pollen should be brought from one form to another, as in the common cowslip. From the stigma and anthers standing nearly on the same level, the flowers are perfectly self-fertile when insects are excluded. Owing to the fortunate existence of this variety, it is possible to fertilise its flowers in a legitimate manner with their own pollen, and to cross other flowers in a legitimate manner with pollen from another variety or fresh stock. Thus the offspring from both unions can be compared quite fairly, free from any doubt from the injurious effects of an illegitimate union. The plants on which I experimented had been raised during two successive generations from spontaneously self-fertilised seeds produced by plants under a net; and as the variety is highly self-fertile, its progenitors in Edinburgh may have been self-fertilised during some previous generations. Several flowers on two of my plants were legitimately crossed with pollen from a short-styled common cowslip growing almost wild in my orchard; so that the cross was between plants which had been subjected to considerably different conditions. Several other flowers on the same two plants were allowed to fertilise themselves under a net; and this union, as already explained, is a legitimate one.
The crossed and self-fertilised seeds thus obtained were sown thickly on the opposite sides of three pots, and the seedlings thinned, so that an equal number were left on the two sides. The seedlings during the first year were nearly equal in height, excepting in Pot 3, Table 6/94, in which the self-fertilised plants had a decided advantage. In the autumn the plants were bedded out, in their pots; owing to this circumstance, and to many plants growing in each pot, they did not flourish, and none were very productive in seeds. But the conditions were perfectly equal and fair for both sides. In the following spring I record in my notes that in two of the pots the crossed plants are “incomparably the finest in general appearance,” and in all three pots they flowered before the self-fertilised. When in full flower the tallest flower-stem on each side of each pot was measured, and the number of the flower-stems on both sides counted, as shown in Table 6/94. The plants were left uncovered, and as other plants were growing close by, the flowers no doubt were crossed by insects. When the capsules were ripe they were gathered and counted, and the result is likewise shown in Table 6/94. TABLE 6/94. Primula veris (equal-styled, red-flowered variety). Heights of plants measured in inches.
Column 1: Number (Name) of Pot. Column 2: Height of tallest flower-stem: crossed Plants. Column 3: Number of Flower-stems: crossed Plants. Column 4: Number of good capsules: crossed Plants. Column 5: Height of tallest flower-stem: self-fertilised Plants. Column 6: Number of Flower-stems: self-fertilised Plants. Column 7: Number of good capsules: self-fertilised Plants. Pot 1 : 10 : 14 : 163 : 6 4/8 : 6 : 6. Pot 2 : 8 4/8 : 12 : * : 5 : 2 : 0. *Several, not counted. Pot 3 : 7 4/8 : 7 : 43 : 10 4/8 : 5 : 26. Totals : 26.0 : 33 : 206 : 22.0 : 13 : 32. The average height of the three tallest flower-stems on the crossed plants is 8.66 inches, and that of the three on the self-fertilised plants 7.33 inches; or as 100 to 85.
All the crossed plants together produced thirty-three flower-stems, whilst the self-fertilised bore only thirteen. The number of the capsules were counted only on the plants in Pots 1 and 3, for the self-fertilised plants in Pot 2 produced none; therefore those on the crossed plants on the opposite side were not counted. Capsules not containing any good seeds were rejected. The crossed plants in the above two pots produced 206, and the self-fertilised in the same pots only 32 capsules; or as 100 to 15. Judging from the previous generations, the extreme unproductiveness of the self-fertilised plants in this experiment was wholly due to their having been subjected to unfavourable conditions, and to severe competition with the crossed plants; for had they grown separately in good soil, it is almost certain that they would have produced a large number of capsules. The seeds were counted in twenty capsules from the crossed plants, and they averaged 24.75; whilst in twenty capsules from the self-fertilised plants the average was 17.65; or as 100 to 71. Moreover, the seeds from the self-fertilised plants were not nearly so fine as those from the crossed plants. If we consider together the number of capsules produced and the average number of contained seeds, the fertility of the crossed plants to the self-fertilised plants was as 100 to 11. We thus see what a great effect, as far as fertility is concerned, was produced by a cross between the two varieties, which had been long exposed to different conditions, in comparison with self-fertilisation; the fertilisation having been in both cases of the legitimate order.
Primula sinensis. As the Chinese primrose is a heterostyled or dimorphic plant, like the common cowslip, it might have been expected that the flowers of both forms when illegitimately fertilised with their own pollen or with that from flowers on another plant of the same form, would have yielded less seed than the legitimately crossed flowers; and that the seedlings raised from illegitimately self-fertilised seeds would have been somewhat dwarfed and less fertile, in comparison with the seedlings from legitimately crossed seeds. This holds good in relation to the fertility of the flowers; but to my surprise there was no difference in growth between the offspring from a legitimate union between two distinct plants, and from an illegitimate union whether between the flowers on the same plant, or between distinct plants of the same form. But I have shown, in the paper before referred to, that in England this plant is in an abnormal condition, such as, judging from analogous cases, would tend to render a cross between two individuals of no benefit to the offspring. Our plants have been commonly raised from self-fertilised seeds; and the seedlings have generally been subjected to nearly uniform conditions in pots in greenhouses. Moreover, many of the plants are now varying and changing their character, so as to become in a greater or less degree equal-styled, and in consequence highly self-fertile. From the analogy of Primula veris there can hardly be a doubt that if a plant of Primula sinensis could have been procured direct from China, and if it had been crossed with one of our English varieties, the offspring would have shown wonderful superiority in height and fertility (though probably not in the beauty of their flowers) over our ordinary plants.
My first experiment consisted in fertilising many flowers on long-styled and short-styled plants with their own pollen, and other flowers on the same plants with pollen taken from distinct plants belonging to the same form; so that all the unions were illegitimate. There was no uniform and marked difference in the number of seeds obtained from these two modes of self-fertilisation, both of which were illegitimate. The two lots of seeds from both forms were sown thickly on opposite sides of four pots, and numerous plants thus raised. But there was no difference in their growth, excepting in one pot, in which the offspring from the illegitimate union of two long-styled plants exceeded in a decided manner in height the offspring of flowers on the same plants fertilised with their own pollen. But in all four pots the plants raised from the union of distinct plants belonging to the same form, flowered before the offspring from the self-fertilised flowers.
Some long-styled and short-styled plants were now raised from purchased seeds, and flowers on both forms were legitimately crossed with pollen from a distinct plant; and other flowers on both forms were illegitimately fertilised with pollen from the flowers on the same plant. The seeds were sown on opposite sides of Pots 1 to 4 in Table 6/95; a single plant being left on each side. Several flowers on the illegitimate long-styled and short-styled plants described in the last paragraph, were also legitimately and illegitimately fertilised in the manner just described, and their seeds were sown in Pots 5 to 8 in the same table. As the two sets of seedlings did not differ in any essential manner, their measurements are given in a single table. I should add that the legitimate unions in both cases yielded, as might have been expected, many more seeds than the illegitimate unions. The seedlings whilst half-grown presented no difference in height on the two sides of the several pots. When fully grown they were measured to the tips of their longest leaves, and the result is given in Table 6/95. TABLE 6/95. Primula sinensis. Heights of plants measured in inches. Column 1: Number (Name) of Pot. Column 2: Plants from legitimately Crossed seeds. Column 3: Plants from illegitimately Self-fertilised seeds. Pot 1 : 8 2/8 : 8. From short-styled mother.
Pot 2 : 7 4/8 : 8 5/8. From short-styled mother. Pot 3 : 9 5/8 : 9 3/8. From long-styled mother. Pot 4 : 8 4/8 : 8 2/8. From long-styled mother. Pot 5 : 9 3/8 : 9. From illegitimate short-styled mother. Pot 6 : 9 7/8 : 9 4/8. From illegitimate short-styled mother. Pot 7 : 8 4/8 : 9 4/8. From illegitimate long-styled mother. Pot 8 : 10 4/8 : 10. From illegitimate long-styled mother. Total : 72.13 : 72.25. In six out of the eight pots the legitimately crossed plants exceeded in height by a trifle the illegitimately self-fertilised plants; but the latter exceeded the former in two of the pots in a more strongly marked manner. The average height of the eight legitimately crossed plants is 9.01, and that of the eight illegitimately self-fertilised 9.03 inches, or as 100 to 100.2. The plants on the opposite sides produced, as far as could be judged by the eye, an equal number of flowers. I did not count the capsules or the seeds produced by them; but undoubtedly, judging from many previous observations, the plants derived from the legitimately crossed seeds would have been considerably more fertile than those from the illegitimately self-fertilised seeds. The crossed plants, as in the previous case, flowered before the self-fertilised plants in all the pots except in Pot 2, in which the two sides flowered simultaneously; and this early flowering may, perhaps, be considered as an advantage.
27. POLYGONEAE.--Fagopyrum esculentum. This plant was discovered by Hildebrand to be heterostyled, that is, to present, like the species of Primula, a long-styled and a short-styled form, which are adapted for reciprocal fertilisation. Therefore the following comparison of the growth of the crossed and self-fertilised seedlings is not fair, for we do not know whether the difference in their heights may not be wholly due to the illegitimate fertilisation of the self-fertilised flowers. I obtained seeds by legitimately crossing flowers on long-styled and short-styled plants, and by fertilising other flowers on both forms with pollen from the same plant. Rather more seeds were obtained by the former than by the latter process; and the legitimately crossed seeds were heavier than an equal number of the illegitimately self-fertilised seeds, in the ratio of 100 to 82. Crossed and self-fertilised seeds from the short-styled parents, after germinating on sand, were planted in pairs on the opposite sides of a large pot; and two similar lots of seeds from long-styled parents were planted in a like manner on the opposite sides of two other pots. In all three pots the legitimately crossed seedlings, when a few inches in height, were taller than the self-fertilised; and in all three pots they flowered before them by one or two days. When fully grown they were all cut down close to the ground, and as I was pressed for time, they were placed in a long row, the cut end of one plant touching the tip of another, and the total length of the legitimately crossed plants was 47 feet 7 inches, and of the illegitimately self-fertilised plants 32 feet 8 inches. Therefore the average height of the fifteen crossed plants in all three pots was 38.06 inches, and that of the fifteen self-fertilised plants 26.13 inches; or as 100 to 69.
28. CHENOPODIACEAE.--Beta vulgaris. A single plant, no others growing in the same garden, was left to fertilise itself, and the self-fertilised seeds were collected. Seeds were also collected from a plant growing in the midst of a large bed in another garden; and as the incoherent pollen is abundant, the seeds of this plant will almost certainly have been the product of a crossed between distinct plants by means of the wind. Some of the two lots of seeds were sown on the opposite sides of two very large pots; and the young seedlings were thinned, so that an equal but considerable number was left on the two sides. These plants were thus subjected to very severe competition, as well as to poor conditions. The remaining seeds were sown out of doors in good soil in two long and not closely adjoining rows, so that these seedlings were placed under favourable conditions, and were not subjected to any mutual competition. The self-fertilised seeds in the open ground came up very badly; and on removing the soil in two or three places, it was found that many had sprouted under ground and had then died. No such case had been observed before. Owing to the large number of seedlings which thus perished, the surviving self-fertilised plants grew thinly in the row, and thus had an advantage over the crossed plants, which grew very thickly in the other row. The young plants in the two rows were protected by a little straw during the winter, and those in the two large pots were placed in the greenhouse.
There was no difference between the two lots in the pots until the ensuing spring, when they had grown a little, and then some of the crossed plants were finer and taller than any of the self-fertilised. When in full flower their stems were measured, and the measurements are given in Table 6/96. TABLE 6/96. Beta vulgaris. Heights of flower stems measured in inches. Column 1: Number (Name) of Pot. Column 2: Crossed Plants. Column 3: Self-fertilised Plants. Pot 1 : 34 6/8 : 36. Pot 1 : 30 : 20 1/8. Pot 1 : 33 6/8 : 32 2/8. Pot 1 : 34 4/8 : 32. Pot 2 : 42 3/8 : 42 1/8. Pot 2 : 33 1/8 : 26 4/8. Pot 2 : 31 2/8 : 29 2/8. Pot 2 : 33 : 20 2/8. Total : 272.75 : 238.50. The average height of the eight crossed plants is here 34.09, and that of the eight self-fertilised plants 29.81; or as 100 to 87. With respect to the plants in the open ground, each long row was divided into half, so as to diminish the chance of any accidental advantage in one part of either row; and the four tallest plants in the two halves of the two rows were carefully selected and measured. The eight tallest crossed plants averaged 30.92, and the eight tallest self-fertilised 30.7 inches in height, or as 100 to 99; so that they were practically equal. But we should bear in mind that the trial was not quite fair, as the self-fertilised plants had a great advantage over the crossed in being much less crowded in their own row, owing to the large number of seeds which had perished under ground after sprouting. Nor were the lots in the two rows subjected to any mutual competition.
29. CANNACEAE.--Canna warscewiczi. In most or all the species belonging to this genus, the pollen is shed before the flower expands, and adheres in a mass to the foliaceous pistil close beneath the stigmatic surface. As the edge of this mass generally touches the edge of the stigma, and as it was ascertained by trials purposely made that a very few pollen-grains suffice for fertilisation, the present species and probably all the others of the genus are highly self-fertile. Exceptions occasionally occur in which, from the stamen being slightly shorter than usual, the pollen is deposited a little beneath the stigmatic surface, and such flowers drop off unimpregnated unless they are artificially fertilised. Sometimes, though rarely, the stamen is a little longer than usual, and then the whole stigmatic surface gets thickly covered with pollen. As some pollen is generally deposited in contact with the edge of the stigma, certain authors have concluded that the flowers are invariably self-fertilised. This is an extraordinary conclusion, for it implies that a great amount of pollen is produced for no purpose. On this view, also, the large size of the stigmatic surface is an unintelligible feature in the structure of the flower, as well as the relative position of all the parts, which is such that when insects visit the flowers to suck the copious nectar, they cannot fail to carry pollen from one flower to another. (6/7. Delpino has described ‘Bot. Zeitung’ 1867 page 277 and ‘Scientific Opinion’ 1870 page 135, the structure of the flowers in this genus, but he was mistaken in thinking that self-fertilisation is impossible, at least in the case of the present species. Dr. Dickie and Professor Faivre state that the flowers are fertilised in the bud, and that self-fertilisation is inevitable. I presume that they were misled by the pollen being deposited at a very early period on the pistil: see ‘Journal of Linnean Society Botany’ volume 10 page 55 and ‘Variabilité des Espèces’ 1868 page 158.)
According to Delpino, bees eagerly visit the flowers in North Italy, but I have never seen any insect visiting the flowers of the present species in my hothouse, although many plants grew there during several years. Nevertheless these plants produced plenty of seed, as they likewise did when covered by a net; they are therefore fully capable of self-fertilisation, and have probably been self-fertilised in this country for many generations. As they are cultivated in pots, and are not exposed to competition with surrounding plants, they have also been subjected for a considerable time to somewhat uniform conditions. This, therefore, is a case exactly parallel with that of the common pea, in which we have no right to expect much or any good from intercrossing plants thus descended and thus treated; and no good did follow, excepting that the cross-fertilised flowers yielded rather more seeds than the self-fertilised. This species was one of the earlier ones on which I experimented, and as I had not then raised any self-fertilised plants for several successive generations under uniform conditions, I did not know or even suspect that such treatment would interfere with the advantages to be gained from a cross. I was therefore much surprised at the crossed plants not growing more vigorously than the self-fertilised, and a large number of plants were raised, notwithstanding that the present species is an extremely troublesome one to experiment on. The seeds, even those which have been long soaked in water, will not germinate well on bare sand; and those that were sown in pots (which plan I was forced to follow) germinated at very unequal intervals of time; so that it was difficult to get pairs of the same exact age, and many seedlings had to be pulled up and thrown away. My experiments were continued during three successive generations; and in each generation the self-fertilised plants were again self-fertilised, their early progenitors in this country having probably been self-fertilised for many previous generations. In each generation, also, the crossed plants were fertilised with pollen from another crossed plant.
Of the flowers which were crossed in the three generations, taken together, a rather larger proportion yielded capsules than did those which were self-fertilised. The seeds were counted in forty-seven capsules from the crossed flowers, and they contained on an average 9.95 seeds; whereas forty-eight capsules from the self-fertilised flowers contained on an average 8.45 seeds; or as 100 to 85. The seeds from the crossed flowers were not heavier, on the contrary a little lighter, than those from the self-fertilised flowers, as was thrice ascertained. On one occasion I weighed 200 of the crossed and 106 of the self-fertilised seeds, and the relative weight of an equal number was as 100 for the crossed to 101.5 for the self-fertilised. With other plants, when the seeds from the self-fertilised flowers were heavier than those from the crossed flowers, this appeared to be due generally to fewer having been produced by the self-fertilised flowers, and to their having been in consequence better nourished. But in the present instance the seeds from the crossed capsules were separated into two lots,--namely, those from the capsules containing over fourteen seeds, and those from the capsules containing under fourteen seeds, and the seeds from the more productive capsules were the heavier of the two; so that the above explanation here fails.
As pollen is deposited at a very early age on the pistil, generally in contact with the stigma, some flowers whilst still in bud were castrated for my first experiment, and were afterwards fertilised with pollen from a distinct plant. Other flowers were fertilised with their own pollen. From the seeds thus obtained, I succeeded in rearing only three pairs of plants of equal age. The three crossed plants averaged 32.79 inches, and the three self-fertilised 32.08 inches in height; so that they were nearly equal, the crossed having a slight advantage. As the same result followed in all three generations, it would be superfluous to give the heights of all the plants, and I will give only the averages. In order to raise crossed and self-fertilised plants of the second generation, some flowers on the above crossed plants were crossed within twenty-four hours after they had expanded with pollen from a distinct plant; and this interval would probably not be too great to allow of cross-fertilisation being effectual. Some flowers on the self-fertilised plants of the last generation were also self-fertilised. From these two lots of seeds, ten crossed and twelve self-fertilised plants of equal ages were raised; and these were measured when fully grown. The crossed averaged 36.98, and the self-fertilised averaged 37.42 inches in height; so that here again the two lots were nearly equal; but the self-fertilised had a slight advantage.
In order to raise plants of the third generation, a better plan was followed, and flowers on the crossed plants of the second generation were selected in which the stamens were too short to reach the stigmas, so that they could not possibly have been self-fertilised. These flowers were crossed with pollen from a distinct plant. Flowers on the self-fertilised plants of the second generation were again self-fertilised. From the two lots of seeds thus obtained, twenty-one crossed and nineteen self-fertilised plants of equal age, and forming the third generation, were raised in fourteen large pots. They were measured when fully grown, and by an odd chance the average height of the two lots was exactly the same, namely, 35.96 inches; so that neither side had the least advantage over the other. To test this result, all the plants on both sides in ten out of the above fourteen pots were cut down after they had flowered, and in the ensuing year the stems were again measured; and now the crossed plants exceeded by a little (namely, 1.7 inches) the self-fertilised. They were again cut down, and on their flowering for the third time, the self-fertilised plants had a slight advantage (namely, 1.54 inches) over the crossed. Hence the result arrived at with these plants during the previous trials was confirmed, namely, that neither lot had any decided advantage over the other. It may, however, be worth mentioning that the self-fertilised plants showed some tendency to flower before the crossed plants: this occurred with all three pairs of the first generation; and with the cut down plants of the third generation, a self-fertilised plant flowered first in nine out of the twelve pots, whilst in the remaining three pots a crossed plant flowered first.
Text from Project Gutenberg, public domain in the United States.