The Effects of Cross & Self-Fertilisation in the Vegetable Kingdom · Charles Darwin
PLANTS RAISED FROM BRAZILIAN SEED.
Fritz Muller sent me from South Brazil seeds of plants which were there absolutely sterile when fertilised with pollen from the same plant, but were perfectly fertile when fertilised with pollen from any other plant. The plants raised by me in England from these seeds were examined by Professor Asa Gray, and pronounced to belong to E. Californica, with which they were identical in general appearance. Two of these plants were covered by a net, and were found not to be so completely self-sterile as in Brazil. But I shall recur to this subject in another part of this work. Here it will suffice to state that eight flowers on these two plants, fertilised with pollen from another plant under the net, produced eight fine capsules, each containing on an average about eighty seeds. Eight flowers on these same plants, fertilised with their own pollen, produced seven capsules, which contained on an average only twelve seeds, with a maximum in one of sixteen seeds. Therefore the cross-fertilised capsules, compared with the self-fertilised, yielded seeds in the ratio of about 100 to 15. These plants of Brazilian parentage differed also in a marked manner from the English plants in producing extremely few spontaneously self-fertilised capsules under a net.
Crossed and self-fertilised seeds from the above plants, after germinating on bare sand, were planted in pairs on the opposite sides of five large pots. The seedlings thus raised were the grandchildren of the plants which grew in Brazil; the parents having been grown in England. As the grandparents in Brazil absolutely require cross-fertilisation in order to yield any seeds, I expected that self-fertilisation would have proved very injurious to these seedlings, and that the crossed ones would have been greatly superior in height and vigour to those raised from self-fertilised flowers. But the result showed that my anticipation was erroneous; for as in the last experiment with plants of the English stock, so in the present one, the self-fertilised plants exceeded the crossed by a little in height. It will be sufficient to state that the fourteen crossed plants averaged 44.64, and the fourteen self-fertilised 45.12 inches in height; or as 100 to 101. THE EFFECTS OF A CROSS WITH A FRESH STOCK. I now tried a different experiment. Eight flowers on the self-fertilised plants of the last experiment (i.e., grandchildren of the plants which grew in Brazil) were again fertilised with pollen from the same plant, and produced five capsules, containing on an average 27.4 seeds, with a maximum in one of forty-two seeds. The seedlings raised from these seeds formed the second SELF-FERTILISED generation of the Brazilian stock.
Eight flowers on one of the crossed plants of the last experiment were crossed with pollen from another grandchild, and produced five capsules. These contained on an average 31.6 seeds, with a maximum in one of forty-nine seeds. The seedlings raised from these seeds may be called the INTERCROSSED. Lastly, eight other flowers on the crossed plants of the last experiment were fertilised with pollen from a plant of the English stock, growing in my garden, and which must have been exposed during many previous generations to very different conditions from those to which the Brazilian progenitors of the mother-plant had been subjected. These eight flowers produced only four capsules, containing on an average 63.2 seeds, with a maximum in one of ninety. The plants raised from these seeds may be called the ENGLISH-CROSSED. As far as the above averages can be trusted from so few capsules, the English-crossed capsules contained twice as many seeds as the intercrossed, and rather more than twice as many as the self-fertilised capsules. The plants which yielded these capsules were grown in pots in the greenhouse, so that their absolute productiveness must not be compared with that of plants growing out of doors.
The English-crossed to the self-fertilised plants, as 100 to 118. The English-crossed to the intercrossed plants, as 100 to 100. The intercrossed to the self-fertilised plants, as 100 to 118. We thus see that in weight, as in height, the self-fertilised plants had a decided advantage over the English-crossed and intercrossed plants. The remaining seeds of the three kinds, whether or not in a state of germination, were sown in three long parallel rows in the open ground; and here again the self-fertilised seedlings exceeded in height by between 2 and 3 inches the seedlings in the two other rows, which were of nearly equal heights. The three rows were left unprotected throughout the winter, and all the plants were killed, with the exception of two of the self-fertilised; so that as far as this little bit of evidence goes, some of the self-fertilised plants were more hardy than any of the crossed plants of either lot. We thus see that the self-fertilised plants which were grown in the nine pots were superior in height (as 116 to 100), and in weight (as 118 to 100), and apparently in hardiness, to the intercrossed plants derived from a cross between the grandchildren of the Brazilian stock. The superiority is here much more strongly marked than in the second trial with the plants of the English stock, in which the self-fertilised were to the crossed in height as 101 to 100. It is a far more remarkable fact--if we bear in mind the effects of crossing plants with pollen from a fresh stock in the cases of Ipomoea, Mimulus, Brassica, and Iberis--that the self-fertilised plants exceeded in height (as 109 to 100), and in weight (as 118 to 100), the offspring of the Brazilian stock crossed by the English stock; the two stocks having been long subjected to widely different conditions.
The forty-five capsules (the four empty ones included) from the English-crossed plants contained 2747 seeds. The forty-five capsules (the five empty ones included) from the intercrossed plants contained 2240 seeds. The forty-five capsules (the nine empty ones included) from the self-fertilised plants contained 1746.7 seeds. The reader should remember that these capsules are the product of cross-fertilisation, effected by the bees; and that the difference in the number of the contained seeds must depend on the constitution of the plants;--that is, on whether they were derived from a cross with a distinct stock, or from a cross between plants of the same stock, or from self-fertilisation. From the above facts we obtain the following ratios:-- Number of seeds contained in an equal number of naturally fertilised capsules produced:-- By the English-crossed and self-fertilised plants, as 100 to 63. By the English-crossed and intercrossed plants, as 100 to 81. By the intercrossed and self-fertilised plants, as 100 to 78.
But to have ascertained the productiveness of the three lots of plants, it would have been necessary to know how many capsules were produced by the same number of plants. The three long rows, however, were not of quite equal lengths, and the plants were much crowded, so that it would have been extremely difficult to have ascertained how many capsules were produced by them, even if I had been willing to undertake so laborious a task as to collect and count all the capsules. But this was feasible with the plants grown in pots in the greenhouse; and although these were much less fertile than those growing out of doors, their relative fertility appeared, after carefully observing them, to be the same. The nineteen plants of the English-crossed stock in the pots produced altogether 240 capsules; the intercrossed plants (calculated as nineteen) produced 137.22 capsules; and the nineteen self-fertilised plants, 152 capsules. Now, knowing the number of seeds contained in forty-five capsules of each lot, it is easy to calculate the relative numbers of seeds produced by an equal number of the plants of the three lots. Number of seeds produced by an equal number of naturally-fertilised plants:-- Plants of English-crossed and self-fertilised parentage, as 100 to 40 seeds. Plants of English-crossed and intercrossed parentage, as 100 to 45 seeds.
Plants of intercrossed and self-fertilised parentage, as 100 to 89 seeds. The superiority in productiveness of the intercrossed plants (that is, the product of a cross between the grandchildren of the plants which grew in Brazil) over the self-fertilised, small as it is, is wholly due to the larger average number of seeds contained in the capsules; for the intercrossed plants produced fewer capsules in the greenhouse than did the self-fertilised plants. The great superiority in productiveness of the English-crossed over the self-fertilised plants is shown by the larger number of capsules produced, the larger average number of contained seeds, and the smaller number of empty capsules. As the English-crossed and intercrossed plants were the offspring of crosses in every previous generation (as must have been the case from the flowers being sterile with their own pollen), we may conclude that the great superiority in productiveness of the English-crossed over the intercrossed plants is due to the two parents of the former having been long subjected to different conditions. The English-crossed plants, though so superior in productiveness, were, as we have seen, decidedly inferior in height and weight to the self-fertilised, and only equal to, or hardly superior to, the intercrossed plants. Therefore, the whole advantage of a cross with a distinct stock is here confined to productiveness, and I have met with no similar case.
8. RESEDACEAE.--Reseda lutea. Seeds collected from wild plants growing in this neighbourhood were sown in the kitchen-garden; and several of the seedlings thus raised were covered with a net. Of these, some were found (as will hereafter be more fully described) to be absolutely sterile when left to fertilise themselves spontaneously, although plenty of pollen fell on their stigmas; and they were equally sterile when artificially and repeatedly fertilised with their own pollen; whilst other plants produced a few spontaneously self-fertilised capsules. The remaining plants were left uncovered, and as pollen was carried from plant to plant by the hive and humble-bees which incessantly visit the flowers, they produced an abundance of capsules. Of the necessity of pollen being carried from one plant to another, I had ample evidence in the case of this species and of R. odorata; for those plants, which set no seeds or very few as long as they were protected from insects, became loaded with capsules immediately that they were uncovered. Seeds from the flowers spontaneously self-fertilised under the net, and from flowers naturally crossed by the bees, were sown on opposite sides of five large pots. The seedlings were thinned as soon as they appeared above ground, so that an equal number were left on the two sides. After a time the pots were plunged into the open ground. The same number of plants of crossed and self-fertilised parentage were measured up to the summits of their flower-stems, with the result given in Table 4/35. Those which did not produce flower-stems were not measured.
TABLE 4/35. Reseda lutea, in pots. Heights of plants to the summits of the flower-stems measured in inches. Column 1: Number (Name) of Pot. Column 2: Crossed Plants. Column 3: Self-fertilised Plants. Pot 1 : 21 : 12 7/8. Pot 1 : 14 2/8 : 16. Pot 1 : 19 1/8 : 11 7/8. Pot 1 : 7 : 15 2/8. Pot 1 : 15 1/8 : 19 1/8. Pot 2 : 20 4/8 : 12 4/8. Pot 2 : 17 3/8 : 16 2/8. Pot 2 : 23 7/8 : 16 2/8. Pot 2 : 17 1/8 : 13 3/8. Pot 2 : 20 6/8 : 13 5/8. Pot 3 : 16 1/8 : 14 4/8. Pot 3 : 17 6/8 : 19 4/8. Pot 3 : 16 2/8 : 20 7/8. Pot 3 : 10 : 7 7/8. Pot 3 : 10 : 17 6/8. Pot 4 : 22 1/8 : 9. Pot 4 : 19 : 11 4/8. Pot 4 : 18 7/8 : 11. Pot 4 : 16 4/8 : 16. Pot 4 : 19 2/8 : 16 3/8. Pot 5 : 25 2/8 : 14 6/8. Pot 5 : 22 : 16. Pot 5 : 8 6/8 : 14 3/8. Pot 5 : 14 2/8 : 14 2/8. Total : 412.25 : 350.86. The average height of the twenty-four crossed plants is here 17.17 inches, and that of the same number of self-fertilised plants 14.61; or as 100 to 85. Of the crossed plants all but five flowered, whilst several of the self-fertilised did not do so. The above pairs, whilst still in flower, but with some capsules already formed, were afterwards cut down and weighed. The crossed weighed 90.5 ounces; and an equal number of the self-fertilised only 19 ounces, or as 100 to 21; and this is an astonishing difference.
TABLE 4/37. Reseda odorata (seedlings from a highly self-fertile plant). Heights of plants to the summits of the leafy stems, flower-stems not included, measured in inches. Column 1: Number (Name) of Pot. Column 2: Crossed Plants. Column 3: Self-fertilised Plants. Pot 1 : 20 7/8 : 22 4/8. Pot 1 : 34 7/8 : 28 5/8. Pot 1 : 26 6/8 : 23 2/8. Pot 1 : 32 6/8 : 30 4/8. Pot 2 : 34 3/8 : 28 5/8. Pot 2 : 34 5/8 : 30 5/8. Pot 2 : 11 6/8 : 23. Pot 2 : 33 3/8 : 30 1/8. Pot 3 : 17 7/8 : 4 4/8. Pot 3 : 27 : 25. Pot 3 : 30 1/8 : 26 3/8. Pot 3 : 30 2/8 : 25 1/8. Pot 4 : 21 5/8 : 22 6/8. Pot 4 : 28 : 25 4/8. Pot 4 : 32 5/8 : 15 1/8. Pot 4 : 32 3/8 : 24 6/8. Pot 5 : 21 : 11 6/8. Pot 5 : 25 2/8 : 19 7/8. Pot 5 : 26 6/8 : 10 4/8. Total : 522.25 : 428.50. The average height of the nineteen crossed plants is here 27.48, and that of the nineteen self-fertilised 22.55 inches; or as 100 to 82. All these plants were cut down in the early autumn and weighed: the crossed weighed 11.5 ounces, and the self-fertilised 7.75 ounces, or as 100 to 67. These two lots having been left freely exposed to the visits of insects, did not present any difference to the eye in the number of seed-capsules which they produced.
The remainder of the same two lots of seeds were sown in two adjoining rows in the open ground; so that the plants were exposed to only moderate competition. The eight tallest on each side were measured, as shown in Table 4/38. TABLE 4/38. Reseda odorata, growing in the open ground. Heights of plants measured in inches. Column 1: Crossed Plants. Column 2: Self-fertilised Plants. 24 4/8 : 26 5/8. 27 2/8 : 25 7/8. 24 : 25. 26 6/8 : 28 3/8. 25 : 29 7/8. 26 2/8 : 25 7/8. 27 2/8 : 26 7/8. 25 1/8 : 28 2/8. Total : 206.13 : 216.75 The average height of the eight crossed plants is 25.76, and that of the eight self-fertilised 27.09; or as 100 to 105. We here have the anomalous result of the self-fertilised plants being a little taller than the crossed; of which fact I can offer no explanation. It is of course possible, but not probable, that the labels may have been interchanged by accident.
Another experiment was now tried: all the self-fertilised capsules, though very few in number, were gathered from one of the semi-self-sterile plants under a net; and as several flowers on this same plant had been fertilised with pollen from a distinct individual, crossed seeds were thus obtained. I expected that the seedlings from this semi-self-sterile plant would have profited in a higher degree from a cross, than did the seedlings from the fully self-fertile plants. But my anticipation was quite wrong, for they profited in a less degree. An analogous result followed in the case of Eschscholtzia, in which the offspring of the plants of Brazilian parentage (which were partially self-sterile) did not profit more from a cross, than did the plants of the far more self-fertile English stock. The above two lots of crossed and self-fertilised seeds from the same plant of Reseda odorata, after germinating on sand, were planted on opposite sides of five pots, and measured as in the last case, with the result in Table 4/39. TABLE 4/39. Reseda odorata (seedlings from a semi-self-sterile plant). Heights of plants to the summits of the leafy stems, flower-stems not included, measured in inches. Column 1: Number (Name) of Pot. Column 2: Crossed Plants. Column 3: Self-fertilised Plants. Pot 1 : 33 4/8 : 31. Pot 1 : 30 6/8 : 28. Pot 1 : 29 6/8 : 13 2/8. Pot 1 : 20 : 32.
Pot 2 : 22 : 21 6/8. Pot 2 : 33 4/8 : 26 6/8. Pot 2 : 31 2/8 : 25 2/8. Pot 2 : 32 4/8 : 30 4/8. Pot 3 : 30 1/8 : 17 2/8. Pot 3 : 32 1/8 : 29 6/8. Pot 3 : 31 4/8 : 24 6/8. Pot 3 : 32 2/8 : 34 2/8. Pot 4 : 19 1/8 : 20 6/8. Pot 4 : 30 1/8 : 32 6/8. Pot 4 : 24 3/8 : 31 4/8. Pot 4 : 30 6/8 : 36 6/8. Pot 5 : 34 6/8 : 24 5/8. Pot 5 : 37 1/8 : 34. Pot 5 : 31 2/8 : 22 2/8. Pot 5 : 33 : 37 1/8. Total : 599.75 : 554.25. The average height of the twenty crossed plants is here 29.98, and that of the twenty self-fertilised 27.71 inches; or as 100 to 92. These plants were then cut down and weighed; and the crossed in this case exceeded the self-fertilised in weight by a mere trifle, namely, in the ratio of 100 to 99. The two lots, left freely exposed to insects, seemed to be equally fertile. The remainder of the seed was sown in two adjoining rows in the open ground; and the eight tallest plants in each row were measured, with the result in Table 4/40.
TABLE 4/40. Reseda odorata, (seedlings from a semi-self-sterile plant, planted in the open ground). Heights of plants measured in inches. Column 1: Crossed Plants. Column 2: Self-fertilised Plants. 28 2/8 : 22 3/8. 22 4/8 : 24 3/8. 25 7/8 : 23 4/8. 25 3/8 : 21 4/8. 29 4/8 : 22 5/8. 27 1/8 : 27 3/8. 22 4/8 : 27 3/8. 26 2/8 : 19 2/8. Total : 207.38 : 188.38. The average height of the eight crossed plants is here 25.92, and that of the eight self-fertilised plants 23.54 inches; or as 100 to 90. 9. VIOLACEAE.--Viola tricolor. Whilst the flowers of the common cultivated heartsease are young, the anthers shed their pollen into a little semi-cylindrical passage, formed by the basal portion of the lower petal, and surrounded by papillae. The pollen thus collected lies close beneath the stigma, but can seldom gain access into its cavity, except by the aid of insects, which pass their proboscides down this passage into the nectary. (4/5. The flowers of this plant have been fully described by Sprengel, Hildebrand, Delpino, and H. Muller. The latter author sums up all the previous observations in his ‘Befruchtung der Blumen’ and in ‘Nature’ November 20, 1873 page 44. See also Mr. A.W. Bennett in ‘Nature’ May 15, 1873 page 50 and some remarks by Mr. Kitchener ibid page 143. The facts which follow on the effects of covering up a plant of V. tricolor have been quoted by Sir J. Lubbock in his ‘British Wild Flowers’ etc. page 62.) Consequently when I covered up a large plant of a cultivated variety, it set only eighteen capsules, and most of these contained very few good seeds--several from only one to three; whereas an equally fine uncovered plant of the same variety, growing close by, produced 105 fine capsules.
The few flowers which produce capsules when insects are excluded, are perhaps fertilised by the curling inwards of the petals as their wither, for by this means pollen-grains adhering to the papillae might be inserted into the cavity of the stigma. But it is more probable that their fertilisation is effected, as Mr. Bennett suggests, by Thrips and certain minute beetles which haunt the flowers, and which cannot be excluded by any net. Humble-bees are the usual fertilisers; but I have more than once seen flies (Rhingia rostrata) at work, with the under sides of their bodies, heads and legs dusted with pollen; and having marked the flowers which they visited, I found them after a few days fertilised. (4/6. I should add that this fly apparently did not suck the nectar, but was attracted by the papillae which surround the stigma. Hermann Muller also saw a small bee, an Andrena, which could not reach the nectar, repeatedly inserting its proboscis beneath the stigma, where the papillae are situated; so that these papillae must be in some way attractive to insects. A writer asserts ‘Zoologist’ volume 3-4 page 1225, that a moth (Plusia) frequently visits the flowers of the pansy.
Hive-bees do not ordinarily visit them, but a case has been recorded ‘Gardeners’ Chronicle’ 1844 page 374, of these bees doing so. Hermann Muller has also seen the hive-bee at work, but only on the wild small-flowered form. He gives a list ‘Nature’ 1873 page 45, of all the insects which he has seen visiting both the large and small-flowered forms. From his account, I suspect that the flowers of plants in a state of nature are visited more frequently by insects than those of the cultivated varieties. He has seen several butterflies sucking the flowers of wild plants, and this I have never observed in gardens, though I have watched the flowers during many years.) It is curious for how long a time the flowers of the heartsease and of some other plants may be watched without an insect being seen to visit them. During the summer of 1841, I observed many times daily for more than a fortnight some large clumps of heartsease growing in my garden, before I saw a single humble-bee at work. During another summer I did the same, but at last saw some dark-coloured humble-bees visiting on three successive days almost every flower in several clumps; and almost all these flowers quickly withered and produced fine capsules. I presume that a certain state of the atmosphere is necessary for the secretion of nectar, and that as soon as this occurs the insects discover the fact by the odour emitted, and immediately frequent the flowers.
TABLE 4/41. Viola tricolor. Heights of plants measured in inches. Column 1: Number (Name) of Pot. Column 2: Crossed Plants. Column 3: Self-fertilised Plants. Pot 1 : 8 2/8 : 0 2/8. Pot 1 : 7 4/8 : 2 4/8. Pot 1 : 5 : 1 2/8. Pot 2 : 5 : 6. Pot 2 : 4 : 4. Pot 2 : 4 4/8 : 3 1/8. Pot 3 : 9 4/8 : 3 1/8. Pot 3 : 3 3/8 : 1 7/8. Pot 3 : 8 4/8 : 0 5/8. Pot 4 : 4 7/8 : 2 1/8. Pot 4 : 4 2/8 : 1 6/8. Pot 4 : 4 : 2 1/8. Pot 5 : 6 : 3. Pot 5 : 3 3/8 : 1 4/8. Total : 78.13 : 33.25. The average height of the fourteen crossed plants is here 5.58 inches, and that of the fourteen self-fertilised 2.37; or as 100 to 42. In four out of the five pots, a crossed plant flowered before any one of the self-fertilised; as likewise occurred with the pair raised during the previous year. These plants without being disturbed were now turned out of their pots and planted in the open ground, so as to form five separate clumps. Early in the following summer (1869) they flowered profusely, and being visited by humble-bees set many capsules, which were carefully collected from all the plants on both sides. The crossed plants produced 167 capsules, and the self-fertilised only 17; or as 100 to 10. So that the crossed plants were more than twice the height of the self-fertilised, generally flowered first, and produced ten times as many naturally fertilised capsules.
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