The Effects of Cross & Self-Fertilisation in the Vegetable Kingdom · Charles Darwin
PLANTS RAISED FROM BRAZILIAN SEED.
By the early part of the summer of 1870 the crossed plants in all the five clumps had grown and spread so much more than the self-fertilised, that any comparison between them was superfluous. The crossed plants were covered with a sheet of bloom, whilst only a single self-fertilised plant, which was much finer than any of its brethren, flowered. The crossed and self-fertilised plants had now grown all matted together on the respective sides of the superficial partitions still separating them; and in the clump which included the finest self-fertilised plant, I estimated that the surface covered by the crossed plants was about nine times as large as that covered by the self-fertilised plants. The extraordinary superiority of the crossed over the self-fertilised plants in all five clumps, was no doubt due to the crossed plants at first having had a decided advantage over the self-fertilised, and then robbing them more and more of their food during the succeeding seasons. But we should remember that the same result would follow in a state of nature even to a greater degree; for my plants grew in ground kept clear of weeds, so that the self-fertilised had to compete only with the crossed plants; whereas the whole surface of the ground is naturally covered with various kinds of plants, all of which have to struggle together for existence.
The ensuing winter was very severe, and in the following spring (1871) the plants were again examined. All the self-fertilised were now dead, with the exception of a single branch on one plant, which bore on its summit a minute rosette of leaves about as large as a pea. On the other hand, all the crossed plants without exception were growing vigorously. So that the self-fertilised plants, besides their inferiority in other respects, were more tender. Another experiment was now tried for the sake of ascertaining how far the superiority of the crossed plants, or to speak more correctly, the inferiority of the self-fertilised plants, would be transmitted to their offspring. The one crossed and one self-fertilised plant, which were first raised, had been turned out of their pot and planted in the open ground. Both produced an abundance of very fine capsules, from which fact we may safely conclude that they had been cross-fertilised by insects. Seeds from both, after germinating on sand, were planted in pairs on the opposite sides of three pots. The naturally crossed seedlings derived from the crossed plants flowered in all three pots before the naturally crossed seedlings derived from the self-fertilised plants. When both lots were in full flower, the two tallest plants on each side of each pot were measured, and the result is shown in Table 4/42.
TABLE 4/42. Viola tricolor: seedlings from crossed and self-fertilised plants, the parents of both sets having been left to be naturally fertilised. Heights of plants measured in inches. Column 1: Number (Name) of Pot. Column 2: Naturally Crossed Plants from artificially crossed Plants. Column 3: Naturally Crossed Plants from Self-fertilised Plants. Pot 1 : 12 1/8 : 9 6/8. Pot 1 : 11 6/8 : 8 3/8. Pot 2 : 13 2/8 : 9 6/8. Pot 2 : 10 : 11 4/8. Pot 3 : 14 4/8 : 11 1/8. Pot 3 : 13 6/8 : 11 3/8. Total : 75.38 : 61.88. The average height of the six tallest plants derived from the crossed plants is 12.56 inches; and that of the six tallest plants derived from the self-fertilised plants is 10.31 inches; or as 100 to 82. We here see a considerable difference in height between the two sets, though very far from equalling that in the previous trials between the offspring from crossed and self-fertilised flowers. This difference must be attributed to the latter set of plants having inherited a weak constitution from their parents, the offspring of self-fertilised flowers; notwithstanding that the parents themselves had been freely intercrossed with other plants by the aid of insects. 10. RANUNCULACEAE.--Adonis aestivalis.
The results of my experiments on this plant are hardly worth giving, as I remark in my notes made at the time, “seedlings, from some unknown cause, all miserably unhealthy.” Nor did they ever become healthy; yet I feel bound to give the present case, as it is opposed to the general results at which I have arrived. Fifteen flowers were crossed and all produced fruit, containing on an average 32.5 seeds; nineteen flowers were fertilised with their own pollen, and they likewise all yielded fruit, containing a rather larger average of 34.5 seeds; or as 100 to 106. Seedlings were raised from these seeds. In one of the pots all the self-fertilised plants died whilst quite young; in the two others, the measurements were as follows: TABLE 4/43. Adonis aestivalis. Heights of plants measured in inches. Column 1: Number (Name) of Pot. Column 2: Crossed Plants. Column 3: Self-fertilised Plants. Pot 1 : 14 : 13 4/8. Pot 1 : 13 4/8 : 13 4/8. Pot 2 : 16 2/8 : 15 2/8. Pot 2 : 13 2/8 : 15. Total : 57.00 : 57.25. The average height of the four crossed plants is 14.25, and that of the four self-fertilised plants 14.31; or as 100 to 100.4; so that they were in fact of equal height. According to Professor H. Hoffman, this plant is proterandrous (4/7. ‘Zur Speciesfrage’ 1875 page 11.); nevertheless it yields plenty of seeds when protected from insects.
Delphinium consolida. It has been said in the case of this plant, as of so many others, that the flowers are fertilised in the bud, and that distinct plants or varieties can never naturally intercross. (4/8. Decaisne ‘Comptes-Rendus’ July 1863 page 5.) But this is an error, as we may infer, firstly from the flowers being proterandrous,--the mature stamens bending up, one after the other, into the passage which leads to the nectary, and afterwards the mature pistils bending in the same direction; secondly, from the number of humble-bees which visit the flowers (4/9. Their structure is described by H. Muller ‘Befruchtung’ etc., page 122.); and thirdly, from the greater fertility of the flowers when crossed with pollen from a distinct plant than when spontaneously self-fertilised. In the year 1863 I enclosed a large branch in a net, and crossed five flowers with pollen from a distinct plant; these yielded capsules containing on an average 35.2 very fine seeds, with a maximum of forty-two in one capsule. Thirty-two other flowers on the same branch produced twenty-eight spontaneously self-fertilised capsules, containing on an average 17.2 seeds, with a maximum in one of thirty-six seeds. But six of these capsules were very poor, yielding only from one to five seeds; if these are excluded, the remaining twenty-two capsules give an average of 20.9 seeds, though many of these seeds were small. The fairest ratio, therefore, for the number of seeds produced by a cross and by spontaneous self-fertilisation is as 100 to 59. These seeds were not sown, as I had too many other experiments in progress.
In the summer of 1867, which was a very unfavourable one, I again crossed several flowers under a net with pollen from a distinct plant, and fertilised other flowers on the same plant with their own pollen. The former yielded a much larger proportion of capsules than the latter; and many of the seeds in the self-fertilised capsules, though numerous, were so poor that an equal number of seeds from the crossed and self-fertilised capsules were in weight as 100 to 45. The two lots were allowed to germinate on sand, and pairs were planted on the opposite sides of four pots. When nearly two-thirds grown they were measured, as shown in Table 4/44. TABLE 4/44. Delphinium consolida. Heights of plants measured in inches. Column 1: Number (Name) of Pot. Column 2: Crossed Plants. Column 3: Self-fertilised Plants. Pot 1 : 11 : 11. Pot 2 : 19 : 16 2/8. Pot 2 : 16 2/8 : 11 4/8. Pot 3 : 26 : 22. Pot 4 : 9 4/8 : 8 2/8. Pot 4 : 8 : 6 4/8. Total : 89.75 : 75.50.
The six crossed plants here average 14.95, and the six self-fertilised 12.50 inches in height; or as 100 to 84. When fully grown they were again measured, but from want of time only a single plant on each side was measured; so that I have thought it best to give the earlier measurements. At the later period the three tallest crossed plants still exceeded considerably in height the three tallest self-fertilised, but not in quite so great a degree as before. The pots were left uncovered in the greenhouse, but whether the flowers were intercrossed by bees or self-fertilised I do not know. The six crossed plants produced 282 mature and immature capsules, whilst the six self-fertilised plants produced only 159; or as 100 to 56. So that the crossed plants were very much more productive than the self-fertilised. 11. CARYOPHYLLACEAE.--Viscaria oculata.
Twelve flowers were crossed with pollen from another plant, and yielded ten capsules, containing by weight 5.77 grains of seeds. Eighteen flowers were fertilised with their own pollen and yielded twelve capsules, containing by weight 2.63 grains. Therefore the seeds from an equal number of crossed and self-fertilised flowers would have been in weight as 100 to 38. I had previously selected a medium-sized capsule from each lot, and counted the seeds in both; the crossed one contained 284, and the self-fertilised one 126 seeds; or as 100 to 44. These seeds were sown on opposite sides of three pots, and several seedlings raised; but only the tallest flower-stem of one plant on each side was measured. The three on the crossed side averaged 32.5 inches, and the three on the self-fertilised side 34 inches in height; or as 100 to 104. But this trial was on much too small a scale to be trusted; the plants also grew so unequally that one of the three flower-stems on the crossed plants was very nearly twice as tall as that on one of the others; and one of the three flower-stems on the self-fertilised plants exceeded in an equal degree one of the others. In the following year the experiment was repeated on a larger scale: ten flowers were crossed on a new set of plants and yielded ten capsules containing by weight 6.54 grains of seed. Eighteen spontaneously self-fertilised capsules were gathered, of which two contained no seed; the other sixteen contained by weight 6.07 grains of seed. Therefore the weight of seed from an equal number of crossed and spontaneously self-fertilised flowers (instead of artificially fertilised as in the previous case) was as 100 to 58.
The seeds after germinating on sand were planted in pairs on the opposite sides of four pots, with all the remaining seeds sown crowded in the opposite sides of a fifth pot; in this latter pot only the tallest plant on each side was measured. Until the seedlings had grown about 5 inches in height no difference could be perceived in the two lots. Both lots flowered at nearly the same time. When they had almost done flowering, the tallest flower-stem on each plant was measured, as shown in Table 4/45. TABLE 4/45. Viscaria oculata. Tallest flower-stem on each plant measured in inches. Column 1: Number (Name) of Pot. Column 2: Crossed Plants. Column 3: Self-fertilised Plants. Pot 1 : 19 : 32 3/8. Pot 1 : 33 : 38. Pot 1 : 41 : 38. Pot 1 : 41 : 28 7/8. Pot 2 : 37 4/8 : 36. Pot 2 : 36 4/8 : 32 3/8. Pot 2 : 38 : 35 6/8. Pot 3 : 44 4/8 : 36. Pot 3 : 39 4/8 : 20 7/8. Pot 3 : 39 : 30 5/8. Pot 4 : 30 2/8 : 36. Pot 4 : 31 : 39. Pot 4 : 33 1/8 : 29. Pot 4 : 24 : 38 4/8. Pot 5 : 30 2/8 : 32. Crowded.
Total : 517.63 : 503.36. The fifteen crossed plants here average 34.5, and the fifteen self-fertilised 33.55 inches in height; or as 100 to 97. So that the excess of height of the crossed plants is quite insignificant. In productiveness, however, the difference was much more plainly marked. All the capsules were gathered from both lots of plants (except from the crowded and unproductive ones in Pot 5), and at the close of the season the few remaining flowers were added in. The fourteen crossed plants produced 381, whilst the fourteen self-fertilised plants produced only 293 capsules and flowers; or as 100 to 77. Dianthus caryophyllus. The common carnation is strongly proterandrous, and therefore depends to a large extent upon insects for fertilisation. I have seen only humble-bees visiting the flowers, but I dare say other insects likewise do so. It is notorious that if pure seed is desired, the greatest care is necessary to prevent the varieties which grow in the same garden from intercrossing. (4/10. ‘Gardeners’ Chronicle’ 1847 page 268.) The pollen is generally shed and lost before the two stigmas in the same flower diverge and are ready to be fertilised. I was therefore often forced to use for self-fertilisation pollen from the same plant instead of from the same flower. But on two occasions, when I attended to this point, I was not able to detect any marked difference in the number of seeds produced by these two forms of self-fertilisation.
Several single-flowered carnations were planted in good soil, and were all covered with a net. Eight flowers were crossed with pollen from a distinct plant and yielded six capsules, containing on an average 88.6 seeds, with a maximum in one of 112 seeds. Eight other flowers were self-fertilised in the manner above described, and yielded seven capsules containing on an average 82 seeds, with a maximum in one of 112 seeds. So that there was very little difference in the number of seeds produced by cross-fertilisation and self-fertilisation, namely, as 100 to 92. As these plants were covered by a net, they produced spontaneously only a few capsules containing any seeds, and these few may perhaps be attributed to the action of Thrips and other minute insects which haunt the flowers. A large majority of the spontaneously self-fertilised capsules produced by several plants contained no seeds, or only a single one. Excluding these latter capsules, I counted the seeds in eighteen of the finest ones, and these contained on an average 18 seeds. One of the plants was spontaneously self-fertile in a higher degree than any of the others. On another occasion a single covered-up plant produced spontaneously eighteen capsules, but only two of these contained any seed, namely 10 and 15.
Text from Project Gutenberg, public domain in the United States.