The Different Forms of Flowers on Plants of the Same Species - Charles Robert Darwin (ebook reader screen TXT) 📗

and produced very few flower-stems; and although they must have been legitimately fertilised by insects, they yielded capsules, compared with those produced by the surrounding legitimate plants, in the ratio only of 5 to 100! It is therefore certain that illegitimate fertilisation, continued during successive generations, affects the powers of growth and fertility of P. veris to an extraordinary degree; more especially when the plants are exposed to ordinary conditions of life, instead of being protected in a greenhouse.


[EQUAL-STYLED RED VARIETY OF Primula veris.
Mr. Scott has described a plant of this kind growing in the Botanic Garden of Edinburgh. (5/12. 'Proceedings of the Linnean Society' volume 8 1864 page 105.) He states that it was highly self-fertile, although insects were excluded; and he explains this fact by showing, first, that the anthers and stigma are in close apposition, and that the stamens in length, position and size of their pollen-grains resemble those of the short-styled form, whilst the pistil resembles that of the long-styled form both in length and in the structure of the stigma. Hence the self-union of this variety is, in fact, a legitimate union, and consequently is highly fertile. Mr. Scott further states that this variety yielded very few seeds when fertilised by either the long- or short- styled common cowslip, and, again, that both forms of the latter, when fertilised by the equal-styled variety, likewise produced very few seeds. But his experiments with the cowslip were few, and my results do not confirm his in any uniform manner.
I raised twenty plants from self-fertilised seed sent me by Mr. Scott; and they all produced red flowers, varying slightly in tint. Of these, two were strictly long-styled both in structure and in function; for their reproductive powers were tested by crosses with both forms of the common cowslip. Six plants were equal-styled; but on the same plant the pistil varied a good deal in length during different seasons. This was likewise the case, according to Mr. Scott, with the parent-plant. Lastly, twelve plants were in appearance short-styled; but they varied much more in the length of their pistils than ordinary short- styled cowslips, and they differed widely from the latter in their powers of reproduction. Their pistils had become short-styled in structure, whilst remaining long-styled in function. Short-styled cowslips, when insects are excluded, are extremely barren: for instance, on one occasion six fine plants produced only about 50 seeds (that is, less than the product of two good capsules), and on another occasion not a single capsule. Now, when the above twelve apparently short-styled seedlings were similarly treated, nearly all produced a great abundance of capsules, containing numerous seeds, which germinated remarkably well. Moreover three of these plants, which during the first year were furnished with quite short pistils, on the following year produced pistils of extraordinary length. The greater number, therefore, of these short-styled plants could not be distinguished in function from the equal- styled variety. The anthers in the six equal-styled and in the apparently twelve short-styled plants were seated high up in the corolla, as in the true short- styled cowslip; and the pollen-grains resembled those of the same form in their large size, but were mingled with a few shrivelled grains. In function this pollen was identical with that of the short-styled cowslip; for ten long-styled flowers of the common cowslip, legitimately fertilised with pollen from a true equal-styled variety, produced six capsules, containing on an average 34.4 seeds; whilst seven capsules on a short-styled cowslip illegitimately fertilised with pollen from the equal-styled variety, yielded an average of only 14.5 seeds.
As the equal-styled plants differ from one another in their powers of reproduction, and as this is an important subject, I will give a few details with respect to five of them. First, an equal-styled plant, protected from insects (as was done in all the following cases, with one stated exception), spontaneously produced numerous capsules, five of which gave an average of 44.8 seeds, with a maximum in one capsule of 57. But six capsules, the product of fertilisation with pollen from a short-styled cowslip (and this is a legitimate union), gave an average of 28.5 seeds, with a maximum of 49; and this is a much lower average than might have been expected. Secondly, nine capsules from another equal-styled plant, which had not been protected from insects, but probably was self-fertilised, gave an average of 45.2 seeds, with a maximum of 58. Thirdly, another plant which had a very short pistil in 1865, produced spontaneously many capsules, six of which contained an average of 33.9 seeds, with a maximum of 38. In 1866 this same plant had a pistil of wonderful length; for it projected quite above the anthers, and the stigma resembled that of the long-styled form. In this condition it produced spontaneously a vast number of fine capsules, six of which contained almost exactly the same average number as before, namely 34.3, with a maximum of 38. Four flowers on this plant, legitimately fertilised with pollen from a short-styled cowslip, yielded capsules with an average of 30.2 seeds. Fourthly another short-styled plant spontaneously produced in 1865 an abundance of capsules, ten of which contained an average of 35.6 seeds, with a maximum of 54. In 1866 this same plant had become in all respects long-styled, and ten capsules gave almost exactly the same average as before, namely 35.1 seeds, with a maximum of 47. Eight flowers on this plant, legitimately fertilised with pollen from a short-styled cowslip, produced six capsules, with the high average of 53 seeds, and the high maximum of 67. Eight flowers were also fertilised with pollen from a long-styled cowslip (this being an illegitimate union), and produced seven capsules, containing an average of 24.4 seeds, with a maximum of 32. The fifth and last plant remained in the same condition during both years: it had a pistil rather longer than that of the true short-styled form, with the stigma smooth, as it ought to be in this form, but abnormal in shape, like a much-elongated inverted cone. It produced spontaneously many capsules, five of which, in 1865, gave an average of only 15.6 seeds; and in 1866 ten capsules still gave an average only a little higher, namely of 22.1, with a maximum of 30. Sixteen flowers were fertilised with pollen from a long-styled cowslip, and produced 12 capsules, with an average of 24.9 seeds, and a maximum of 42. Eight flowers were fertilised with pollen from a short-styled cowslip, but yielded only two capsules, containing 18 and 23 seeds. Hence this plant, in function and partially in structure, was in an almost exactly intermediate state between the long-styled and short-styled form, but inclining towards the short-styled; and this accounts for the low average of seeds which it produced when spontaneously self-fertilised.
The foregoing five plants thus differ much from one another in the nature of their fertility. In two individuals a great difference in the length of the pistil during two succeeding years made no difference in the number of seeds produced. As all five plants possessed the male organs of the short-styled form in a perfect state, and the female organs of the long-styled form in a more or less complete state, they spontaneously produced a surprising number of capsules, which generally contained a large average of remarkably fine seeds. With ordinary cowslips LEGITIMATELY FERTILISED, I once obtained from plants cultivated in the greenhouse the high average, from seven capsules, of 58.7 seeds, with a maximum in one capsule of 87 seeds; but from plants grown out of doors I never obtained a higher average than 41 seeds. Now two of the equal- styled plants, grown out of doors and spontaneously SELF-FERTILISED, gave averages of 44 and 45 seeds; but this high fertility may perhaps be in part attributed to the stigma receiving pollen from the surrounding anthers at exactly the right period. Two of these plants, fertilised with pollen from a short-styled cowslip (and this in fact is a legitimate union), gave a lower average than when self-fertilised. On the other hand, another plant, when similarly fertilised by a cowslip, yielded the unusually high average of 53 seeds, with a maximum of 67. Lastly, as we have just seen, one of these plants was in an almost exactly intermediate condition in its female organs between the long- and short-styled forms, and consequently, when self-fertilised, yielded a low average of seed. If we add together all the experiments which I made on the equal-styled plants, 41 spontaneously self-fertilised capsules (insects having been excluded) gave an average of 34 seeds, which is exactly the same number as the parent-plant yielded in Edinburgh. Thirty-four flowers, fertilised with pollen from the short-styled cowslip (and this is an analogous union), produced 17 capsules, containing an average of 33.8 seeds. It is a rather singular circumstance, for which I cannot account, that 20 flowers, artificially fertilised on one occasion with pollen from the same plants yielded only ten capsules, containing the low average of 26.7 seeds.
As bearing on inheritance, it may be added that 72 seedlings were raised from one of the red-flowered, strictly equal-styled, self-fertilised plants descended from the similarly characterised Edinburgh plant. These 72 plants were therefore grandchildren of the Edinburgh plant, and they all bore, as in the first generation, red flowers, with the exception of one plant, which reverted in colour to the common cowslip. In regard to structure, nine plants were truly long-styled and had their stamens seated low down in the corolla in the proper position; the remaining 63 plants were equal-styled, though the stigma in about a dozen of them stood a little below the anthers. We thus see that the anomalous combination in the same flower, of the male and female sexual organs which properly exist in the two distinct forms, was inherited with much force. Thirty- six seedlings were also raised from long and short-styled common cowslips, crossed with pollen from the equal-styled variety. Of these plants one alone was equal-styled, 20 were short-styled, but with the pistil in three of them rather too long, and the remaining 15 were long-styled. In this case we have an illustration of the difference between simple inheritance and prepotency of transmission; for the equal-styled variety, when self-fertilised, transmits its character, as we have just seen, with much force, but when crossed with the common cowslip cannot withstand the greater power of transmission of the latter.


PULMONARIA.
I have little to say on this genus. I obtained seeds of P. officinalis from a garden where the long-styled form alone grew, and raised 11 seedlings, which were all long-styled. These plants were named for me by Dr. Hooker. They differed, as has been shown, from the plants belonging to this species which in Germany were experimented on by Hildebrand (5/13. 'Botanische Zeitung' 1865 page 13.); for he found that the long-styled form was absolutely sterile with its own pollen, whilst my long-styled seedlings and the parent-plants yielded a fair supply of seed when self-fertilised. Plants of the long-styled form of Pulmonaria angustifolia were, like Hildebrand's plants, absolutely sterile with their own pollen, so that I could never procure a single seed. On the other hand, the short-styled plants of this species, differently from those of P. officinalis, were fertile with their own pollen in a quite remarkable degree for a heterostyled plant. From seeds carefully self-fertilised I raised 18 plants, of which 13 proved short-styled and 5 long-styled.
Polygonum fagopyrum.
From flowers on long-styled plants fertilised illegitimately with pollen from the same plant, 49 seedlings were raised, and these consisted of 45 long-styled and 4 short-styled. From flowers on short-styled plants illegitimately fertilised with pollen from the same plant 33 seedlings were raised, and these consisted of 20 short-styled and 13 long-styled. So that the usual rule of illegitimately fertilised long-styled plants tending much more strongly than short-styled plants to reproduce their own form here holds good.