The Different Forms of Flowers on Plants of the Same Species by Charles Robert Darwin (top fiction books of all time txt) 📖

seeds; so that these illegitimate plants were sterile when legitimately crossed.
I succeeded in raising from the above seven short-styled illegitimate plants, fertilised with their own-form pollen, only six plants--grandchildren of the first union. These, like their parents, were of low stature, and had so poor a constitution that four died before flowering. With ordinary plants it has been a rare event with me to have more than a single plant die out of a large lot. The two grandchildren which lived and flowered were short-styled; and twelve of their flowers were fertilised with their own-form pollen and produced twelve capsules containing an average of 28.2 seeds; so that these two plants, though belonging to so weakly a set, were rather more fertile than their parents, and perhaps not in any degree sterile. Four flowers on the same two grandchildren were legitimately fertilised by a long-styled illegitimate plant, and produced four capsules, containing only 32.2 seeds instead of about 64 seeds, which is the normal average for legitimate short-styled plants legitimately crossed.
By looking back, it will be seen that I raised at first from a short-styled plant fertilised with its own-form pollen one long-styled and seven short-styled illegitimate seedlings. These seedlings were legitimately intercrossed, and from their seed fifteen plants were raised, grandchildren of the first illegitimate union, and to my surprise all proved short-styled. Twelve short-styled flowers borne by these grandchildren were illegitimately fertilised with pollen taken from other plants of the same lot, and produced eight capsules which contained an average of 21.8 seeds, with a maximum of 35. These figures are rather below the normal standard for such a union. Six flowers were also legitimately fertilised with pollen from an illegitimate long-styled plant and produced only three capsules, containing on an average 23.6 seeds, with a maximum of 35. Such a union in the case of a legitimate plant ought to have yielded an average of 64 seeds, with a possible maximum of 73 seeds.


SUMMARY ON THE TRANSMISSION OF FORM, CONSTITUTION, AND FERTILITY OF THE ILLEGITIMATE OFFSPRING OF Primula Sinensis.
In regard to the long-styled plants, their illegitimate offspring, of which fifty-two were raised in the course of two generations, were all long-styled. (5/7. Dr. Hildebrand, who first called attention to this subject 'Botanische Zeitung' 1864 page 5, raised from a similar illegitimate union seventeen plants, of which fourteen were long-styled and three short-styled. From a short-styled plant illegitimately fertilised with its own pollen he raised fourteen plants, of which eleven were short-styled and three long-styled.) These plants grew vigorously; but the flowers in one instance were small, appearing as if they had reverted to the wild state. In the first illegitimate generation they were perfectly fertile, and in the second their fertility was only very slightly impaired. With respect to the short-styled plants, twenty-four out of twenty- five of their illegitimate offspring were short-styled. They were dwarfed in stature, and one lot of grandchildren had so poor a constitution that four out of six plants perished before flowering. The two survivors, when illegitimately fertilised with their own-form pollen, were rather less fertile than they ought to have been; but their loss of fertility was clearly shown in a special and unexpected manner, namely, when legitimately fertilised by other illegitimate plants: thus altogether eighteen flowers were fertilised in this manner, and yielded twelve capsules, which included on an average only 28.5 seeds, with a maximum of 45. Now a legitimate short-styled plant would have yielded, when legitimately fertilised, an average of 64 seeds, with a possible maximum of 74. This particular kind of infertility will perhaps be best appreciated by a simile: we may assume that with mankind six children would be born on an average from an ordinary marriage; but that only three would be born from an incestuous marriage. According to the analogy of Primula Sinensis, the children of such incestuous marriages, if they continued to marry incestuously, would have their sterility only slightly increased; but their fertility would not be restored by a proper marriage; for if two children, both of incestuous origin, but in no degree related to each other, were to marry, the marriage would of course be strictly legitimate, nevertheless they would not give birth to more than half the full and proper number of children.
[EQUAL-STYLED VARIETY OF Primula Sinensis.
As any variation in the structure of the reproductive organs, combined with changed function, is a rare event, the following cases are worth giving in detail. My attention was first called to the subject by observing, in 1862, a long-styled plant, descended from a self-fertilised long-styled parent, which had some of its flowers in an anomalous state, namely, with the stamens placed low down in the corolla as in the ordinary long-styled form, but with the pistils so short that the stigmas stood on a level with the anthers. These stigmas were nearly as globular and as smooth as in the short-styled form, instead of being elongated and rough as in the long-styled form. Here, then, we have combined in the same flower, the short stamens of the long-styled form with a pistil closely resembling that of the short-styled form. But the structure varied much even on the same umbel: for in two flowers the pistil was intermediate in length between that of the long and that of the short-styled form, with the stigma elongated as in the former, and smooth as in the latter; and in three other flowers the structure was in all respects like that of the long-styled form. These modifications appeared to me so remarkable that I fertilised eight of the flowers with their own pollen, and obtained five capsules, which contained on an average 43 seeds; and this number shows that the flowers had become abnormally fertile in comparison with those of ordinary long- styled plants when self-fertilised. I was thus led to examine the plants in several small collections, and the result showed that the equal-styled variety was not rare.
TABLE 5.31. Primula Sinensis. Preponderance of long-styled over the short-styled form.
Column 1: Name of owner or place. Column 2: Long-styled form. Column 3: Short-styled form. Column 4: equal-styled variety.
Mr. Horwood : 0 : 0 : 17. Mr. Duck : 20 : 0 : 9. Baston : 30 : 18 : 15. Chichester : 12 : 9 : 2. Holwood : 42 : 12 : 0. High Elms : 16 : 0 : 0. Westerham : 1 : 5 : 0. My own plants from purchased seeds : 13 : 7 : 0. Total : 134 : 51 : 43.
In a state of nature the long and short-styled forms would no doubt occur in nearly equal numbers, as I infer from the analogy of the other heterostyled species of Primula, and from having raised the two forms of the present species in exactly the same number from flowers which had been LEGITIMATELY crossed. The preponderance in Table 5.31 of the long-styled form over the short-styled (in the proportion of 134 to 51) results from gardeners generally collecting seed from self-fertilised flowers; and the long-styled flowers produce spontaneously much more seed (as shown in the first chapter) than the short-styled, owing to the anthers of the long-styled form being placed low down in the corolla, so that, when the flowers fall off, the anthers are dragged over the stigma; and we now also know that long-styled plants, when self-fertilised, very generally reproduce long-styled offspring. From the consideration of this table, it occurred to me in the year 1862, that almost all the plants of the Chinese primrose cultivated in England would sooner or later become long-styled or equal-styled; and now, at the close of 1876, I have had five small collections of plants examined, and almost all consisted of long-styled, with some more or less well-characterised equal-styled plants, but with not one short-styled.
With respect to the equal-styled plants in the table, Mr. Horwood raised from purchased seeds four plants, which he remembered were certainly not long-styled, but either short or equal-styled, probably the latter. These four plants were kept separate and allowed to fertilise themselves; from their seed the seventeen plants in the table were raised, all of which proved equal-styled. The stamens stood low down in the corolla as in the long-styled form; and the stigmas, which were globular and smooth, were either completely surrounded by the anthers, or stood close above them. My son William made drawings for me, by the aid of the camera, of the pollen of one of the above equal-styled plants; and, in accordance with the position of the stamens, the grains resembled in their small size those of the long-styled form. He also examined pollen from two equal- styled plants at Southampton; and in both of them the grains differed extremely in size in the same anthers, a large number being small and shrivelled, whilst many were fully as large as those of the short-styled form and rather more globular. It is probable that the large size of these grains was due, not to their having assumed the character of the short-styled form, but to monstrosity; for Max Wichura has observed pollen-grains of monstrous size in certain hybrids. The vast number of the small shrivelled grains in the above two cases explains the fact that, though equal-styled plants are generally fertile in a high degree, yet some of them yield few seeds. I may add that my son compared, in 1875, the grains from two white-flowered plants, in both of which the pistil projected above the anthers, but neither were properly long-styled or equal- styled; and in the one in which the stigma projected most, the grains were in diameter to those in the other plant, in which the stigma projected less, as 100 to 88; whereas the difference between the grains from perfectly characterised long-styled and short-styled plants is as 100 to 57. So that these two plants were in an intermediate condition. To return to the 17 plants in the first line of Table 5.31: from the relative position of their stigmas and anthers, they could hardly fail to fertilise themselves; and accordingly four of them spontaneously yielded no less than 180 capsules; of these Mr. Horwood selected eight fine capsules for sowing; and they included on an average 54.8 seeds, with a maximum of 72. He gave me thirty other capsules, taken by hazard, of which twenty-seven contained good seeds, averaging 35.5, with a maximum of 70; but if six poor capsules, each with less than 13 seeds, be excluded, the average rises to 42.5. These are higher numbers than could be expected from either well- characterised form if self-fertilised; and this high degree of fertility accords with the view that the male organs belonged to one form, and the female organs partially to the other form; so that a self-union in the case of the equal- styled variety is in fact a legitimate union.
The seed saved from the above seventeen self-fertilised equal-styled plants produced sixteen plants, which all proved equal-styled, and resembled their parents in all the above-specified respects. The stamens, however, in one plant were seated higher up the tube of the corolla than in the true long-styled form; in another plant almost all the anthers were contabescent. These sixteen plants were the grandchildren of the four original plants, which it is believed were equal-styled; so that this abnormal condition was faithfully transmitted, probably through three, and certainly through two generations. The fertility of one of these grandchildren was carefully observed: six flowers were fertilised with pollen from the same flower, and produced six capsules, containing on an average 68 seeds, with a maximum of 82, and a minimum of 40. Thirteen capsules spontaneously self-fertilised yielded an average of 53.2 seeds, with the astonishing maximum in one of 97 seeds. In no legitimate union has so high an average as 68 seeds been observed by me, or nearly so high a maximum as 82 and