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

more productive of the two forms. The results of my trials on the fertility of the two forms, when legitimately and illegitimately fertilised, are given in Table 1.9.
TABLE 1.9. Primula vulgaris.
Column 1: Nature of the Union. Column 2: Number of Flowers fertilised. Column 3: Number of good Capsules produced. Column 4: Maximum Number of Seeds in any one Capsule. Column 5: Minimum Number of Seeds in any one Capsule. Column 6: Average Number of Seeds per Capsule.
Long-styled by pollen of short-styled. Legitimate union : 12 : 11 : 77 : 47 : 66.9.
Long-styled by own-form pollen. Illegitimate union : 21 : 14 : 66 : 30 : 52.2.
Short-styled by pollen of long-styled. Legitimate union: 8 : 7 : 75 : 48 : 65.0.
Short-styled by own-form pollen. Illegitimate union : 18 : 7 : 43 : 5 : 18.8 (This average is perhaps rather too low).
SUMMARY:
The two legitimate unions together : 20 : 18 : 77 : 47 : 66.0.
The two illegitimate unions together : 39 : 21 : 66 : 5 : 35.5 (This average is perhaps rather too low).
We may infer from this table that the fertility of the two legitimate unions taken together is to that of the two illegitimate unions together, as judged by the proportional number of flowers which when fertilised in the two methods yielded capsules, as 100 to 60. If we judge by the average number of seeds per capsule produced by the two kinds of unions, the ratio is as 100 to 54; but this latter figure is perhaps rather too low. It is surprising how rarely insects can be seen during the day visiting the flowers, but I have occasionally observed small kinds of bees at work; I suppose, therefore, that they are commonly fertilised by nocturnal Lepidoptera. The long-styled plants when protected from insects yield a considerable number of capsules, and they thus differ remarkably from the same form of the cowslip, which is quite sterile under the same circumstances. Twenty-three spontaneously self-fertilised capsules from this form contained, on an average, 19.2 seeds. The short-styled plants produced fewer spontaneously self-fertilised capsules, and fourteen of them contained only 6.2 seeds per capsule. The self-fertilisation of both forms was probably aided by Thrips, which abounded within the flowers; but these minute insects could not have placed nearly sufficient pollen on the stigmas, as the spontaneously self-fertilised capsules contained much fewer seeds, on an average, than those (as may be seen in Table 1.9.) which were artificially fertilised with their own-form pollen. But this difference may perhaps be attributed in part to the flowers in the table having been fertilised with pollen from a distinct plant belonging to the same form; whilst those which were spontaneously self-fertilised no doubt generally received their own pollen. In a future part of this volume some observations will be given on the fertility of a red-coloured variety of the primrose.
Primula Sinensis.
In the long-styled form the pistil is about twice as long as that of the short- styled, and the stamens differ in a corresponding, but reversed, manner. The stigma is considerably more elongated and rougher than that of the short-styled, which is smooth and almost spherical, being somewhat depressed on the summit; but the stigma varies much in all its characters, the result, probably, of cultivation. The pollen-grains of the short-styled form, according to Hildebrand, are 7 divisions of the micrometer in length and 5 in breadth; whereas those of the long-styled are only 4 in length and 3 in breadth. (1/10. After the appearance of my paper this author published some excellent observations on the present species 'Botanische Zeitung' January 1, 1864, and he shows that I erred greatly about the size of the pollen-grains in the two forms. I suppose that by mistake I measured twice over pollen-grains from the same form.) The grains, therefore, of the short-styled are to those of the long- styled in length as 100 to 57. Hildebrand also remarked, as I had done in the case of P. veris, that the smaller grains from the long-styled are much more transparent than the larger ones from the short-styled form. We shall hereafter see that this cultivated plant varies much in its dimorphic condition and is often equal-styled. Some individuals may be said to be sub-heterostyled; thus in two white-flowered plants the pistil projected above the stamens, but in one of them it was longer and had a more elongated and rougher stigma, than in the other; and the pollen-grains from the latter were to those from the plant with a more elongated pistil only as 100 to 88 in diameter, instead of as 100 to 57. The corolla of the long-styled and short-styled form differs in shape, in the same manner as in P. veris. The long-styled plants tend to flower before the short-styled. When both forms were legitimately fertilised, the capsules from the short-styled plants contained, on an average, more seeds than those from the long-styled, in the ratio of 12.2 to 9.3 by weight, that is, as 100 to 78. In Table 1.10 we have the results of two sets of experiments tried at different periods.
TABLE 1.10. Primula Sinensis.
Column 1: Nature of the Union. Column 2: Number of Flowers fertilised. Column 3: Number of good Capsules produced. Column 4: Average Weight of Seeds per Capsule. ... Column 5: Average Number of Seeds per Capsule as ascertained on a subsequent occasion.
Long-styled by pollen of short-styled. Legitimate union : 24 : 16 : 0.58 :: 50.
Long-styled by own-form pollen. Illegitimate union : 20 : 13 : 0.45 :: 35.
Short-styled by pollen of long-styled. Legitimate union: 8 : 8 : 0.76 :: 64.
Short-styled by own-form pollen. Illegitimate union : 7 : 4 : 0.23 :: 25.
SUMMARY:
The two legitimate unions together : 32 : 24 : 0.64 :: 57.
The two illegitimate unions together : 27 : 17 : 0.40 :: 30.
The fertility, therefore, of the two legitimate unions together to that of the two illegitimate unions, as judged by the proportional number of flowers which yielded capsules, is as 100 to 84. Judging by the average weight of seeds per capsule produced by the two kinds of unions, the ratio is as 100 to 63. On another occasion a large number of flowers of both forms were fertilised in the same manner, but no account of their number was kept. The seeds, however, were carefully counted, and the averages are shown in the right hand column. The ratio for the number of seeds produced by the two legitimate compared with the two illegitimate unions is here 100 to 53, which is probably more accurate than the foregoing one of 100 to 63.
TABLE 1.11. Primula Sinensis (from Hildebrand).
Column 1: Nature of the Union. Column 2: Number of Flowers fertilised. Column 3: Number of good Capsules produced. Column 4: Average Number of Seeds per Capsule.
Long-styled by pollen of short-styled. Legitimate union : 14 : 14 : 41.
Long-styled by own-form pollen, from a distinct plant. Illegitimate union : 26 : 26 : 18.
Long-styled by pollen from same flower. Illegitimate union : 27 : 21 : 17.
Short-styled by pollen of long-styled. Legitimate union: 14 : 14 : 44.
Short-styled by own-form pollen, from a distinct plant. Illegitimate union : 16 : 16 : 20.
Short-styled by pollen from the same flower. Illegitimate union : 21 : 11 : 8.
SUMMARY:
The two legitimate unions together : 28 : 28 : 43.
The two illegitimate unions together (own-form pollen): 42 : 42 : 18.
The two illegitimate unions together (pollen from the same flower ): 48 : 32 : 13.
Hildebrand in the paper above referred to gives the results of his experiments on the present species; and these are shown in a condensed form in Table 1.11. Besides using for the illegitimate unions pollen from a distinct plant of the same form, as was always done by me, he tried, in addition, the effects of the plant's own pollen. He counted the seeds.
It is remarkable that here all the flowers which were fertilised legitimately, as well as those fertilised illegitimately with pollen from a distinct plant belonging to the same form, yielded capsules; and from this fact it might be inferred that the two forms were reciprocally much more fertile in his case than in mine. But his illegitimately fertilised capsules from both forms contained fewer seeds relatively to the legitimately fertilised capsules than in my experiments; for the ratio in his case is as 42 to 100, instead of, as in mine, as 53 to 100. Fertility is a very variable element with most plants, being determined by the conditions to which they are subjected, of which fact I have observed striking instances with the present species; and this may account for the difference between my results and those of Hildebrand. His plants were kept in a room, and perhaps were grown in too small pots or under some other unfavourable conditions, for his capsules in almost every case contained a smaller number of seeds than mine, as may be seen by comparing the right hand columns in Tables 1.10 and 1.11.
The most interesting point in Hildebrand's experiments is the difference in the effects of illegitimate fertilisation with a flower's own pollen, and with that from a distinct plant of the same form. In the latter case all the flowers produced capsules, whilst only 67 out of 100 of those fertilised with their own pollen produced capsules. The self-fertilised capsules also contained seeds, as compared with capsules from flowers fertilised with pollen from a distinct plant of the same form, in the ratio of 72 to 100.
In order to ascertain how far the present species was spontaneously self- fertile, five long-styled plants were protected by me from insects; and they bore up to a given period 147 flowers which set 62 capsules; but many of these soon fell off, showing that they had not been properly fertilised. At the same time five short-styled plants were similarly treated, and they bore 116 flowers which ultimately produced only seven capsules. On another occasion 13 protected long-styled plants yielded by weight 25.9 grains of spontaneously self- fertilised seeds. At the same time seven protected short-styled plants yielded only half-a-grain weight of seeds. Therefore the long-styled plants yielded nearly 24 times as many spontaneously self-fertilised seeds as did the same number of short-styled plants. The chief cause of this great difference appears to be, that when the corolla of a long-styled plant falls off, the anthers, from being situated near the bottom of the tube are necessarily dragged over the stigma and leave pollen on it, as I saw when I hastened the fall of nearly withered flowers; whereas in the short-styled flowers, the stamens are seated at the mouth of the corolla, and in falling off do not brush over the lowly-seated stigmas. Hildebrand likewise protected some long-styled and short-styled plants, but neither ever yielded a single capsule. He thinks that the difference in our results may be accounted for by his plants having been kept in a room and never having been shaken; but this explanation seems to me doubtful; his plants were in a less fertile condition than mine, as shown by the difference in the number of seeds produced, and it is highly probable that their lessened fertility would have interfered with especial force with their capacity for producing self- fertilised seeds.
[Primula auricula. (1/11. According to Kerner our garden auriculas are descended from P. pubescens, Jacq., which is a hybrid between the true P. auricula and hirsuta. This hybrid has now been propagated for about 300 years, and produces, when legitimately fertilised, a large number of seeds; the long-styled forms yielding an average number of 73, and the short-styled 98 seeds per capsule: see his "Geschichte