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

capsules; and each of these ought to have contained on an average 52.2 seeds, instead of only 11.5 seeds. When these plants were illegitimately and legitimately fertilised with pollen from the common primrose, the average numbers were increased, but were far from attaining the normal standards. So it was when both forms of the common primrose were fertilised with pollen from these illegitimate plants; and this shows that their male as well as their female organs were in a deteriorated condition. The sterility of these plants was shown in another way, namely, by their not producing any capsules when the access of all insects (except such minute ones as Thrips) was prevented; for under these circumstances the common long-styled primrose produces a considerable number of capsules. There can, therefore, be no doubt that the fertility of these plants was greatly impaired. The loss is not correlated with the colour of the flower; and it was to ascertain this point that I made so many experiments. As the parent-plant growing in Edinburgh was found by Mr. Scott to be in a high degree sterile, it may have transmitted a similar tendency to its offspring, independently of their illegitimate birth. I am, however, inclined to attribute some weight to the illegitimacy of their descent, both from the analogy of other cases, and more especially from the fact that when the plants were LEGITIMATELY fertilised with pollen of the common primrose they yielded an average, as may be seen in the table, of only 5 more seeds than when ILLEGITIMATELY fertilised with the same pollen. Now we know that it is eminently characteristic of the illegitimate offspring of Primula Sinensis that they yield but few more seeds when legitimately fertilised than when fertilised with their own-form pollen.
Primula veris, Brit. Fl.
Var. officinalis of Linn., P. officinalis OF Jacq.
Seeds from the short-styled form of the cowslip fertilised with pollen from the same form germinate so badly that I raised from three successive sowings only fourteen plants, which consisted of nine short-styled and five long-styled plants. Hence the short-styled form of the cowslip, when self-fertilised, does not transmit the same form nearly so truly as does that of P. Sinensis. From the long-styled form, always fertilised with its own-form pollen, I raised in the first generation three long-styled plants,--from their seed 53 long-styled grandchildren,--from their seed 4 long-styled great-grandchildren,--from their seed 20 long-styled great-great-grandchildren,--and lastly, from their seed 8 long-styled and 2 short-styled great-great-great-grandchildren. In this last generation short-styled plants appeared for the first time in the course of the six generations,--the parent long-styled plant which was fertilised with pollen from another plant of the same form being counted as the first generation. Their appearance may be attributed to atavism. From two other long-styled plants, fertilised with their own-form pollen, 72 plants were raised, which consisted of 68 long-styled and 4 short-styled. So that altogether 162 plants were raised from illegitimately fertilised long-styled cowslips, and these consisted of 156 long-styled and 6 short-styled plants.
We will now turn to the fertility and powers of growth possessed by the illegitimate plants. From a short-styled plant, fertilised with its own-form pollen, one short-styled and two long-styled plants, and from a long-styled plant similarly fertilised three long-styled plants were at first raised. The fertility of these six illegitimate plants was carefully observed; but I must premise that I cannot give any satisfactory standard of comparison as far as the number of the seeds is concerned; for though I counted the seeds of many legitimate plants fertilised legitimately and illegitimately, the number varied so greatly during successive seasons that no one standard will serve well for illegitimate unions made during different seasons. Moreover the seeds in the same capsule frequently differ so much in size that it is scarcely possible to decide which ought to be counted as good seed. There remains as the best standard of comparison the proportional number of fertilised flowers which produce capsules containing any seed.
First, for the one illegitimate short-styled plant. In the course of three seasons 27 flowers were illegitimately fertilised with pollen from the same plant, and they yielded only a single capsule, which, however, contained a rather large number of seeds for a union of this nature, namely, 23. As a standard of comparison I may state that during the same three seasons 44 flowers borne by legitimate short-styled plants were self-fertilised, and yielded 26 capsules; so that the fact of the 27 flowers on the illegitimate plant having produced only one capsule proves how sterile it was. To show that the conditions of life were favourable, I will add that numerous plants of this and other species of Primula all produced an abundance of capsules whilst growing close by in the same soil with the present and following plants. The sterility of the above illegitimate short-styled plant depended on both the male and female organs being in a deteriorated condition. This was manifestly the case with the pollen; for many of the anthers were shrivelled or contabescent. Nevertheless some of the anthers contained pollen, with which I succeeded in fertilising some flowers on the illegitimate long-styled plants immediately to be described. Four flowers on this same short-styled plant were likewise LEGITIMATELY fertilised with pollen from one of the following long-styled plants; but only one capsule was produced, containing 26 seeds; and this is a very low number for a legitimate union.
With respect to the five illegitimate long-styled plants of the first generation, derived from the above self-fertilised short-styled and long-styled parents, their fertility was observed during the same three years. These five plants, when self-fertilised, differed considerably from one another in their degree of fertility, as was the case with the illegitimate long-styled plants of Lythrum salicaria; and their fertility varied much according to the season. I may premise, as a standard of comparison, that during the same years 56 flowers on legitimate long-styled plants of the same age and grown in the same soil, were fertilised with their own pollen, and yielded 27 capsules; that is, 48 per cent. On one of the five illegitimate long-styled plants 36 flowers were self- fertilised in the course of the three years, but they did not produce a single capsule. Many of the anthers on this plant were contabescent; but some seemed to contain sound pollen. Nor were the female organs quite impotent; for I obtained from a LEGITIMATE cross one capsule with good seed. On a second illegitimate long-styled plant 44 flowers were fertilised during the same years with their own pollen, but they produced only a single capsule. The third and fourth plants were in a very slight degree more productive. The fifth and last plant was decidedly more fertile; for 42 self-fertilised flowers yielded 11 capsules. Altogether, in the course of the three years, no less than 160 flowers on these five illegitimate long-styled plants were fertilised with their own pollen, but they yielded only 22 capsules. According to the standard above given, they ought to have yielded 80 capsules. These 22 capsules contained on an average 15.1 seeds. I believe, subject to the doubts before specified, that with legitimate plants the average number from a union of this nature would have been above 20 seeds. Twenty-four flowers on these same five illegitimate long-styled plants were legitimately fertilised with pollen from the above-described illegitimate short-styled plant, and produced only 9 capsules, which is an extremely small number for a legitimate union. These 9 capsules, however, contained an average of 38 apparently good seeds, which is as large a number as legitimate plants sometimes yield. But this high average was almost certainly false; and I mention the case for the sake of showing the difficulty of arriving at a fair result; for this average mainly depended on two capsules containing the extraordinary numbers of 75 and 56 seeds; these seeds, however, though I felt bound to count them, were so poor that, judging from trials made in other cases, I do not suppose that one would have germinated; and therefore they ought not to have been included. Lastly, 20 flowers were legitimately fertilised with pollen from a legitimate plant, and this increased their fertility; for they produced 10 capsules. Yet this is but a very small proportion for a legitimate union.
There can, therefore, be no doubt that these five long-styled plants and the one short-styled plant of the first illegitimate generation were extremely sterile. Their sterility was shown, as in the case of hybrids, in another way, namely, by their flowering profusely, and especially by the long endurance of the flowers. For instance, I fertilised many flowers on these plants, and fifteen days afterwards (namely on March 22nd) I fertilised numerous long-styled and short- styled flowers on common cowslips growing close by. These latter flowers, on April 8th, were withered, whilst most of the illegitimate flowers remained quite fresh for several days subsequently; so that some of these illegitimate plants, after being fertilised, remained in full bloom for above a month.
We will now turn to the fertility of the 53 illegitimate long-styled grandchildren, descended from the long-styled plant which was first fertilised with its own pollen. The pollen in two of these plants included a multitude of small and shrivelled grains. Nevertheless they were not very sterile; for 25 flowers, fertilised with their own pollen, produced 15 capsules, containing an average of 16.3 seeds. As already stated, the probable average with legitimate plants for a union of this nature is rather above 20 seeds. These plants were remarkably healthy and vigorous, as long as they were kept under highly favourable conditions in pots in the greenhouse; and such treatment greatly increases the fertility of the cowslip. When these same plants were planted during the next year (which, however, was an unfavourable one), out of doors in good soil, 20 self-fertilised flowers produced only 5 capsules, containing extremely few and wretched seeds.
Four long-styled great-grandchildren were raised from the self-fertilised grandchildren, and were kept under the same highly favourable conditions in the greenhouse; 10 of their flowers were fertilised with own-form pollen and yielded the large proportion of 6 capsules, containing on an average 18.7 seeds. From these seeds 20 long-styled great-great-grandchildren were raised, which were likewise kept in the greenhouse. Thirty of their flowers were fertilised with their own pollen and yielded 17 capsules, containing on an average no less than 32, mostly fine seeds. It appears, therefore, that the fertility of these plants of the fourth illegitimate generation, as long as they were kept under highly favourable conditions, had not decreased, but had rather increased. The result, however, was widely different when they were planted out of doors in good soil, where other cowslips grew vigorously and were completely fertile; for these illegitimate plants now became much dwarfed in stature and extremely sterile, notwithstanding that they were exposed to the visits of insects, and must have been legitimately fertilised by the surrounding legitimate plants. A whole row of these plants of the fourth illegitimate generation, thus freely exposed and legitimately fertilised, produced only 3 capsules, containing on an average only 17 seeds. During the ensuing winter almost all these plants died, and the few survivors were miserably unhealthy, whilst the surrounding legitimate plants were not in the least injured.
The seeds from the great-great-grandchildren were sown, and 8 long-styled and 2 short-styled plants of the fifth illegitimate generation raised. These whilst still in the greenhouse produced smaller leaves and shorter flower-stalks than some legitimate plants with which they grew in competition; but it should be observed that the latter were the product of a cross with a fresh stock,--a circumstance which by itself would have added much to their vigour. (5/11. For full details of this experiment, see my 'Effects of Cross and Self- fertilisation' 1876 page 220.) When these illegitimate plants were transferred to fairly good soil out of doors, they became during the two following years much more dwarfed in stature