The Different Forms of Flowers on Plants of the Same Species, Charles Robert Darwin [tohfa e dulha read online .TXT] 📗

is the case. A good observer, resting his belief on the usual kind of evidence, states that in Linum Austriacum (which is heterostyled, and is considered by Planchon as a variety of L. perenne) the anthers open the evening before the expansion of the flowers, and that the stigmas are then almost always fertilised. (3/5. H. Lecoq 'Etudes sur la Geogr. Bot.' 1856 tome 5 page 325.) Now we know positively that, so far from Linum perenne being fertilised by its own pollen in the bud, its own pollen is as powerless on the stigma as so much inorganic dust.
Linum flavum.
The pistil of the long-styled form of this species is nearly twice as long as that of the short-styled; the stigmas are longer and the papillae coarser. In the short-styled form the stigmas diverge and pass out between the filaments, as in the previous species. The stamens in the two forms differ in length; and, what is singular, the anthers of the longer stamens are not so long as those of the other form; so that in the short-styled form both the stigmas and the anthers are shorter than in the long-styled form. The pollen-grains of the two forms do not differ in size. As this species is propagated by cuttings, generally all the plants in the same garden belong to the same form. I have inquired, but have never heard of its seeding in this country. Certainly my own plants never produced a single seed as long as I possessed only one of the two forms. After considerable search I procured both forms, but from want of time only a few experiments were made. Two plants of the two forms were planted some way apart in my garden, and were not covered by nets. Three flowers on the long- styled plant were legitimately fertilised with pollen from the short-styled plant, and one of them set a fine capsule. No other capsules were produced by this plant. Three flowers on the short-styled plant were legitimately fertilised with pollen from the long-styled, and all three produced capsules, containing respectively no less than 8, 9, and 10 seeds. Three other flowers on this plant, which had not been artificially fertilised, produced capsules containing 5, 1, and 5 seeds; and it is quite possible that pollen may have been brought to them by insects from the long-styled plant growing in the same garden. Nevertheless, as they did not yield half the number of seeds compared with the other flowers on the same plant which had been artificially and legitimately fertilised, and as the short-styled plants of the two previous species apparently evince some slight capacity for fertilisation with their own-form pollen, these three capsules may have been the product of self-fertilisation.
Besides the three species now described, the yellow-flowered L. corymbiferum is certainly heterostyled, as is, according to Planchon, L. salsoloides. (3/6. Hooker's 'London Journal of Botany' 1848 volume 7 page 174.) This botanist is the only one who seems to have inferred that heterostylism might have some important functional bearing. Dr. Alefeld, who has made a special study of the genus, says that about half of the sixty-five species known to him are heterostyled. (3/7. 'Botanische Zeitung' September 18, 1863 page 281.) This is the case with L. trigynum, which differs so much from the other species that it has been formed by him into a distinct genus. (3/8. It is not improbable that the allied genus, Hugonia, is heterostyled, for one species is said by Planchon (Hooker's 'London Journal of Botany' 1848 volume 7 page 525) to be provided with "staminibus exsertis;" another with "stylis staminibus longioribus," and another has "stamina 5, majora, stylos longe superantia.") According to the same author, none of the species which inhabit America and the Cape of Good Hope are heterostyled.
I have examined only three homostyled species, namely, L. usitatissimum, angustifolium, and catharticum. I raised 111 plants of a variety of the first- named species, and these, when protected under a net, all produced plenty of seed. The flowers, according to H. Muller, are frequented by bees and moths. (3/9. 'Die Befruchtung der Blumen' etc. page 168.) With respect to L. catharticum, the same author shows that the flowers are so constructed that they can freely fertilise themselves; but if visited by insects they might be cross- fertilised. He has, however, only once seen the flowers thus visited during the day; but it may be suspected that they are frequented during the night by small moths for the sake of the five minute drops of nectar secreted. Lastly, L. Lewisii is said by Planchon to bear on the same plant flowers with stamens and pistils of the same height, and others with the pistils either longer or shorter than the stamens. This case formerly appeared to me an extraordinary one; but I am now inclined to believe that it is one merely of great variability. (3/10. Planchon in Hooker's 'London Journal of Botany' 1848 volume 7 page 175. See on this subject Asa Gray in 'American Journal of Science' volume 36 September 1863 page 284.)
PULMONARIA (BORAGINEAE).
Pulmonaria officinalis.
Hildebrand has published a full account of this heterostyled plant. (3/11. 'Botanische Zeitung' 1865 January 13 page 13.) The pistil of the long-styled form is twice as long as that of the short-styled; and the stamens differ in a corresponding, though converse, manner. There is no marked difference in the shape or state of surface of the stigma in the two forms. The pollen-grains of the short-styled form are to those of the long-styled as 9 to 7, or as 100 to 78, in length, and as 7 to 6 in breadth. They do not differ in the appearance of their contents. The corolla of the one form differs in shape from that of the other in nearly the same manner as in Primula; but besides this difference the flowers of the short-styled are generally the larger of the two. Hildebrand collected on the Siebengebirge, ten wild long-styled and ten short-styled plants. The former bore 289 flowers, of which 186 (i.e. 64 per cent) had set fruit, yielding 1.88 seed per fruit. The ten short-styled plants bore 373 flowers, of which 262 (i.e. 70 per cent) had set fruit, yielding 1.86 seed per fruit. So that the short-styled plants produced many more flowers, and these set a rather larger proportion of fruit, but the fruits themselves yielded a slightly lower average number of seeds than did the long-styled plants. The results of Hildebrand's experiments on the fertility of the two forms are given in Table 3.19.
TABLE 3.19. Pulmonaria officinalis (from Hildebrand).
Column 1: Nature of the Union. Column 2: Number of Flowers fertilised. Column 3: Number of Fruits produced. Column 4: Average Number of Seeds per Fruit.
Long-styled by pollen of short-styled. Legitimate union : 14 : 10 : 1.30.
Long-styled 14 by own-pollen, and 16 by pollen of other plant of same form. Illegitimate union : 30 : 0 : 0.
Short-styled by pollen of long-styled. Legitimate union: 16 : 14 : 1.57.
Short-styled 11 by own-pollen, 14 by pollen of other plant of same form. Illegitimate union : 25 : 0 : 0.
In the summer of 1864, before I had heard of Hildebrand's experiments, I noticed some long-styled plants of this species (named for me by Dr. Hooker) growing by themselves in a garden in Surrey; and to my surprise about half the flowers had set fruit, several of which contained 2, and one contained even 3 seeds. These seeds were sown in my garden and eleven seedlings thus raised, all of which proved long-styled, in accordance with the usual rule in such cases. Two years afterwards the plants were left uncovered, no other plant of the same genus growing in my garden, and the flowers were visited by many bees. They set an abundance of seeds: for instance, I gathered from a single plant rather less than half of the seeds which it had produced, and they numbered 47. Therefore this illegitimately fertilised plant must have produced about 100 seeds; that is, thrice as many as one of the wild long-styled plants collected on the Siebengebirge by Hildebrand, and which, no doubt, had been legitimately fertilised. In the following year one of my plants was covered by a net, and even under these unfavourable conditions it produced spontaneously a few seeds. It should be observed that as the flowers stand either almost horizontally or hang considerably downwards, pollen from the short stamens would be likely to fall on the stigma. We thus see that the English long-styled plants when illegitimately fertilised were highly fertile, whilst the German plants similarly treated by Hildebrand were completely sterile. How to account for this wide discordance in our results I know not. Hildebrand cultivated his plants in pots and kept them for a time in the house, whilst mine were grown out of doors; and he thinks that this difference of treatment may have caused the difference in our results. But this does not appear to me nearly a sufficient cause, although his plants were slightly less productive than the wild ones growing on the Siebengbirge. My plants exhibited no tendency to become equal-styled, so as to lose their proper long-styled character, as not rarely happens under cultivation with several heterostyled species of Primula; but it would appear that they had been greatly affected in function, either by long-continued cultivation or by some other cause. We shall see in a future chapter that heterostyled plants illegitimately fertilised during several successive generations sometimes become more self-fertile; and this may have been the case with my stock of the present species of Pulmonaria; but in this case we must assume that the long-styled plants were at first sufficiently fertile to yield some seed, instead of being absolutely self-sterile like the German plants.
Pulmonaria angustifolia.
Seedlings of this plant, raised from plants growing wild in the Isle of Wight, were named for me by Dr. Hooker. It is so closely allied to the last species, differing chiefly in the shape and spotting of the leaves, that the two have been considered by several eminent botanists--for instance, Bentham--as mere varieties. But, as we shall presently see, good evidence can be assigned for ranking them as distinct. Owing to the doubts on this head, I tried whether the two would mutually fertilise one another. Twelve short-styled flowers of P. angustifolia were legitimately fertilised with pollen from long-styled plants of P. officinalis (which, as we have just seen, are moderately self-fertile), but they did not produce a single fruit. Thirty-six long-styled flowers of P. angustifolia were also illegitimately fertilised during two seasons with pollen from the long-styled P. officinalis, but all these flowers dropped off unimpregnated. Had the plants been mere varieties of the same species these illegitimate crosses would probably have yielded some seeds, judging from my success in illegitimately fertilising the long-styled flowers of P. officinalis; and the twelve legitimate crosses, instead of yielding no fruit, would almost certainly have yielded a considerable number, namely, about nine, judging from the results given in Table 3.20. Therefore P. officinalis and angustifolia appear to be good and distinct species, in conformity with other important functional differences between them, immediately to be described.
TABLE 3.20. Pulmonaria angustifolia.
Column 1: Nature of the Union. Column 2: Number of Flowers fertilised. Column 3: Number of Fruits produced. Column 4: Average Number of Seeds per Fruit.
Long-styled by pollen of short-styled. Legitimate union : 18 : 9 : 2.11.
Long-styled by own-form pollen. Illegitimate union : 18 : 0 : 0.
Short-styled by pollen of long-styled. Legitimate union: 18 : 15 : 2.60.
Short-styled by own-form pollen. Illegitimate union : 12 : 7 : 1.86.
The long-styled and short-styled flowers of P. angustifolia differ from one another in structure in nearly the same manner as those of P. officinalis. But in Figure 3.6 a slight bulging of the corolla in the long-styled