On this view, also, the large size of the stigmatic surface is an unintelligible feature in the structure of the flower, as well as the relative position of all the parts, which is such that when insects visit the flowers to suck the copious nectar, they cannot fail to carry pollen from one flower to another. (6/7. Delpino has described 'Bot. Zeitung' 1867 page 277 and 'Scientific Opinion' 1870 page 135, the structure of the flowers in this genus, but he was mistaken in thinking that self-fertilisation is impossible, at least in the case of the present species. Dr. Dickie and Professor Faivre state that the flowers are fertilised in the bud, and that self-fertilisation is inevitable. I presume that they were misled by the pollen being deposited at a very early period on the pistil: see 'Journal of Linnean Society Botany' volume 10 page 55 and 'Variabilité des Espèces' 1868 page 158.)

According to Delpino, bees eagerly visit the flowers in North Italy, but I have never seen any insect visiting the flowers of the present species in my hothouse, although many plants grew there during several years. Nevertheless these plants produced plenty of seed, as they likewise did when covered by a net; they are therefore fully capable of self-fertilisation, and have probably been self-fertilised in this country for many generations. As they are cultivated in pots, and are not exposed to competition with surrounding plants, they have also been subjected for a considerable time to somewhat uniform conditions. This, therefore, is a case exactly parallel with that of the common pea, in which we have no right to expect much or any good from intercrossing plants thus descended and thus treated; and no good did follow, excepting that the cross-fertilised flowers yielded rather more seeds than the self-fertilised. This species was one of the earlier ones on which I experimented, and as I had not then raised any self-fertilised plants for several successive generations under uniform conditions, I did not know or even suspect that such treatment would interfere with the advantages to be gained from a cross. I was therefore much surprised at the crossed plants not growing more vigorously than the self-fertilised, and a large number of plants were raised, notwithstanding that the present species is an extremely troublesome one to experiment on. The seeds, even those which have been long soaked in water, will not germinate well on bare sand; and those that were sown in pots (which plan I was forced to follow) germinated at very unequal intervals of time; so that it was difficult to get pairs of the same exact age, and many seedlings had to be pulled up and thrown away. My experiments were continued during three successive generations; and in each generation the self-fertilised plants were again self-fertilised, their early progenitors in this country having probably been self-fertilised for many previous generations. In each generation, also, the crossed plants were fertilised with pollen from another crossed plant.

Of the flowers which were crossed in the three generations, taken together, a rather larger proportion yielded capsules than did those which were self-fertilised. The seeds were counted in forty-seven capsules from the crossed flowers, and they contained on an average 9.95 seeds; whereas forty-eight capsules from the self-fertilised flowers contained on an average 8.45 seeds; or as 100 to 85. The seeds from the crossed flowers were not heavier, on the contrary a little lighter, than those from the self-fertilised flowers, as was thrice ascertained. On one occasion I weighed 200 of the crossed and 106 of the self-fertilised seeds, and the relative weight of an equal number was as 100 for the crossed to 101.5 for the self-fertilised. With other plants, when the seeds from the self-fertilised flowers were heavier than those from the crossed flowers, this appeared to be due generally to fewer having been produced by the self-fertilised flowers, and to their having been in consequence better nourished.

Charles Darwin

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