The Effects of Cross & Self-Fertilisation in the Vegetable Kingdom · Charles Darwin
MEANS OF FERTILISATION.
Medicago lupulina (Leguminosae).--On account of the danger of losing the seeds, I was forced to gather the pods before they were quite ripe; 150 flower-heads on plants visited by bees yielded pods weighing 101 grains; whilst 150 heads on protected plants yielded pods weighing 77 grains. The inequality would probably have been greater if the mature seeds could have been all safely collected and compared. Ig. Urban (Keimung, Bluthen, etc., bei Medicago 1873) has described the means of fertilisation in this genus, as has the Reverend G. Henslow in the ‘Journal of the Linnean Society Botany’ volume 9 1866 pages 327 and 355. Nicotiana tabacum (Solanaceae).--Fully self-fertile. Ipomoea purpurea (Convolvulaceae).--Highly self-fertile. Leptosiphon androsaceus (Polemoniacae).--Plants under a net produced a good many capsules. Primula mollis (Primulaceae).--A non-dimorphic species, self-fertile: J. Scott, in ‘Journal of the Linnean Society Botany’ volume 8 1864 page 120. Nolana prostrata (Nolanaceae).--Plants covered up in the greenhouse, yielded seeds by weight compared with uncovered plants, the flowers of which were visited by many bees, in the ratio of 100 to 61. Ajuga reptans (Labiatae).--Set a good many seeds; but none of the stems under a net produced so many as several uncovered stems growing closely by. Euphrasia officinalis (Scrophulariaceae).--Covered-up plants produced plenty of seed; whether less than the exposed plants I cannot say. I saw two small Dipterous insects (Dolichopos nigripennis and Empis chioptera) repeatedly sucking the flowers; as they crawled into them, they rubbed against the bristles which project from the anthers, and became dusted with pollen.
Veronica agrestis (Scrophulariaceae).--Covered-up plants produced an abundance of seeds. I do not know whether any insects visit the flowers; but I have observed Syrphidae repeatedly covered with pollen visiting the flowers of V. hederaefolia and chamoedrys. Mimulus luteus (Scrophulariaceae).--Highly self-fertile. Calceolaria (greenhouse variety) (Scrophulariaceae).--Highly self-fertile. Verbascum thapsus (Scrophulariaceae).--Highly self-fertile. Verbascum lychnitis.--Highly self-fertile. Vandellia nummularifolia (Scrophulariaceae).--Perfect flowers produce a good many capsules. Bartsia odontites (Scrophulariaceae).--Covered-up plants produced a good many seeds; but several of these were shrivelled, nor were they so numerous as those produced by unprotected plants, which were incessantly visited by hive and humble-bees. Specularia speculum (Lobeliaceae).--Covered plants produced almost as many capsules as the uncovered. Lactuca sativa (Compositae).--Covered plants produced some seeds, but the summer was wet and unfavourable. Galium aparine (Rubiaceae).--Covered plants produced quite as many seeds as the uncovered. Apium petroselinum (Umbelliferae).--Covered plants apparently were as productive as the uncovered. Zea mays (Gramineae).--A single plant in the greenhouse produced a good many grains. Canna warscewiczi (Marantaceae).--Highly self-fertile. Orchidaceae.--In Europe Ophrys apifera is as regularly self-fertilised as is any cleistogene flower. In the United States, South Africa, and Australia there are a few species which are perfectly self-fertile. These several cases are given in the second edition of my work on the Fertilisation of Orchids. Allium cepa (blood red var.) (Liliaceae).--Four flower-heads were covered with a net, and they produced somewhat fewer and smaller capsules than those on the uncovered heads. The capsules were counted on one uncovered head, and were 289 in number; whilst those on a fine head from under the net were only 199.]
Each of these lists contains by a mere accident the same number of genera, namely, forty-nine. The genera in the first list include sixty-five species, and those in the second sixty species; the Orchideae in both being excluded. If the genera in this latter order, as well as in the Asclepiadae and Apocynaceae, had been included, the number of species which are sterile if insects are excluded would have been greatly increased; but the lists are confined to species which were actually experimented on. The results can be considered as only approximately accurate, for fertility is so variable a character, that each species ought to have been tried many times. The above number of species, namely, 125, is as nothing to the host of living plants; but the mere fact of more than half of them being sterile within the specified degree, when insects are excluded, is a striking one; for whenever pollen has to be carried from the anthers to the stigma in order to ensure full fertility, there is at least a good chance of cross-fertilisation. I do not, however, believe that if all known plants were tried in the same manner, half would be found to be sterile within the specified limits; for many flowers were selected for experiment which presented some remarkable structure; and such flowers often require insect-aid. Thus out of the forty-nine genera in the first list, about thirty-two have flowers which are asymmetrical or present some remarkable peculiarity; whilst in the second list, including species which are fully or moderately fertile when insects were excluded, only about twenty-one out of the forty-nine are asymmetrical or present any remarkable peculiarity.
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