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Show 392 MEA.NS OF CHOSS-FERTILISATION. CHAP. X. after the interval of sev ral hours, pollen fro1n the same species be placed on the stigma, tho effects of the fonner are wholly obliterated, excepting in so1ne rare cases. If two varieties are treated in the same n1anner, the result is analogous, though of directly opposite nature ; for pollen from any other variety is often or generally prepotent over that from the same flower. I will-give some instances: the pollen of Mi1nulus luteus regularly falls on the stigma of its own flower, for the plant is highly fertile when insects are excluded.. Now several flowers on a remarkably constant whitish variety were fertilised without being castrated with pollen from a yellowish variety; and of the twenty-eight seedlings thus raised, every one bore yellowish flowers, so that the pollen of the yellow variety completely overwhelmed that of the mother-plant. Again, lberis umbellata is spontaneously self-fertile, and I saw an abundance of pollen from their own flowers on the stig1nas ; nevertheless, of thirty seedlings raised fro1n non-castrated flowers of a crimson variety crossed with pollen fr01n a pink variety, twenty-four bore pink flowers, like those of the n1ale or pollen-bearing parent. In these two cases flowers were fertilised with pollen from a distinct variety, and this was shown to be prepotent by the charactet of tho offspring. Nearly similar results often follow when two or more self-fertile varieties are allowed to grow near one another and are visited by insects. The common cabbage produces a large number of flowers on the sa1ne stalk:, and when insects are excluded these set many capsules, moderately rich in seeds. I planted a white l.Cohl-rabi, a purple Kohl-rabi, a Portsmouth broccoli, a Brussels sprout, and a S~1gar-loaf cabbage near tog ther and loft them uncovered. Seeds collected from each kind were sown in separate beds; and the majority of the seedlings in CHAP. X. PRE~OTENT POLLEN. 393 all five beds were mongrelised in the most complicated manner, some taking more after one variety, and some after another. Tho effects of the Kohl-rabi were particularly plain in the enlarged stems of n1any of the seedlings. Altogether 233 plants were raised, of which 155 were n1ongr~lised in the plainest manner, and of the remaining 78 not half were absolutely pure. I repeated the experiment by planting ncar together two varieties of cabbage with purple-green and whitegreen lacinated leaves; and of the 325 seedlings l·aised from the purple-green yariety, 165 had whitegreen and 160 purple-green leaves. Of the 466 seedlings raised fro1n the white-green variety, 220 had purple-green and 246 white-green leaves. These cases show how largely pollen from a neighbouring variety of the cabbage effaces the action of the plant's own pollen. We should boar in mind that pollen must be carried by the bees from flower to flower on the same large branching stem much more abundantly than from plant to plant; and in the case of plants the flowers of which are in some degree dichogamous, those on the same stem would be of different a.o·es and b ' would thus be as ready for mutual fertilisation as the flowers on distinct plants, were it not for the prepotency of pollen from another variety.* Several varieties of the radish (Raphanus sativ~ts), which is moderately self-fertile when insects are ex· eluded, were in flower at the same time in my garden. Seed was collected from one of them, and out of twentytwo seedlings thus raised only twelve were true to their kind.t * A Writer in the 'Gardeners' Chronicle' (1855, p. 7:-30) sa.ys ItBha t he. planted a bed of turnips n rass~ca rapa l and of rape (B. apus) elose togetho1·, and f::owed the seeds of tlte former. The result was that l"Carccly one seedling was tr110 to its J,ind, and several clo:'lcly r0semblcd rape. t Duhamel, as quoted by God .. |
