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Show 388 MEANS OF CROSS-FERTILISATION. CHAP. X. It should be re1nombored that in two of the cases in which highly self-fertile varieties appeared a1nongst n1y experimental plants, namely, with Mi1nnlus and Nicotiana, snch varieties were greatly benefited by a cross with a fresh stock or with a slightly different variety ; and this likewise was tho case with the cultivated varieties of Pisum ~ativun~ and Lathyrus odoratus, which have been long propagated by selffertilisation. Therefore until the contrary is distinctly proved, I n1ust believe that as a g neral rule s1nall and inconspicuous flowers are occasionally intercrossocl by insects; and that after long-continued self-fertilisation, if they are crossed with pollen brought from a plant growing under somewhat different co11ditions, or descended from one thus growing, their offspring would profit greatly. It cannot be admitted, under our present state of knowledge, that self-fertilisation continued during 1nany successive generations is ever the most beneficial method of reproduction. The Means which favour or ens.ure Flowers being fertilised with Pollen fron~ a distinct Plant.-Wo have seen in four cases that seedlings raised from a cross between flowers on the same plant, even on plants appearing distinct- from having been propagated by stolons or cuttings, were not superior to seedlings from self-fertilised flowers; and in a fifth case (Digitalis) superior only . in a slight degree. Therefore we might expect that with plants growing in a state of nature a cross between the flowers on distinct individuals, and not n1orely between the flowers on the sa1ne plant, would generally Reen, by humble-bees. Tinzmann (as quoteu in 'Gardm10r::;' Chronicle,' 1~46, p. 183) found that some of the vtuietics did not bcnr seed when fertilised wit.lt pollen from tile same van·e t y, but were fertile with that from un· other variety. CHAP. x. MEANS OF CROSS-FERTILISATION. 389 or often be effected by some means. The fact of bees and of some Diptcra visiting the flowers of the same species as long as they can, instead of promiscuous! y visiting various species, favours the intorcrossing of distinct plants. On the other hand, insects usually search a large number of flowers on the same plant before they fly to another, and this is opposed to crossfertilisation. The extraordinary number of flowers which bees are able to search within a very short space of time, as will be shown in a future chapter, increases the chance of cross-fertilisation ; as does the faf'.t that they are not able to perceive without entering a ~ower whether other bees have exhausted the nectar. For instance, H. Muller found* that four-fifths of the ~owers of La1nium album which a humble-bee visited had been already exhausted of their nectar. In order that distinct plants should be intercrossed, it is of course indispensable that two or 1nore individuals should grow near one another; and this is generally the case. Thus A. de Oandolle remarks that in ascending a mountain t~e individuals of the same species do not commonly disappear ne~r its upper li1nit quite gradually, but rather abruptly. This fact can hardly be explained by the nature of the conditions, as these graduate away in an insensible manner, and it probably depends in large. part on vigorous seedlings being produced only as h1gh up the mountain as many individuals can subsist together . With respect to dimcious plants, distinct individuals must always fertilise each other. With monmcious plants, as pollen has to be carried from flower to flower, there will always be a good chance of its being carried from pl~~t to plant. Delpino has also observed t the * 'Die Befruchtung,' &o. p. 311. . t 'Ult. Osservazioni,' &c., part ii. fasc. ii. p. 337. |