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Show 440 GENERAL RESULTS. CHAP. XH has been warmly disputed by others. A recent and competent judge, Prof. Asa Gray,* leans t~ tho side of Andrew ICnight, which seems to me, fron1 such evidence as I have been able to collect, tho n1ore probable vie'Vv, notwithstanding Inany. ~pp~seu facts. The 1neans for favouring cross-fortlhsatiOD: and preventing self-fertilisation, or ~onvorsely for ~avouring self-fertilisation and prevent1ng to a certtnn extent cross-fertilisation, are wonderfully cliversifiou; ancl it is remarkable that these differ widely in clos Jy allied plants,t-in the ~p~cies of the smne genus, ~nd som~times in the individuals of the same species. It Is uot rare to find hermaphrodite plants and others with separated sexes within the same genus; and it is common to find some of the sp cies dichogamons and others maturing their sexual elements simultaneously. The dichogamous genus Saxifraga contains proterandrous and proterogynous speci s. + Several genera include both heterosty led (dimorphic or trimorphic fonns) and homosty led species. Ophrys offers a remarkable instance of one species having its structure 1nanifestly adapted for self-fertilisation, and other species as 1nanifestly adapted for cross-fcrtili. ation. ~orne con -generic species aro q nite sterile and others quite fertile with their own poll n. From those sevo~·al causes we often find within the same genus speCies which do not produce seeds, while others produce .an abundance, when insects are excluded. Soinc spec1es bear cleistogon flowers which cannot be crossed, as well as perfect flowers, whilst others in the snme g·enus * ' Darwinifma: Essays and Rovie,vs pertaining to Darwinism,' 1876, p. 3;3~. + Hildebrand has insisted stl·'ongly to this effect in hi::; valuable obscrvatious on the fer-tilisatinn of the Gramino::n: 'Mon: ttshcricht K. Almd. Berlin,' Oct. l 72, p. 7G3. · . , ! Dr. E1tglcr, 'Bot. ZCLtung, 1868, p. 83:-1. CHAP. XII. GENERAL RESULTS. 441 never produce cleistoo·ene flowei·s So · · t e:- • me spemes ex1 under two forn1s, the one bearing conspicuous flowers adapt?d for cross-fertilisation, the other bearing inconspicuous flowers adapted for self-fertilistion whilst other species in. the ::;arne genus present only ~ single form. Even With the individuals of the same species th~ degree of self-sterility varies greatly, as in Reseda: With polygan1ous plants, the distribution of the sexes d.iffers i~ the individuals of the same species. The relatrve penod at which the sexual elements in the same flower arc mature, differs in tho varieties of Pelargonium ; a~d Car~iere gives several cases,* showing that the penod vanes according to the temperature to which the plants are exposed. This extraordinary diyersity in the means for favouring or preventing cross- and self-fertilisation in closely allied fonns, probably depends on the results of both processes being highly beneficial to the species, but directly opposed in many ways to one~another and dependent on variable conditions. Self-fertilisation assures the production of a large supply of seeds; and the necessity or advantage of this will be determined by the average length of life of the plant, which largely depends on the ainount of destruction suffered by the seeds and seedlings. This destruction follows fron1 the n1ost various and variable causes, such as the presence of animals of several kinds, and the growth of surrounding plants. The possibility of cross-fertilisation depe11:ds n1ainly on the presence and number of certain insects, often of insects belonging to special groups, and on the degree to which they are attracted to the flowers of any particular species in preference to other fiowers,-all circumstances likely to change. * 'Dos Y ttrietes,' 18G5, p. ao. |
