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Show 3G6 MEANS OF FERTILISA'riON. CIIAI>, X. CHAPTER X. MEANS OF FERTILISATION. Sterility ann fertility of phl..nts when in. ccts arc cxc1udod-Tho monns by wl1ich flowers are cro~s-fertiliscd-Strncturos favourable to selffertilisation- Hdation between the structure and conspicuousness of flowers, tho vis its of insects, and tho a<lvnntagcs of cross-fertilisation- Tho means by which flowers :ne fertilised with pollen from a distinct plant-Greater fcrtllising power of such pollen -Anemophilous ~<pccies-Convorsion of anomophilous speci ·s into ontomophilous- Origin of r- ctnr-Anemophilous phmt:s gon crnJly have tlwir sexes Bopara.ted -Conversion of diclinous into henuapbrodite :flowers-rrrocs often have their sexes separated. IN the introduetory chapter I briefly specified the various means by which cross-fertilisation is favoured or ensured, namely, the separation of the sexcs,- the 1naturity of tho n1ale and fc1nale sexual olon1onts at different periods, - tho hetorostylod or dimorphic and trimorphic condition of certain plants,- many n1echanical contrivances,-the more or less complete inefficiency of a flower's own pollen on the stignut,-ancl the prepotency of pollen fron1 any other individual over that fro1n the san1o plant. Son1e of those points require further consideration; but for full details I nrust refer tho reader to the several excellent 'Yorks n1entioned in tho introduction. I will in the first place give two lists : tho fir~t, of plants which arc either quitE~ sterile or produce less than <:tbout half the full complement of seeds, when jnsects are excluded; and a second list of plants which, when thus treated, arc fully fertile or produce at least half the full complement CHAP. X. MEANS OF FERTILISATION. 357 of seecls. Those lists have boon con1pilou fro1n th · several previous tables, with some additional cases fron1 1ny own observations and those of others. Tho spoeio. are arrangou nearly in tho orclor followed by Lindley in his ' V cgotablo J(jngclom.' The reader shoul<l observe that the sterility or fortiljty of the plants in those two lists depends on two ·wholly distinct causes; namely, tho absence or presence of the proper means by which pollen is applied to the stigma, and its los: or greater efficiency when thus applied. As it is obvious that with plants in which tho sexes are separate, pollen must be carried by some means from flower to flower, such species are excluded from the lists; as are likewise dimorphic and tri1norphic plants, in whicl1 the same necessity occurs to a limited extent. Experience has proved to me that, independently of the exclusion of insects, the seed-bearing power of a phtnt is not lessened by covering it while in flower under e:t thin net supported on a frame; ancl· this might indeed have been inferred from the consiJ.eration of the two following lists, as they include a considerable nun1ber of species belonging to tho sa1no genera, some of which are quite sterile and others quite fertile when protected by a net from the access of insects. List of Plants which,. when Insects are excluded, are eithe1· quite sterile, or prod,uce, as far as I co~~zd Judge, less than half the nu1nber of Seeds produced by unprotected Plants. Passijlora alatct, racemosa, cmrulea, eclulis, laurifulir.t, and some individuals of P. quoclrangularis (Passifl.oracero), are quito sterile under those conditions: see 'Variation of Animals and Plants under Domestication,' chap. xvii. 2nd edit. vol. ii. p. 118. Viola canina (Violacoce).-Perfect flowers quite sterile unless fertilised by bees, or artificially fertilised. |
