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Show 380 MEANS OF CROSS-FERTILISATION. CHAP. X. From the various facts now given, it is evident that most flowers are adapted in an admirable Inanner for cross-fertilisation. Nevertheless, the greater number likewise present structures which aro Inanifestly adapted, though not in so striking a rn.annor, for selffertilisation. The chief of these is their honnaphrodite condition; that is, their including within the same corolla both the male and female roprod ucti vo organs. These often stand close together and are n1ature at the same time ; so that pollen from the smne flower cannot fail to be depositeu at th proper period on the stigma. There are also various details of structure .-1dapted for self-fertilisation.* Such strnctnres are best shown in those curious cases discovered by H. Muller, in which a species exists under two forms,one bearing conspicuous flowers fitted for cross-fertilisation, and the other. sinaller flowers fitted for self-fertilisation, with many parts in the latter slightly modified for this special purpose. t \s two objects in most resp cts opposed, namely, cross-fertilisation and self-fertilisation, have in many cases to be gained, we can understand the co-existence in so many flowers of structures which appear at :first sight unnecessarily coinplex and of an opposed nature. We can thus understand the great contrast in structure between eleistogene flowers, which are adapted exclusively for self-fertilisation, and ordinary flowers on the same plant, which are adapted so as to allow of at least occasional cross-fertilisation.t Tho former are always * H. Muller,' Die Bcfruchtun!Y' &c. p. 448. 5 ' t '~ature,' 1873, pp. 44, 433. t Fntz Miillor has discovered in. the an~mal kingdom (' J enmsche Ze1tschr.' B. iv. p. 451) a case curiously analogous to that of the phwts which bear cleistog( •ne and perfect ~?wer_s. 1:-!e finds in the ue~t:-; of 1 ermites,_1 ~ Brazil males and females wit imperf'e ct win~s, w1 l l·C h d0 not leave the nests and propa_gaje th~ spedes in a cleistogcne manner CuAP. X. CROSS-FERTILISATION. 381 n1inute, .completely closed, with their petals more or less rudimentary and never brightly coloured; thev never secret.e nec. tar, .n ever are odoriferou.s , h ave very" small anthers which p1oduce only a few gra1ns f 11 d h . . o po en, a. n t. md r hst 1gmas are but little developed . B ean·n g. 1~ min t at some flowers are cross-fertilised b the wind (called anemophilous bv Delpino) and yth b . , , o ers y Insects (called entomophilous ), we can further und:rstand,_ .as was point~d out by me several years ago, the great contrast In appearance between these ~wo classes of flowers. Anemophilous flowers resemble ~n many r~spects cleistogene flowers, but differ widely ~n not being closed, i.n pr_oducing an extraordinary ~mount ~f pollen which Is always incoherent, and In the stigma ofte~ being large I y developed or plumose. We certainly owe the beauty and odour of our flowe.rs and the storage of a large supply of honey to the existence of insects. On the Relation . between the Structure and Conspicuousness of Flowers, the Vis£ts of Insects, and the Advantages of Gross-fertilisation. It. has already been shown that there is no close relation between the n urn ber of seeds produced by flower.s when .crossed and self-fertilised, and the degree to whiCh theu offspring are affected by the two ·process~ s. I have also given reasons for believing that the Inefficiency of a plant's own pollen is in most cases b~t only if a fully-developed queen a~ter swarming does not enter : e old nest. The fullv-devu-oped males and fl'males are ;~mged, and individuals from distinct. nests can ltan.lly fail often 0 , ln~ercro~s. In the act of 8 " armmg they are destroyed in almost infinite numbers by a ilost of enemies, so that a queen may often f<til to enter an old nest· and tlten the imperfectly deve~ loped males and females propagate and keep up the stock. * 'Journal of Linn. Soc.' vol. vii. Bot. 1863, p. 77. |