482 



BOTANY 



TART II 



type in various Avays, which Avill be described in the special part 

 below. 



In Angiosperms, on the other hand, a union of the two sexes in 

 the one flower, which is thus hermaphrodite, and the investment of 

 the flower by coloured leaves (diff'ering in appearance from the foliage 

 leaves), forming a PERIANTH, is the rule (Figs. 425, 426). In contrast 

 to the UNISEXUAL or diclinous flower with the sporophylls arranged 

 spirally on an elongated axis, which is characteristic of the Gymno- 

 sperms, the perianth leaves and sporophylls in the Angiosj)erms are 

 usually borne in whorls on a greatly shortened axis. The arrange- 

 ment OF THE FLORAL LEAVES IN WHORLS, THE COLOURED PERIANTH, 

 AND THE HERMAPHRODITE NATURE OF THE FLOWERS ARE THUS 



CHARACTERISTIC OF Angiosperms, although these features do not 



apply without exceptions to 



all angiospermic flowers. 



4 V 1 These difterences depend 



J' f) /'~'^'^7~i. on the important factor of 



the means of pollination. 



>/'S^ 



Fio. 425. — Flowet of Pcwunia peregrina, in longitudinal 

 section, k and c, Perianth ; a, androecium ; g, 

 gynaeceum. (i nat. size.) 



Fig. 426. — Flower of Acorns Calamus, 

 pg, perigone ; a, androecium ; g, 

 gynaeceum. (Enlarged. After 

 Engler.) 



When, as is the case with the Gymnosperms and the catkinate flowers 

 of Angiosperms, this function is performed by the wind, the elongation 

 of the axis and the absence of an investment of leaves around the 

 female receptive organ are advantageous. When, on the other hand, 

 pollination is eff'ected by insects or birds, the conspicuousness given 

 by the presence of a perianth and other attractions, such as scent or 

 sweet-tasting substances, are necessary. The form of the flower, the 

 arrangement of the sporophylls in it, and the place at which nectar is 

 secreted must be adapted to the visiting insects (cf. p. 493, oecology 

 of the flower). It is to this that the variety of form and colour 

 exhibited in the flowers of Angiosperms must be ascribed. 



The monoecious and dioecious conditions which are the rule in the gynmospermic 

 flower also occur in Angiosperms ; they are, however, much less common than the 

 hermaphrodite condition. Tiie association of hermaphrodite and unisexual flowers 

 on the same plant leads in certain Angiosperms to what is known as polyg.\my. 

 When hermaphrodite and unisexual flowers are distributed on distinct individuals 



