526 A TEXTBOOK OF BOTANY iCh. XIII 



synergida9, three antipodal cells and the fusion nucleus 

 (Figs. 190, 250). If fertilization occurs, the stimulus given 

 by the union of the second sperm nucleus with the fusion 

 nucleus (page 353), causes a resumption of the formation of 

 prothallus or endosperm, which, with the developing embryo, 

 fills the sac, — this arrangement representing apparently an 

 adaptation of economy which prevents the waste of endo- 

 sperm if no embryo is formed. Third, the pollen grain 

 (microspore) germinates more simply than in Gymnosperms, 

 omitting the prothallial cells (Fig. 188). Fourth, the embryo 

 possesses normally one or two cotyledons instead of several. 

 Fifth, the Angiosperms have utilized, both for pollination and 

 dissemination, other agencies in addition to the wind, upon 

 which the Gymnosperms almost wholly rely. In contiguity 

 with the stamens and pistils, they have developed nectaries, 

 of which the position is clearly shown by bright and con- 

 trasting leaves (petals) ; and the structural arrangements 

 are such that insects and small birds thus induced to visit 

 the flowers transfer the pollen. Further, in correlation with 

 a fuller utilization of these visits, the monoecious and dioecious 

 arrangements of the Gymnosperms are largely abandoned in 

 favor of the aggregation of stamens and pistils into the same 

 flowers (hermaphrodite), while various arrangements, struc- 

 tural and physiological (pages 286-294), prevent self-polli- 

 nation and ensure the advantageous cross-pollination. 

 Dissemination also is effected largely by the aid of animals, 

 in fitness whereto various structural features appear, — 

 hooks which catch in the fur, edible fruits with protected seeds 

 which are eaten, and other arrangements (pages 356-66). 

 These features are especially connected with fruits, of which 

 the development begins in the flower, wherein are produced 

 profound structural changes, such as enlarged receptacles, in- 

 ferior ovaries, and the like. Not all, however, of the multi- 

 form morphological modifications of flowers and fruits are to be 

 associated with pollination and dissemination, for many are 

 undoubtedly purely structural and mutational (pages 12-13). 



