352 THE FLOWER 



in the region of the micropyle, which the tube enters 

 then pushing between the cells of the nucellus anc 

 entering the embryo sac. Here the tip bursts, setting 

 free the two male gametes. These are elongated anc 

 often curved or slightly twisted spirally, recalling th( 

 spirally twisted male gametes of the lower plants 

 They appear to wriggle actively in the sac, one fusing 

 with the egg, the other with the secondary nucleus X5! 

 the sac (Fig. 59, D). 



The zygote or " fertilised egg " gives rise to the 

 embryo of the new plant, the secondary nucleus o; 

 the sac, to which three nuclei have now contributed 

 divides to form a new tissue, the endosperm, which fills 

 the sac, and at the expense of which the embryo grows 

 The endosperm nucleus is of the nature of a zygote 

 nucleus, being formed by the fusion of (a) the sistei 

 nucleus of the functional female gamete, (b) a corre- 

 sponding antipodal nucleus, (c) a male gamete. This 

 " triple fusion " is most unusual, and leads to irregulari- 

 ties in division of the endosperm " zygote " nucleus 

 The endosperm may thus be regarded as an " abnorm- 

 ally " formed embryo, which is sacrificed to the feeding 

 of the normal embryo. 



Varieties of Floral Form and Structure. The variety 

 of form and structure among flowers is exceedingly 

 great, and seed plants are classified largely by mean 

 of these differences. Thus the floral leaves of eac 

 kind, particularly the stamens and carpels, vary fron 

 a large and indefinite number (buttercup) to a sma 

 and definite number (apple, cherry, corncockle). Th 

 floral leaves in each whorl may be quite free and separat 

 from one another x (buttercup and Fig. 60, A) or the] 



1 This is expressed by the prefix apo- (Greek av6, away from 

 Thus the buttercup flower is apocarpous. 



