352 THE FLOWER 



in the region of the micropyle, which the tube enters, 

 then pushing between the cells of the nucellus and 

 entering the embryo sac. Here the tip bursts, setting 

 free the two male gametes. These are elongated and 

 often curved or shghtly twisted spirally, recaUing the 

 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 of 

 the sac (Fig. 59, D). 



The zygote or " fertiUsed egg " gives rise to the 

 embryo of the new plant, the secondary nucleus of 

 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 («) the sister 

 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 means 

 of these differences. Thus the floral leaves of each : 

 kind, particularly the stamens and carpels, vary from 

 a large and indefinite number (buttercup) to a small 

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

 floral leaves in each whorl may be quite free and separate 

 from one another ' (buttercup and Fig. 60, A) or they 



" This is expressed by the prefix apo- (Greek &n6, away from). 

 Thus the buttercup flower is apocarpous. 



