190 



STRUCTURAL BOTANY 



embryo now enters on a period of rest. Its further 

 development is postponed until the germination of the 

 seed, and will be described below. In the meantime 

 we must shortly consider the changes which take place 



in other parts of the 

 ovule during its conver- 

 sion into a seed. 



cot 



g. Endosperm 

 In the Lilies the union 

 of the two polar nuclei 

 of the embryo-sac takes 

 place comparatively late 

 in fact, not till after 

 they have been joined by 

 the male, vermiform, 

 nucleus from the pollen- 

 tube (Fig. 86). The 

 further changes about to 

 be described, leading to 

 the formation of the en- 

 dosperm, only take place 



FIG. 90. -Older embryo of same if tnis curious process 

 seen m section. The basal cell n ,.,. , . , J 



is still larger than before. Its fertilisation has been 



enormous nucleus has two nu- accomplished. The de- 



cleolic^ cells which will form velopment of the endo- 

 the cotyledon : st t disc of cells , , 



from one side of which the stem Sperm usually starts 



arises later ; r, radicle. Magni- before that of the embryo. 



The triple nucleus of 



the embryo-sac divides, but without any formation of 

 cell-wall between the daughter-nuclei. The division 

 is repeated again and again in the same manner (see 

 Fig. 87). Soon the nuclei thus formed arrange them- 

 selves in the layer of protoplasm lining the wall of 

 the sac, and there they still go on dividing. All 

 this time the embryo-sac is increasing enormously 



