ANGIOSPERM^E 



371 



zation, as is the case in many Gymnosperms. This is especially true 

 of many Orchids, where the whole development of the ovules may 

 take place subsequent to pollination. 



..cot 



ft. 



FIG. 339. Naias flexilis. Development of embryo. A-E, longitudinal sections 

 (X 250). F, transverse section of older embryo (X 200) ; SMS, suspensor-cell ; 

 k, a free endosperm-nucleus. 



The Embryo 



The Embryo (Figs. 339-341) may remain very rudimentary, as in the Orchids, 

 where it is a nearly globular mass of perfectly undifferentiated tissue. On the 

 other hand, the embryo in the ripe seed 

 may be large, and completely fill the 

 embryo-sac, as in Naias and Sagittaria, 

 and the organs of the young sporophyte 

 are well developed. 



The fertilized egg usually divides by a 

 transverse wall into two cells, of which the 

 basal one, which is in contact with the 

 upper end of the embryo-sac, does not 

 divide further, but may become much 

 enlarged, and serve as an organ of absorp- 

 tion. The outer cell may at once develop 

 into the young embryo, or it may undergo 

 several transverse divisions, and form a 

 suspensor with the embryo at the apex 

 (Fig. 339). The latter, in typical cases, 

 develops the single cotyledon from the 

 apical portion, while the root arises from 

 the region which is in contact with the 

 suspensor. The stem-apex is lateral in 

 origin, and is first recognizable at a late 

 stage in the development of the embryo. In this respect the Monocotyledons 

 resemble Isoetes. 



FIG. 340. Naias flexilis, older em- 

 bryos. (A, X 200; B, X 70) ; cot, 

 cotyledon; at, stem-apex; r, root; 

 sws, suspeusor. 



