VEGETATIVE REPRODUCTION. 243 



The sporangium forms the body of the ovule (fig. 254). In 

 the interior "the mother cells of the megaspores are differen- 

 tiated early, the outer tissues forming the wall of the sporan- 

 gium (fig. 254). In a few ovules as many as 20 to 40 mega- 

 spores begin to develop ; in most only one to four. Even 

 when several megaspores begin to form it is rare for more 

 than one to reach perfection ; the remainder disappear 

 almost completely. 



341. Indehiscence. — The megaspore never escapes from 

 the sporangium ; a condition which necessitates many adapta- 

 tions. (See further ^[ 358, 414). The protection of the 

 megaspore by the sporangium renders a thick wall unneces- 

 sary. For this reason the megaspore looks more like a cavity 

 in the ovule than like a spore. Because an embryo appears 

 later inside this apparent cavity, the megaspore of seed plants 

 has long been called the embryo-sac. 



342. Integuments. — The sporangium is surrounded by 

 one or two integuments. These arise as outgrowths from 



the tissues adjacent. If the spo- jf~^- -nc _ 



rangium is to have two coats, the jtfPNt Ip*\ 



inner appears first as a low ring i»|fgf /^•^f' l '' ill J 



around its base gradually growing ^L \ - djr 

 up around it j the outer shortly . ^"^ 



appears in the same way (fig. 255). 



Fig. 255. — Two very young ovules 



These integuments, as well as the of the California poppy {E*ck- 



scholtzia), seen from the outside. 

 Sporangium, Often grOW linsym- B, somewhat older than -7. nc, 



the sporangium ; j'c, the inner in- 

 metncally, SO that at the maturity tegument; fr, the outer integu- 



. ment; fn, the stalk. Magnified 



Of the megaspore the OVUle IS often 140 diam.— After Duchartre. 



variously curved (figs. 254, 255, 256). The megaspore it- 

 self may be distorted by this means so as to lose still more 

 its likeness to a spore. 



343. Location. — Ovules are borne either upon the axis 

 itself or upon the carpels. When they are borne upon 

 the axis they may be either uncovered, as in the yew 



