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 ^j^[ 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- 

 rangium is to have two coats, the 



inner appears first as a low ring 

 around its base gradually growing 

 up around it ; 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 (Esck- 



scholtzia), seen from the outside. 



Sporangium, Often grOW imsym- B, somewhat older than A. nc, 



the sporangium ; jc, the inner in- 



metriCally, SO that at the maturity tegument; /r, the outer integu- 

 ment ; f>i, 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 



