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 « r 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 emhryo-sac. 



342. Integuments. — The sporangium is surrounded by 

 one or two integuments. These arise as outgrowths from 



the tissues adjacent. If the spo- ^-^ w 



rangium is to have two coats, the *r* | L 



inner appears first as a low ring A, V /'/'■Jf""' I 



around its base gradually growing , ^"y 



up around it ; the outer shortly , ^^^ 



appears in the same way (fig. 255). 



_, . 11 1 Fig. 255.— Two very young ovules 



I hese integuments, as well as the of the California puppy ,/„/,. 



scholtzia I, seen from the outside. 

 sporangium, often grow unsvm- /•'. somewhat older than 



. '. the sporangium ; fc, the inner in- 



metncally, so that at the maturity tegument; /■>■, the outer integu 



ment; ///, the -ulk. Magnified 

 Of the megaspore the OVUle is Often [4odiam Mtei 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 



