VEGETATIVE REPRODUCTION. 243 



The s|)orangium forms the body of the ovule (fig. 254). In 

 the interior the mother cells of the miegaspores are diffeien- 

 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 Ijegin to develop ; in most only one lo four. E\'en 

 when several megaspores liegin to form it is rare for more 

 than one to reach perfection ; the remainder disappear 

 almost completely. 



341. Indehiscence. — The megaspore never esca])es 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 tliis apparent cavity, the megaspore of seed plants 

 has long been called the embryo-sac. 



342. Integuments. — The sporangium is surrounded Ijy 

 one or two iutegumenls. These arise as outgrowths from 



the tissues adjacent. If the sjio- ^-„ ^ _^ 



rangium is to ha\'e two coats, the w'""'A Wf" \ 



inner appears first as a low ring %;■-;'/ /""--iP J 



around its base gradually growing %. \ , 'r'"/ 



up around it ; the outer shortly . 



appears in the same way (fig. 255). 



I'iG. 2S5- — iwo very young ovules 



These integuments, as well as the of die California poppy [F.sch- 



S'sholtzia^.^Zftw from the outside. 



sporangium, often grow u^s^■m- p, somewhat older than j. „c. 



^ ' ' the sporangium ; /V, the inner in- 



metrically, so that at the maturit)' legument; />■, the outer Integu- 



^ . , nient;y>, the stalk. Magnified 



of the megaspore the ovule is oltcn Modiam.— .\fter 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 cither upon the axis 

 itself or upon the carpels. Wlieii they are borne upon 

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



