354 



BOTANY 



The Ovule 



The macrosporangium, or ovule, in many of the lower Angiosperms 

 (many Aracese, Peperomia), is formed directly from the apex of the 

 floral axis, as it is in Taxus. Usually it is an outgrowth of the 

 carpel. In either case the tissue from which it grows is called 

 the Placenta. The development of the ovule is very similar to that 



FIG. 319. A-D, Naias flexilis. Development of the ovule. A, section of very young 

 ovule, formed from the axis of the shoot, showing the sub-epidermal archesporial 

 cell (x 400) ; car, the carpel. B, an older ovule, with the first integument, in 1 , 

 and the tapetal cell, t, cut off from the archesporium. C, a still older stage, the 

 tapetal cell and sporogenous cell both divided, the latter into three. D, young 

 ovule inclosed in the ovary (x 200). E, Sparganium simplex, section of mature 

 ovule; in 1 , iri 2 , the integuments; fun, funiculus; ma, embryo-sac. 



of the Gymnosperms. Sometimes but a single integument is present, 

 but more commonly there are two. When the growth of the ovule is 

 alike on all sides, it is symmetrical, "erect," or " orthotropous " ; 

 where growth is stronger on one side it is bent over, " anatropous." 

 More rarely, as in the Cruciferse and Caryophyllacese, it is bent in 

 the middle, or evenly curved, " campylotropous." 



The archesporium can usually be traced back to a single hypoder- 

 mal cell (Fig. 319). This may at once give rise to the embryo-sac 



