164 



MANUAL OF BOTANY 



young embryo down into the tissue filling the cavity of the spore, 

 the so-called endosperm. The epibasal cell forms four octants, 

 and from these arise the growing points of the stem and of two 

 cotyledons. As the growth of the embryo proceeds, a pseudo-foot 

 is developed from the hypocotyledonary region, and the direc- 

 tion of growth of the axis becomes changed, so that the stem 

 gradually curves upwards to emerge from the spore at the crack 

 at its apical region, through which the prothallium is partially 

 protruding. The foot remains embedded in the tissue of the 



endosperm and absorbs 

 Fig. J-1. its contents. The first 



root is developed from 

 cells in the interior of 

 the hypocotyl between 

 the foot and the sus- 

 pensor. 



The other member 

 of this section, Isoetes, 

 differs from Selaginella 

 very markedly in its 

 habit. It has a short 

 tuberous stem some- 

 what lobed externally, 

 from which spring 

 numerous leaves in a 

 cluster. The leaves 

 are long and narrow, 

 some of the cluster be- 

 ing fertile and some 

 sterile. The fertile 

 leaves bear large spo- 

 rangia in a kind of pit 

 upon the upper surface 

 at their base. The 

 microsporophylls are developed later than the macrosporophylls. 

 Each leaf bears a ligule as in Selaginella. Numerous roots 

 spring from the under side or base of the tuberous stem, and 

 these branch dichotomously. 



The stem is monostelic, and grows by a meristem, which con- 

 sists of several large so-called apical cells. The stele is com- 

 posed of a number of fused collateral bundles, which are common 

 and run out into the leaves. Outside the bast of the bundles is 

 a merismatic ring, which forms vascular tissue internally, and 



Fi'/. 921. Isoe/es lacustris. Lake Quill-wort. The 

 stem is small and corm-like, and bears its leaves, 

 wliicli are linear-cylindrical, in tufts. 



