GROUPS OF PLANTS. 



67 



thallus or nutritive layer a few archegonia are borne. It should 

 be stated that sometimes the archegonia are developed very early 

 on the prothallus tissue, but usually they are developed after the 

 spores have escaped from the sporangium. After fertilization 

 of the Qgg a multicellular embryo develops which shows the fol- 

 lowing parts (Fig. 42) : (i) An elongated cell or row of cells 

 which extends into the tissues of the prothalfus for the purpose of 

 obtaining nutriment; (2) a root; and (3) a stem bearing at its 

 tip (4) two leaves, or cotyledons. One of the specially notable 



^ ^ 



Fig. 43. Successive stages in the germination of the microspores of a Selaginella: 

 p and w, cells of the prothallus; s, cells giving rise to sperms. A, B, D, views of spores from 

 the side; C, view from the back; in E the cells surrounding the sperm mother cell are dis- 

 organized; F, two biciliate sperms. After BelajefF. 



characters of the plants of the Selaginella group is, as Ave have 

 seen, the great reduction in size of the gametophyte which in 

 the case of the microspore does not enlarge beyond the wall of 

 the spore, and in the case of the megaspore only partly protrudes 

 be_vond the wall of the spore. 



Isoetes. This is a genus of aquatic or marsh plants known 

 as quillworts. The plants produce a number of filiform roots 

 which penetrate the mud. and a compact tuft of rush-like leaves. 

 The plants are heterosporous, as in Selaginella. The sporangia 

 are borne in the axils of the leaves, the outer leaves bearing the 

 megasporangia and the inner leaves the microsporangia. The 



