GROUPS OF PLANTS. 



57 



leading to the dififcrentiation of the spores seem to be connected 

 with nutrition, those nuclei which are in more favorable positions 

 giving rise to larger and better nourished spores which eventually 

 lead to the formation of the megaspores, and those which are 

 less favorably placed leading to the microspores. 



The subject of heterospory is one of great interest, and when 

 it is pointed out that all of the higher plants are heterosporous 

 the subject has even more interest. 



FILICALES. 



General Characters. On germination the asexual spore 

 in the Filicales or Ferns gives rise to a thallus-like body known as 

 the prothallus which is frequently dorsiventral and in a number 



Fig. 33. Male fern [Aspidium (Nephrcdium or Dryopteris) Filix mas]. A, prothallus 

 of gametophyte as seen from the under (ventral) side showing archegonia (ar), antheridia 

 (an), and rhizoids (rh); B, prothallus showing young plant (sporophyte) which has devel- 

 oped from an oospore and is still connected with the gametophyte, roots (w), and the first 

 leaf (b). After Schenck. 



of cases somewhat heart-shaped, but varies considerably in out- 

 line, being sometimes more or less tuberous. The prothallus is 

 frequently but a few millimeters in diameter and the cells usually 

 contain chloroplasts. On the under or ventral surface rhizoids 

 are usually present (Fig. 33, rh). The sexual organs usually 

 arise on the lower surface (Fig. 33) but they may develop on the 

 upper or dorsal surface or even laterally. A single prothallus 



