62 



STRUCTURAL BOTANY 



the prothallus soon takes the form of a short green 

 filament (see Fig. 29), each cell of which may produce 

 a root-hair. In most Ferns the root-hairs of the pro- 

 thallus remain 

 unicellular ; in a 

 few they become 

 multicellular. 



*P- ^&&3P^ Soon the 



transverse divi- 

 sions of the 

 filament cease, 

 an oblique wall 

 appears in the 

 terminal cell, 

 followed by 



another at right angles to it, and thus a wedge-shaped 

 apical cell is marked out. This goes on cutting off 

 segments to the right and left, the segments divide up 

 further, and soon the young prothallus becomes converted 

 into a flat cellular plate, 

 which for a time remains 

 only one cell thick (see 

 Fig. 30). As growth goes 

 on, the prothallus tends to 

 become heart-shaped, the 



FIG. 29. Very young prothallus of an Aspidium. 

 sp, membrane of spore ; r.h, first root-hair; 

 a, apical cell. Magnified 210. (E. S.) 



FIG. 30. Apex of young prothallus 

 of Aspidium, older than in Fig. 

 29, seen in surface view, a, apical 

 cell. Magnified 210. (E. S.) 



growing-point lying at the 



base of a depression between 



two lobes. This is due to 



the fact that the apical 



meristem does not grow so fast as the older tissue which 



has been produced from it on either side. 



The single apical cell does not long maintain its in- 

 dependence. It soon divides up into a row of equivalent 



