256 



MOSSES AND FERNS 



CHAP. 



cells give rise to the prothallium. In the larger cell divisions 

 take place in three directions and transform it into a nearly 

 globular cell mass, terminated by four quadrant cells, one of 

 which usually becomes the apical cell, much as in the flat pro- 

 thallium. In exceptional cases the first divisions are in one 

 plane and a short filament results. 



As soon as the apical cell is established it grows in pre- 

 cisely the same way as the similar cell in the thallus of a 



Fig. 130. — A^tgiopteris evecia{'H.ofim). Germination of the spores, — A, B, X220; C, XT75; j/, spore 



membrane ; x, apical cell (after Jonkman). 



Liverwort, and produces a thallus of much the same form and 

 structure. As the prothallium grows older, however, a cross- 

 wall forms in the apical cell, and this is followed by a longi- 

 tudinal wall in the outer one, forming two similar cells which, 

 by further longitudinal divisions, may produce a row of marginal 

 initials, and the subsequent growth of the prothallium is due to 

 the divisions and growth of this group of initial cells (Fig. 

 131, A). 



At first the prothallium has a spatulate form, but before the 



