MARATTIALES 



275 



mass, terminated by four quadrant cells, one of which usually 

 becomes the apical cell, much as in the flat prothallium. 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 precisely 

 the same way as the similar cell in the thallus of a Liverwort, 

 and produces a thallus of much the sanie form and structure. 

 As the prothallium grows older, however, a cross-wall forms in 



Fig. 149. — Angiopteris evecta. Germination of the spores, — A, B, X220; C, X175: 

 spj spore membrane; x, apical cell (after Jonkman). 



the apical cell, and this is followed by a longitudinal wall in the 

 outer one, forming two similar cells which, by further longi- 

 tudinal 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. 150, A). 



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

 single apical cell is replaced by the group of marginal initials, 

 the outer cells of the segments grow more rapidly than the 

 inner ones, and the segments project beyond the apical cell, 



