ORGANIC EVOLUTION 



125 



relationship with the simple mosses. The leaves of mosses 

 are better developed, possessing a mid rib, and usually a 



Fig. 74. Three "leafy liverworts" Jungermannia. three species. (Copiedfrom 

 Gray's Manual of Botany, a classical manual of North American plants). 



margin of close knit cells, and are often borne on erect 



stems. 



A comparison of the sporophyte phase in bryophytes 



will be most instructive. It 

 is in this group we first find 

 alternation of generations 

 fully established with an 

 organized sporophyte. While 

 there is reproduction by 

 means of both sex cells and 

 spores among the thallophy- 

 tes, there is rarely the regular 

 alternation between them and 

 never such differentiation of 

 a distinct sporophyte phase as 

 we find there. This phase 

 finds a simple expression in 

 Riccia. From the egg there de- 

 velops a single spherical shell 

 of (ephemeral) sterile cells 

 containing spores (Fig. 75) 



Fig. 75. Section of the simple sporo- 

 phyte of Riccia naians, inclosed with- 

 in the distended wall of the archego- 

 nium). Note that all the cells of the 

 sporophyte are spores save a single 

 peripheral layer that is dotted in the 

 figure (after Chamberlain). 



