MUSCI AND HEPATICAE 369 



out. In any organism with a life-cycle punctuated by the two stages of 

 the spore and the zygote, there are two possibilities of somatic expan- 

 sion, viz. in the diploid sporophyte and in the haploid gametophyte. 

 In the Bryophytes the second alternative has been fully exploited. 

 Their characters depend upon the development of the gametophyte 

 to the highest condition in which it is seen in Land Vegetation. The 

 details of this development run parallel with those of the sporophyte 

 in Vascular Plants, so that the two present a series of analogies. The 



FIG. 313. 



Ricciocarpus natans. Young sporogonia still surrounded by the archegonial 

 wall. The younger ( x 666) shows the wall of the sporogonium shaded, surrounding 

 the sporogenous cells. In the older ( x 560), these are separated as the free 

 spore-mother-cells. (After Garber.) 



most striking are seen in the organs of photo-synthesis, in the conduct- 

 ing tracts, and in the grouping of the organs of sex. In the sporo- 

 phyte of Vascular Plants the typical photo-synthetic organ is the 

 leaf-blade, with its ventilated mesophyll and stomatal control. In 

 the gametophyte of Mosses and Liverworts a similarly ventilated 

 structure is seen in the leaves of some of the larger Mosses (Fig. 300), 

 and in the thallus-structure of the Marchantiales (Fig. 309). These 

 are, however, parts of the gametophyte, and the ventilated structure 

 is here produced mainly by involution of the outer surface, while in 

 Vascular Plants it arises from intercellular splitting of the cell-walls. 

 The physiological end is the same in both cases, but the place and 



B.B. 2 A 



