THE EVOLUTION OF THE SPOROPHYTE. 7 



to exist at present. The simplest known type of sporophyte 

 is that of Riccia (Fig. 3, a), already referred to. Here, although 

 a thin peripheral layer of cells is sterile, much the greater part 

 of the sporophyte is composed of sporogenous tissue, all of 

 whose cells produce spores. 



There are still existing a number of low liverworts which 

 very clearly show the mode of progression from the simple 

 capsule filled with spores, found in Riccia, to the more highly 

 organized sporophyte of the higher forms. The first step in 

 the development of the sporophyte is the separation of the 



FIG. 3. Diagram showing the evolution of the sporophyte in the typical liverworts, a, Riccia; 

 b, Sphaerocarpus ; c, Porella. In Riccia all but the single outer layer of cells produce 

 spores ; in Sphaerocarpus an absorbent organ or foot is present, and some of the sporoge- 

 nous tissue remains sterile, forming the sterile cells, st. ; in Porella a stalk is developed 

 between the foot and the capsule, and the sterile cells within the capsule develop into 

 elaters, el. ; j/., tetrads of spores. 



upper spore-bearing portion from a lower part, which becomes 

 an organ of absorption, the foot, and thus the sporophyte 

 begins to assume the character of an individual plant with both 

 vegetative and reproductive tissues. This stage is well shown 

 by the little liverwort Sphaerocarpus (Fig. 3, b). Here the upper 

 part of the sporophyte becomes differentiated into a globular 

 capsule, with a well-developed wall enclosing the sporogenous 

 tissue, some of whose cells remain undivided and serve to 

 nourish the growing spores derived from the other sporoge- 

 nous cells. 



In the higher liverworts (Fig. 3, c] the foot is well devel- 

 oped, and a stalk is usually formed between it and the cap- 

 sule. In the latter the sterile cells form the curious spirally 



