The Origin of Land Plants 89 



of generations " which characterizes all of the 

 higher plants. 



Evolution of the Gametophyte in Bryophytes. 

 The gametophyte in the simpler liverworts is not 

 strikingly different from some of the algae, and, as 

 we have seen, may be a delicate prostrate thallus, 

 composed of almost uniform cells except for hair- 

 like roots or rhizoids, and the reproductive organs. 

 From this simple type of gametophyte there may be 

 traced several diverging structural types. The 

 gametophyte reaches its highest degree of special- 

 ization in some of the larger mosses where there 

 are developed leafy shoots of considerable size, a 

 foot or more in length occasionally, and these show 

 a specialization of the tissues which may be almost 

 compared to that of the sporophyte of the ferns. 

 The roots, however, never assume the perfect form 

 found in the ferns and seed-plants, but are usually 

 composed of a single cell. The mosses often depend 

 only to a limited extent upon these hair-like roots 

 to supply them with water, absorbing the water 

 readily through the leaves very much as the algae 

 do. Liverworts and mosses together form the 

 group of Bryophytes. 



The apparent inability of the gametophyte to de- 

 velop adequate roots, probably accounts for its fail- 

 ure to reach dimensions at all comparable to those 

 of the sporophyte of the higher plants. Moreover, 

 none of the large gametophytic structures have de- 

 veloped a mechanical system of tissues sufficient 



