222 THE EVOLUTION OF PLANTS 



theory by the absence of facts, and there has 

 been much controversy on the origin of the higher 

 Cryptogams. It is at this point that we enter 

 the region of speculation, while, as we have seen 

 in Chapter IV, quite good evidence for the origin 

 of Seed-plants from the Cryptogams is available. 



The question of the evolution of the higher 

 Spore-plants has turned chiefly on the origin of 

 the alternation of generations which is so uni- 

 versal and striking a feature in their develop- 

 ment. A similar alternation of sexual and 

 asexual phases exists in the class of the Mosses 

 and Liverworts, but here the functions of the 

 two generations are differently allotted. In the 

 Ferns and their allies, the asexual generation 

 does all the important vegetative work, and is 

 what we call the plant; the sexual generation 

 (prothallus) is comparatively unimportant ex- 

 cept as the bearer of the sexual cells. In the 

 Moss-series, on the other hand, the plant, which 

 does the vegetative work, is the sexual genera- 

 tion; the asexual generation is a fruit, which has 

 little more to do than to produce the spores. 

 Thus the Moss-plant corresponds to the Fern- 

 prothallus, and the Moss-ffuit to the Fern-plant. 



The Mosses and Liverworts (together called 

 Bryophytes) are much simpler plants than the 

 Ferns and their allies. Mosses, for example, 

 have no vascular system, though some of them 



