102 EVOLUTION OF PLANTS 



are examined, it is seen that even the lowest mosses 

 are far more complicated than any of these algse. 



The zoospores, or motile non-sexual reproductive cells 

 of the algse, are wanting completely in the mosses, but 

 among the lowest liverworts there have been discovered 

 certain cells which perhaps represent them. In these 

 forms the contents of an ordinary thallus cell are 

 ejected in the form of a unicellular or two-celled body 

 very much like the zoospores of many algse, but desti- 

 tute of cilia. The method of development of these 

 bodies suggests that in them we have the last trace of 

 zoospore formation, the absence of cilia being corre- 

 lated with the terrestrial habit of the liverworts. Spe- 

 cial non-sexual reproductive bodies (buds or gemmse) 

 of an entirely different kind are not uncommon in 

 many of the higher forms, both among the Hepaticee 

 and the true mosses. 



The lower Hepaticse are of especial importance in a 

 study of the origin of the higher plants, as there is good 

 reason to believe that they represent the most primitive 

 of existing chlorophyll-bearing terrestrial plants, and 

 probably have given rise to all the higher types of 

 vegetation. 



The liverworts, in common with the other mosses and 

 the ferns, have the egg-cell borne in a peculiar organ, of 

 very uniform structure in all of them, known as the 

 archegonium (Fig. 26, A, B) ; and on account of this 

 uniformity of structure, mosses and ferns together are 

 often united into one great division, the Archegoniafoe. 

 The archegonium usually has the form of a long- 

 necked flask in whose enlarged base, or venter, is found 

 the egg-cell. The nearest approach to this structure 



