102 EVOLUTION OF PLANTS 
are examined, it is seen that even the lowest mosses 
are far more complicated than any of these alge. 
The zodéspores, or motile non-sexual reproductive cells 
of the alge, 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 zodspores of many alge, but desti- 
tute of cilia. The method of development of these 
bodies suggests that in them we have the last trace of 
zodspore formation, the absence of cilia being corre- 
lated with the terrestrial habit of the liverworts. Spe- 
cial non-sexual reproductive bodies (buds or gemme) 
of an entirely different kind are not uncommon in 
many of the higher forms, both among the Hepatice 
and the true mosses. 
The lower Hepatice 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 Archegoniate. 
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 
