406 BRYOPHYTES (MOSS PLANTS) 
and the air soon dries them out. To live on land a plant must be 
protected against transpiration, and to become large and erect, a 
plant must have structures for connecting it to the ground and 
a stem to support it against the wind. It is believed that the 
land plants came from Algac, and this means that certain Algae 
must have acquired the land habit and in so doing ceased to be 
Algae and became Bryophytes. One can imagine that this trans- 
formation came about by some Algae gradually becoming more 
and more adapted to living on the shore, where they were often 
stranded, until finally they became so modified as to be fitted to 
live permanently on land. 
Liverworts 
The Liverworts are thought to be the group that first acquired 
the land habit, for, as a group, they are less complex than the 
Mosses and are also more like the Algae in their moisture re- 
quirements. While many of them live on land, there are some 
forms which still live in water, and it is, therefore, in the Liver- 
worts that the connection between water forms and land forms 
is most evident. Even most of those Liverworts that live on 
land are not able to endure dry air and hot sunshine, for, in most 
part, they must grow in places that are moist or at least shaded. 
But the Liverworts did much toward establishing the land habit, 
and it is thought that our strictly land plants originated from 
such forms as the Liverworts. 
The plant body of most Liverworts is a flat body, known as 
a thallus, but in some forms it is differentiated into stem- and 
leaf-like structures. The thallus form of plant body, although 
varying much in form according to the species, is usually lobed 
and often branched. Often the thalli are liver-shaped, and 
their shape was once thought to signify that these plants 
possess special virtues in the cure of liver diseases — whence 
the name Liverworts. 
The thallus forms of Liverworts often form mat-like coverings 
on moist soil or on moist rocks, such as the sides of a cliff. Those 
Liverworts having better differentiated plant bodies and re- 
sembling Moss commonly grow on logs and tree trunks in moist 
and shady woods. There are about 4000 species of Liverworts 
and they vary widely in complexity. They are commonly sub- 
