232 THE SIMPLEST LAND PLANTS 



get some idea of how these conditions affected the 

 land migrants, and of the adjustments in structure 

 and function that have taken place. It must be 

 clearly understood, however, that the simplest forms 

 of land plants now existing certainly do not represent 

 the actual stages in the evolution of land vegetation. 

 They represent rather side lines of the evolutionary 

 tree which have become stabilised, and are probably 

 incapable of much further evolution. 



The plants known as Liverworts and Mosses are 

 two groups of green plants, all comparatively small 

 and with tissues of comparatively simple organisation. 

 They resemble one another in the structure of their 

 reproductive organs and in their life histories, but 

 are distinctly different in the form and structure of 

 the plant body. In the last respect the Mosses are 

 decidedly more highly developed than the Liverworts. 

 Both represent comparatively low stages in adapta- 

 tion of the green plant to terrestrial life. Most of 

 them live in situations where the soil and the air are 

 constantly moist, so that they are in little danger of 

 losing water by evaporation so quickly that they dry 

 up. They are not nearly so completely protected as 

 the seed plants against this risk by having an almost 

 impermeable waterproof covering, though many of 

 them are protected to some extent. And they can 

 absorb water over much or over the whole of their 

 surface, which the higher plants cannot do. Some 

 species actually live in places (e.g. rock surfaces, walls, 

 etc.) where they are liable to dry up, and these can 

 remain alive, though dormant, in the dry condition ; 

 when wetted they quickly absorb water and resume 

 active life. 



Liverworts : Pellia. This is a common liverwort 



