84 Plant Life and Evolution 



of their life history is dependent upon the presence 

 of free water. 



Modifications Due to Terrestrial Habit. When 

 the plant exchanges its aquatic environment for 

 life on land it must undergo some radical changes 

 in structure. First of all is the necessity for econ- 

 omizing water, as it is no longer able to take in 

 water at all points, and it must therefore provide 

 both for the absorption of water and for checking 

 undue loss of water through transpiration. Hence 

 have arisen the special water-absorbing roots, 

 and the protection of all the exposed cell walls 

 by a cuticle or impervious membrane, which ma- 

 terially checks the escape of water through 

 evaporation. 



The lowest of these land plants, such as the liver- 

 worts and mosses, are often prostrate in their habit 

 and do not assume the upright position common 

 to most of the higher plants. This prostrate habit, 

 which implies an imperfect development of me- 

 chanical or supporting tissues, recalls the behavior 

 of their algal ancestors. Where the upright position 

 is assumed, it involves a greater or less development 

 of firm tissues, the so-called mechanical or support- 

 ing tissues, which give to the plant sufficient rigid- 

 ity to overcome the force of gravity. With the 

 increasing size of the plant comes the need for rapid 

 transportation of water, and there have arisen in 

 response to this need the characteristic conducting 

 tissues, which reach their highest development in 



