296 EVOLUTION OF PLANTS 



green relatives of fresh water; but the modifications 

 found in the Algse are slight when compared with the 

 profound structural changes exhibited by the Arche- 

 goniates and Spermatophytes, which have become 

 adapted to terrestrial life. 



With the change from the aquatic to the terrestrial 

 environment the tissues have become very much better 

 developed, especially the mechanical tissues which give 

 rigidity and strength to the plant. The difference in 

 the degree to which these are developed in closely 

 related land and water plants is very noticeable, and 

 is of course directly associated with the changed 

 environment. 



The degree of moisture varies extremely over land 

 areas, and those plants which inhabit dry regions have 

 become much changed, so that they are enabled to 

 endure extreme diyness, either by having the surface 

 exposed to the dry atmosphere much reduced through 

 the partial or complete suppression of the leaves, or by 

 having these very perfectly protected against loss of 

 water by means of extremely thick impervious cells upon 

 the outside, or by a thick covering of hairs or scales. 

 Other xerophytes, or dry-region plants, are character- 

 ized by thickened underground stems which serve as 

 reservoirs of water, or remain dormant during the dry 

 period, starting quickly into growth with the advent 

 of the brief rainy season. 



Plants which are subject to extreme cold have devel- 

 oped protective structures similar to those of plants 

 whose growth is checked by drought. These plants, 

 too, often develop underground resting stems, which 

 send up the annual shoots when spring arrives. The 



