296 EVOLUTION OF PLANTS 
green relatives of fresh water; but the modifications 
found in the Algz 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 dryness, either by having the surface 
exposed to the dry atmosphere much reduced through 
the partial or complete suppression ofthe 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 eae 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 
