RELATION TO ENVIRONMENT 



501 



through between their segments. If the light is deficient, as we have 

 already seen, the size of the leaf becomes much increased. 



Xerophytes. The name Xerophyte is given to plants which grow 

 where the moisture is deficient, and which are more or less modified 

 so as to guard against excessive loss of moisture. While xerophytes 

 are especially characteristic of hot, arid regions, they are by no means 

 confined to these. 



Among the lower plants, especially the Green Algae, special resist- 

 ant spores are produced which carry the plant through periods of 

 drought, and the same purpose is served by the spores of many 

 Mosses and Ferns. Among the latter groups, however, there are a 

 good many which can be completely dried up without injury, and 

 may, therefore, be classed as xerophytes. In California, for example, 

 where all vegetation is exposed to prolonged droughts each year, 

 very many of the Mosses and Liverworts become completely dried up 

 without being killed, and a few 

 Ferns like the common Gold-back 

 (Gymnogramme triangular is), and 

 several species of Selaginella, e.g. 

 S. lepidopliylla, also become com- 

 pletely dried up, but on being 

 moistened, the leaves absorb water 

 and quickly become fresh and 

 green again, and the plant at once 

 resumes its growth. 



It is among the Spermatophytes, 

 however, that the most perfectly 

 developed xerophytes are found. 

 The methods by which protection 

 against drouth is attained are 

 various. In typical xerophytic 

 trees and shrubs, especially those 

 with evergreen leaves, the latter 

 are very greatly reduced in size, 

 compared with the leaves of meso- 

 phytes; or where there is no 

 noticeable reduction in size, the epidermis is very thick, so that the 

 leaves are leathery in texture, as in the Oleander or Holly, or the 

 needles of coniferous trees. The small leathery leaves of these 

 xerophytes offer a strong contrast to the large delicate leaves of 

 shade-loving plants, or the exceedingly soft leaves of submersed 

 aquatics. 



The reduction of the leaves is carried so far in many xerophytes 

 that they are degraded to mere scales, quite functionless as assimi- 

 lating organs. Such forms, like the Broom, or Casuarina (Fig. 469), 



FIG. 469. Casuarina 

 The leaves reduced 

 sheaths. 



equisetifolia. 

 to toothed 



