Water Requirements 



129 



intergrading series of forms three primary groups which 

 are conveniently designated (1) xerophytes, (2) meso- 

 phytes, and (3) hydrophytes. 



Xerophytes. - - Plants adjusted to physiological dryness 

 are properly termed xerophytes. In the preceding pages 

 references have been made to 

 the fact that there are a large 

 number of plants both peren- 

 nial and annual which are able 

 to exist in typical desert situ- 

 ations. In general, such plants 

 are tough, often hairy, and 

 they usually possess reduced or 

 leathery leaves. Accompanying 

 these modifications there may 

 be histological adjustments 

 which may serve to check 

 water-loss during the more arid periods, and to accu- 

 mulate or store water when it is more plentiful. Special 

 peculiarities of the epidermis, and of the plant in general, 

 as affecting transpiration, have been discussed. The 

 cactus, yucca, and sage-brush of the southwest are 

 plants possessing the capacity for types of modifications 

 which enable them to persist and to become the dominant 

 vegetation in much of that region. 



Some of the most famous writing papers are those manu- 

 factured in Scotland and England from a widely distrib- 

 uted and much exploited African desert grass known as 

 alfa. This name refers particularly to Stipa tenacissima, 

 which occupies millions of acres in the steppes of northern 

 Algeria. It is a plant too tough even to furnish food for 



FIG. 41. Section of Begonia 

 leaf, showing colorless water- 

 storage tissue adjacent to epi- 

 dermis. [After Coulter.] 



K 



