THE AMERICAN BOTANIST 



9 



Most interesting of xerophytic characteristics are those 

 which are concerned with the retention of moisture. At the 

 outset we find our plants divided into two groups : one, rep- 

 resented by the cacti, without leaves and performing all the 

 work of food making by means of its green stems ; the other, 

 represented by the agave and yucca, retaining its leaves and 

 protecting them from evaporation. While it is probable that 

 certain leafy xerophytes, to be mentioned later, represent the 

 most resistant types of vegetation on the earth, the cacti are 

 eminently successful types and in a drought-enduring contest 

 would undoubtedly outlast nearly all comers. They are ad- 

 mirably adapted for this by their thick and leathery epidermis, 

 through which water escapes very slowly, by their sap which, 

 mixed with mucilage and other gums, is slow to evaporate, 

 but especially by their stems which tend to become cylindrical 

 or spherical, this latter form being the one which presents the 

 least surface for a given bulk and thus presents the least 

 chance possible for evaporation. 



In the leafy xerophytes, the leaves are seldom large and 

 in some even these are dropped when conditions become 

 severe. Long and narrow leaf forms are common and in 

 certain extreme types the leaves are cylindrical with an 

 abundant water tissue suggesting the plan after which cactus 

 stems are constructed. Evaporation from the leaves is re- 

 tarded by a thick epidermis, or by a coat of wax, hairs, or 

 varnish. The small openings in the epidermis, called stoniata, 

 which are necessary for an exchange of gases with the outside 

 air, and through which moisture may escape, are usually on 

 the under, shaded side of the leaf. On hot days the escape 

 of moisture through these may be further hindered by the 

 leaves rolling backward and enclosing them, or the stomata, 

 themselves, may be sunk in pits or grooves in the leaf and 

 protected by wax and hairs. A few partial xerophytes, like 

 our well-known compass plant and the gums of Australia, 



