482 



PRACTICAL BOTANY 



(2) The various kinds of heather, knotgrass (Polygonum 

 aviculare), knawel (^Scleranthus annum), and the rushes are 

 instances of xerophytes with small leaves, exposing compara- 

 tively little surface to the sun and air. 



(3) Century plants (Fig. 62), houseleeks, and aloes are 

 good examples of fleshy-leaved xerophytes. 



(4) Many xerophytes combine in their leaves some of the 

 characteristics of groups (1), (2), and (3). The leaves of 

 cedars, hemlocks, firs, and spruces have a thick epidermis and 



close interior struc- 

 ture, like that shown in 

 Fig. 249, and are also 

 small, exposing little 

 surface. The common 

 purslane, the portu- 

 laca, and the ice plants 

 (Mesembryanthemum) 

 have small and rather 

 fleshy leaves. 



Many xerophytes 

 have extraordinarily 

 developed root systems, as in the case of the mesquite (Sect. 

 27), and so can draw moisture from great depths in the earth. 

 Others have extensive provisions for water storage (Sects. 34, 

 66, 67). Among these the cacti are notable for the amount 

 of water which they can store in their succulent stems, which 

 are often fluted, so as to expand and contract readily. This 

 water has been rapidly absorbed by the spreading, shallow 

 root system from the layer of earth (only an inch or two 

 deep), which is moistened by the rare rains of the desert re- 

 gions where many such cacti grow. Between rains the roots 

 of these cacti serve only for anchorage. 



Xerophytes in general are so constituted as to transpire 

 little at any time, or else to be able, in case of danger from 

 excessive dryness, to reduce the amount of transpiration to a 

 very low value. 



FIG. 363. Cross section of rolled-up leaf of 

 crowberiy (Empetrum nigrum) 



Magnified 



