INFLUENCE OF WATER ON LAND-PLANTS 39 



of a gradual evolution along certain lines or tendencies, 

 the purpose of which is to protect the plant against 

 physical or physiological dryness. 



I. External Xerophytic Tendencies. 



1. Diminution of the Transpiring Surfaces e.g., cypress 

 (Fig. 6). 



2. Increase in Bulk compared with Surface e.g., 

 fleshy-leaved plants, as stonecrop (Fig. 7). 



FIG. 6. CYPRESS, SHOWING 

 CONCRESCENT TYPE OF LEAF. 

 (NATURAL SIZE.) 



FIG. 7. STONECROP (Sedum acre), SHOW- 

 ING CROWDED SUCCULENT LEAVES. 

 (NATURAL SIZE. AFTER SOWERBY.) 



3. Increase in Longevity. In this country land-plants 

 which retain their leaves in winter are almost invariably 

 evergreen xerophytes e.g., holly, ivy, heath, laurel, 

 box, pine. By increasing the longevity of their leaves, 

 plants are spared the necessity of making a complete 

 set each year. This economy is imposed on most 

 xerophytes because they are poorly nourished, and possess 

 none too much nutriment.. 



4. Development of Screening Structures, such as hairs, 

 scales, etc., which prevent the wind from removing 

 moist air from the neighbourhood of the stomata e.g., 

 mullein, cudweed. 



