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.., 
fleshy-leaved plants, as stonecrop (Fig. 7). 
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Fic. 6.—Cypress, sHowine Fic. 7.—StonEorop (Sedum acre), sHow- 
CoNCRESCENT TYPE oF LEAF. ING CROWDED SvuccuLENT LEAvEs. 
(NatTuRAL Sze.) (NaTurAL Size. ArrEeR 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. 
