226 ROUND THE YEAR 



examples. We find all these among the Yorkshire 

 hills and on the sea-coast, but' in no intermediate 

 places. The plant which has been acclimatised to one 

 of these habitats is thereby adapted to face the most 

 serious difficulties of the other. 



Hindered transpiration points to a limited supply of 

 water, which may arise in various ways, (i) Water 

 may be deficient altogether, as in the desert, or 

 on rocks and loose stones. (2) Water may be plenti- 

 ful, but absorbed with difficulty because of low 

 temperature. (3) Water may be plentiful, but 

 absorbed with difficulty because of salts dissolved 

 in it The Crowberry and our other native moorland 

 plants are efficiently protected against contingencies 

 I and 2, of which the second is probably the more 

 frequent and serious. 



A considerable proportion of our moorland plants 

 (Ling, Crowberry, Nardus, Juncus, Cranberry, etc.) 

 are evergreen. In this the moorland flora resembles 

 that of high northern regions. I am not quite certain 

 why a large part of the more conspicuous plants of 

 cold regions should be evergreen. The explanation 

 may be that even during the summer the conditions 

 require checks to transpiration, and that these checks, 

 once developed, enable the plant to endure winter con- 

 ditions without loss of leaves. The glossy cuticle, the 

 simple form, the reduced size, the dry texture, and 

 the concealed stomates, which are common features of 

 leaves exposed to sun and wind, form a great part of 

 the special equipment of an evergreen. 



But there are puzzling exceptions to the evergreen 

 habit. Take the British Heaths (Ertcacea) for 



