iv ON PLANT LIFE 147 



transpiration and thus adapting plants to dry 

 situations are by the development of hairs, 

 by the formation of chalky excretions, by 

 the sap becoming saline or viscid, by the leaf 

 becoming more or less rolled up, or protected 

 by a covering of varnish. 



Our English trees are for the most part 

 deciduous. Leaves would be comparatively 

 useless in winter when growth is stopped by 

 the cold ; moreover, they would hold the 

 snow, and thus cause the boughs to be broken 

 down. Hence perhaps the glossiness of Ever- 

 green leaves, as, for instance, of the Holly, 

 from which the snow slips off. In warmer 

 climates trees tend to retain their leaves, and 

 some species which are deciduous in the north 

 become evergreen, or nearly so, in the south 

 of Europe. Evergreen leaves are as a rule 

 tougher and thicker than those which drop off 

 in autumn ; they require more protection from 

 the weather. But some evergreen leaves are 

 much longer lived than others ; those of the 

 Evergreen Oak do not survive a second year, 

 those of the Scotch Pine live for three, of the 

 Spruce Fir, Yew, etc., for eight or ten, of the 



