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 
