INTRODUCTION 39 



as a rule, so that they may receive as much light as 

 possible, being generally horizontal, sometimes broad, 

 sometimes narrow. The factor of shade causes plants 

 to be modified, especially in leaf form, to the light 

 conditions. 



Drought, physical or physiological, is another 

 factor which necessitates modifications in the form 

 of the leaf, the leaves of plants exposed to such 

 conditions being usually narrow and often rolled up. 



Cold or heat cause similar modifications to those 

 of drought, especially in regard to root absorption, 

 and this in turn affects the leaves. 



The supply of water to the roots from rain is also 

 responsible for the different arrangement of the 

 leaves in different plants, and these arrangements 

 are also connected with the adaptations of the stem 

 to the same end, hence the mosaic patterns of 

 leaves, the channelled leafstalks, and hollow upper 

 side of the leaf with impressed veins, etc. The 

 habit of the plant, form and arrangement of the stem- 

 leaves are all connected with this factor of water- 

 distribution. In leaves there is also need to protect 

 the water they contain from evaporating, and the 

 stomata from being clogged by water. The surface 

 may be smooth or hairy. 



Leaves also require protection from browsing 

 animals and are sometimes spiny or prickly as in the 

 Sea Holly, or covered with stiff hairs as in the 

 Woundwort, etc., or with stinging hairs as in the 

 Nettle, or they may be leathery, fleshy, etc. 



