INFLUENCE OF ENVIRONMENT ON PLANTS 345 



nutritive processes. The adaptation to their environment 

 which these plants exhibit is thus chiefly in the direction 

 of economising a limited water supply. 



The influence of the environment on the form of the 

 plant can be seen equally well in the case of such plants 

 as grow in Alpine regions, where the cold is usually intense, 

 and the atmosphere for long periods so humid that trans- 

 piration is only occasionally possible, and where consequently 

 the absorption of the raw materials of the food is much 

 impeded. Similar conditions mark the bleak moorlands of 



FIG. 144. TRANSVERSE SECTION OF ROLLED LEAF OF HEATH, st. STOMATA IN THE 



GROOVE. 



temperate climates. These show very great differences 

 between the extremes of temperature which mark summer 

 and winter respectively. The water supply also shows very 

 great variations at different times of the year. The plants 

 are generally of comparatively small size, and bear thick, 

 often rolled-up, leaves which are evergreen. The thick 

 exterior and the general hardness of the leaf are a response 

 to, and a defence against, the cold. In the heaths, which 

 may be regarded as typical moorland plants, transpiration 

 is reduced to a minimum, large air-chambers in the leaf 

 with only a few stomata, and those situated in a deep groove, 

 providing for the aeration of the protoplasts. During the 



