INFLUENCE OF ENVIEONMENT ON PLANTS 339 



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 cold the closing 

 of these almost hidden stomata guards the plant from the 

 evaporation, which, if unchecked, would lead to a loss 

 of heat that would be fatal to it. The metabolism being 



FIG. 146. TBANSVEBSE SECTION OF ROLLED LEAF OF HEATH. 



reduced by the low temperature, the contents of the air 

 reservoirs suffice for such interchanges of gases as are 

 imperative, and for the coincident exhalation of watery 

 vapour by the protoplasts, but as these contents are very 

 slowly renewed the total evaporation is but slight. When, 

 on the other hand, for a part of the year the temperature 

 is high, the spacious reservoirs provide for a very rapid 

 transpiration as soon as the stomata are open, a very 

 large spongy mesophyll abutting on them (fig. 146). The 

 evergreen leaves also are an expression of the struggle 

 against the difficulty of the absorption of food materials, 

 which in such atmospheric conditions is possible for only 



z 2 



