338 VEGETABLE PHYSIOLOGY 



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 transpira- 

 tion is only occasionally possible, and where consequently 

 the absorption of food materials is much impeded. Similar 

 conditions mark the bleak moorlands of 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, 



FIG. 1-44. TRANSVERSE SECTION OF ROLLED LEAF OF HEATH. 



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 might be fatal to it. The metabolism being- 

 reduced by the low temperature, the contents of the air 

 reservoirs suffice for such interchanges of gases as are 



