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, 
Fic, 144.—TRANSVERSE SECTION oF RotuepD Lear or 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 alr-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 
