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 
Fic. 144.—Transverse Suction oF Rotuep Lear or Heavu. st. SrOMATA 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 
