346 VEGETABLE PHYSIOLOGY 
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 meta- 
bolism being 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. 144). 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 
a limited period of the year. By preserving its leaves green 
the plant can take advantage not only of the light of summer, 
but also of those bright sunny days which occur occasionally 
during the cold season, and thus improve every opportunity 
afforded it. 
Some lowland plants show a similar response to their 
environment, the form and structure of different individuals 
of the same species varying to a certain extent, according 
to their advantages or the reverse, under such conditions 
as sunlight or shade, drought or moisture, exposure to or 
protection from cold winds, &c. 
Epiphytic plants show some conspicuous modifications 
of their structure in consequence of their peculiar habit of 
life. They usually live upon the surfaces of trees, to which 
they cling by various means, but from which they derive 
no nourishment except such as is afforded by accumulations 
of débris, &c., upon the trunks. They are not parasitic, 
but merely live upon the tree as other plants grow upon 
rocks or cliffs. Mosses and Liverworts are very largely 
epiphytic, as are certain species of Phanerogams; the 
latter are very specialised forms, and show most adapta- 
tion of form and structure. Perhaps the most remarkable 
