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INFLUENCE OF ENVIRONMENT ON PLANTS 339 
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 
feature about them is their aerial adventitious roots, which 
are given off in some cases from every node of the stem, 
so that each internode has its own supply. These are 
often long cord-like structures, which are of some thickness, 
often contain chloroplasts, and are either covered by a 
