INFLUENCE OF ENVIRONMENT ON PLANTS 337 
lie immediately under the epidermis of a small depression 
of the surface, and which communicate with the exterior 
by a few fine pores which perforate the latter. The 
epidermis of this depression is made up of cells with thin 
non-cuticularised walls. Each so-called gland is in contact 
with the end of a fibro-vascular bundle, whose sheath is 
carried forward over the general mass of delicate cells 
(fig. 148). The depression of the surface is filled with a 
mass of carbonate of lime, which is originally excreted by 
the leaf, and which is held in its place by a few papille 
which project from the epidermis. Such an arrangement 
serves a double purpose; any dew or rain which reaches 
the surface of the leaf is absorbed by the carbonate of lime 
and can make its way slowly into the gland, whence it 
passes into the fibro-vascular system ; while, when the leaf 
is dry the incrusting mineral matter serves as a plug to 
the depression, and reduces transpiration. 
Many plants which inhabit sandy deserts possess 
similar mechanisms; some excrete carbonate of lime, 
others crystalline accumulations of common salt. The 
latter can not only absorb dew and rain but can also con- 
dense and take up moisture from the air. They are found 
occurring in such sandy wastes as are by the seashore or 
near salt lakes. 
Many trees which grow in temperate climates, in poor 
sandy soil on the margin of streams, show a somewhat 
similar mechanism, but the excretion from their leaves takes 
the form of a kind of resinous varnish or balsam which 
can be readily wetted and which can absorb water. In some 
cases so-called glandular hairs discharge a similar function. 
The water which is absorbed in this way is rarely pure, 
but contains traces of sulphuric acid and ammonia, which, 
though trifling in amount, are no doubt of value in the 
nutritive processes. The adaptation to their environ- 
ment 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 
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