344 VEGETABLE PHYSIOLOGY 
upon rocks have leaves which show special structures for 
absorbing water from rain or dew. Several plants, among 
which some species of Saxifrage are conspicuous, possess 
a number of glandular structures upon the teeth of their 
thick narrow leaves. Each consists of a small mass of 
cells with delicate walls, which le 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. Hach 
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. 143). 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 condense 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 
