234 ABSORPTION-CELLS ON LEAVES. 
(cf. fig. 261). The bottom of each depression is made up of cells with very thin 
external walls, and the function of these cells is to suck up the water that flows 
into the cavity. It is obvious that the absorbed water passes thence into the 
enlarged extremities of the branches of the vascular bundles, and may then be 
conducted to other parts of the leaf. Seeing that all these saxifrages have their 
habitat in crevices of rocks on sunny deelivities, they are much exposed to 
desiccation in times of drought. The epidermal cells of the medial area and those 
of the extreme edge are no doubt protected by a very thick cuticle (see fig. 55°); 
but in the case of the thin-walled cells at the bottom of the depression there is the 
danger of as much or even more water escaping through them, in the form of vapour, 
than has been previously taken in during the prevalence of rain. 
In order to prevent this loss of moisture recourse is had to a very remarkable 
contrivance for closing the cavity, viz., an incrustation of carbonate of lime. In 
many saxifrages this crust covers the whole face of the leaf, in others only the 
margin, or the spot where the depression occurs. In the latter case it looks like a lid 
over the cavity. At that spot the crust is always thickened, and sometimes it forms 
a regular stopper which fills up the entire cavity. It rests upon the epidermis of 
the leaf, but is not adnate thereto, and may be removed with a needle. When a 
leaf is bent the crust is ruptured and breaks up into irregular plates and scales, 
and a strong gust of wind would then easily strip off the fragments and blow them 
away. In species subject to this danger, as, for instance, Saaifraga Arzoon, in 
which the rosetted leaves curl strongly upwards and inwards in dry weather, the 
crust of lime is held fast by peculiar plugs which arise from individual epidermal 
cells projecting above the rest in the form of papilla (see fig. 55°). These plugs 
are found principally on the side walls of the cavities, but are also scattered every- 
where on the epidermis of the margin of the leaf. They are so incrusted with the 
lime that the latter cannot easily fall off, and a comparatively strong pressure must 
be applied with the needle to detach it from the substratum. The calcium carbonate 
of which these crusts consist is excreted in solution by the plant from pores occur- 
ring at the bottom of the depressions. The pores are constructed like ordinary 
stomata, but are, as a rule, somewhat bigger, and it is not improbable that, when 
once the lime crust has formed from the excreted solution, they take part in the 
function of transpiration. 
There is scarcely any need for further explanation of the manner in which the 
apparatus here described acts. When rain or dew falls on a saxifrage leaf the 
whole upper surface is moistened directly, whilst the water soaks under the crust 
of lime, and, diffusing itself there, fills in a moment the depressions, and is taken 
up by the absorption-cells situated at the bottom of the latter. The calcareous 
stopper imbedded in each cavity is only upheaved by this process to a trifling 
extent. In dry weather the crust is appressed closely to the epidermal cells, and 
the stopper descends again and impedes the evaporation of water from the thin- 
walled cells within the cavities. 
The absorptive organs on the leaves of Acantholimon, Goniolimon, and a few 
