THE SKELETON OF THE PLANT él 
those of the cereal grasses. The value of this deposit to 
the plant is not very evident; it.appears at first sight to be 
an adaptation enabling the plant to remain upright, but it 
is found that its absence does not render the grasses more 
liable to fall. 
Some cells of the epidermis of certain plants, especially 
among the Nettle family, contain curious ingrowths of 
cellulose, in which there is a very large deposition of 
calcium carbonate. They are known as cystoliths (fig. 49). 
The cell-walls of certain regions of particular plants 
are transformed into mucilage. This material is especially 
prominent in the large brown seaweeds, particularly the 
Fucace@, where it forms the bulk of the internal tissue. It 
occurs also in certain layers of the seed-coat of such seeds as 
linseed, and in certain regions in the sporocarps of Marsilea. 
It is of assistance in the dissemination of the spores of 
this plant, and possibly has a similar value in the cases of 
such seeds as contain it. It differs from cellulose by 
absorbing water greedily, and swelling up considerably. 
It gives a blue colour with iodine and sulphuric acid as 
cellulose does, differing from the latter chiefly in the ease 
with which the absorption of water is brought about. It 
is not clear at present whether mucilage is derived from 
cellulose only, or whether the pectoses take part in its com- 
position, though the latter is probable. The gums are closely 
related to mucilage, and seem to represent a further dis- 
integration of the cell-wall in that direction. Many of the 
gums yield derivatives much like those of pure pectic 
bodies, which suggests that their affinities are rather with 
the latter. In all probability, however, they are all mix- 
tures of the two classes of constituents. 
We see thus that in the construction of the skeleton of 
a complex plant, while its basis is the cell-membranes of 
the several protoplasts, which at first form an almost 
homogeneous tissue, not only does differentiation take 
place in the direction indicated in the last chapter, but 
this differentiation is accompanied by changes in chemical, 
