56 VEGETABLE PHYSIOLOGY 
cell-wall. The chief of these are salts of calcium, usually 
the oxalate, but often the carbonate. Some cell-walls show 
a copious deposit of regular crystals of one of these—such 
are the cells of the bulb scales of the onion, the fibres of 
the bast of Ephedra, among others (fig. 48). In many plants 
copious deposits of silica are formed in the cell-wall, 
especially in the epidermal cells of the Equisetacew, and in 
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 
Fig. 49.—Szction oF Portion 
or Lear oF Ficus, sHow- 
Inq CYSTOLITH (cys) IN LaraE 
Fig. 48.—Crysrats or Cacium CELL OF THE THREE-LAYERED 
OxaLaTE IN Watt or CELL Eprpermis (ep). (pa) Patt- 
or THE Bast ov Ephedra. SADE LAYER. 
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 ag 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 
Fucacece, where it forms the bulk of the internal tissue. It 
occurs also in certain layers of the seed-coats of such seeds as 
linseed, and in certain regions in the sporocarps of Marsilea. 
