240 VEGETABLE PHYSIOLOGY 
as they are used up in nourishing the embryo during the 
early stages of germination. 
It is necessary, however, to mention that thickened 
cell-walls must not always be regarded as stores of food. 
In thickened sclerenchymatous tissue and in ordinary wood- 
cells the deposit must be looked upon as a permanent 
strengthening of the skeleton of the plant. 
These thickened cell-walls are not composed always of 
true cellulose. Our knowledge of their composition is not 
Fie. 114.—Szcrion THRoveH ExtERNAL REGION oF GRAIN 
oF BARLEY. . 
p, pericarp of fruit; 4, testa of seed; al, layer of cells 
containing aleurone grains; am, cells of endosperm ; 
n, nucleus, (After Strasburger.) : 
at all complete, but it extends so far as to show that both 
cellulose and pectic compounds may be present and in very 
different proportions in different cases. Layers of muci- 
lage also are of frequent occurrence. 
Nitrogenous material, like carbohydrate, is stored up in 
various places and in different forms. By far the com- 
monest condition is that of some description of protein. 
The most abundant deposits are found in seeds, in the cells 
of which they usually occur in the form of granules of 
varying sizes and often of complex composition. In certain 
