234 



VEGETABLE PHYSIOLOGY 



of most seeds must be looked upon as reserve food material, 

 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 



-am 



FIG. 114. SECTION THROUGH EXTERNAL REGION OF GRAIN OF BARLEY. 



p, pericarp of fruit; , testa of seed; a I, 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 

 cases, as in fleshy roots, the protein may be dispersed in 

 amorphous form in the substance of the protoplasm, 



