44 NUTRITION OF FARM ANIMALS 



75. Composition of protoplasm. The chemical constitution 

 of living protoplasm is unknown, partly because it is undoubtedly 

 very complex but chiefly because of its instability and the im- 

 possibility of isolating it without at the same time destroying 

 its life. Moreover, it doubtless varies materially in cells of 

 different types. The proteins, perhaps combined with each 

 other into " giant molecules," undoubtedly constitute the 

 basis and predominating ingredient of protoplasm, but certain 

 lipoids (lecithins and cholesterins), ash ingredients (electrolytes), 

 and perhaps glycogen and other carbohydrates, in addition, 

 of course, to water, appear to be also essential constituents. In 

 the cytoplasm, the simple proteins (41) seem to predominate, 

 while the nucleus is especially characterized by the presence of 

 the nucleoproteins (52). 



76. The cell wall. As already indicated, the protoplasm 

 often develops a cell wall. So far as concerns the species of 

 plants which serve as feed for farm animals, it may be said 

 that a vegetable cell is always surrounded by a cell wall the 

 basic ingredient of which is the carbohydrate cellulose, a sub- 

 stance not found in the bodies of the higher animals. 



In the young and growing parts of plants, the cell wall is thin 

 and consists substantially of cellulose only. In certain parts 

 of plants, such as the cotyledons and endosperms of seeds or 

 the tissues of succulent roots and tubers, the cell wall remains 

 comparatively thin even in mature tissue. In other parts of 

 the plant, on the contrary, it becomes very much thickened by 

 the deposition of additional cellulose and especially of substances 

 other than cellulose. These other substances, which appear 

 to be essentially carbohydrates or their derivatives, are of two 

 general kinds. The first of these is the hemicelluloses (18), 

 which are more readily attacked by hydrolyzing agents than 

 pure cellulose and which constitute to a large extent a deposit 

 of reserve material and include both hexosans and pentosans. 

 The second consists of substances belonging to the lignin and 

 cutin groups (19, 20), which serve to impart strength and rigidity 

 along with more or less impermeability to the cell wall. They 

 are, therefore, particularly abundant in older plants as com- 

 pared with younger ones and in those organs which serve to 

 support the plant, such as the stem. The extreme form of the 

 thickened cell wall is seen in wood. A few of the numerous 



