COMPOSITION OF ANIMALS AND OF FEEDING STUFFS 45 



forms of vegetable cells are illustrated in Figs. 43-45 of 

 Chapter XV. 



77. Cell enclosures. In addition to the essential constitu- 

 ents of the cytoplasm and nucleus there are observed in cells 

 a variety of other substances designated as subsidiary ingredi- 

 ents or cell enclosures. These may consist of food material 

 which has entered the cell and is on its way to being incor- 

 porated into the molecules of protoplasm, or, on the other hand, 

 of waste products of cell activity on their way to being ex- 

 creted from the cell into the surrounding medium. Moreover, 

 many cells have the power of storing up surplus food, especially 

 non-nitrogenous substances, as reserve material. Such material 

 is not usually regarded as constituting a part of the protoplasm 

 but as being simply included in it mechanically. 



The most common cell enclosure in the animal is fat, which 

 is contained in large quantity in certain connective tissue cells 

 and constitutes the reserve fuel material of the animal body, 

 the storage of carbohydrates (glycogen) being much more 

 limited in amount. While some important groups of plants 

 also store up large amounts of fat in their seeds, nevertheless 

 the predominating reserve materials in the vegetable kingdom 

 are carbohydrates, including the reserve carbohydrates of the 

 cell wall and, as a cell enclosure, starch. Starch is found in 

 all parts of plants, but is especially abundant in seeds and in 

 the starchy roots and tubers, where large amounts of this sub- 

 stance are stored up. Illustrations of plant cells containing 

 starch are afforded by Figs. 43-45 of Chapter XV. 



Both because of the chemical composition of the cell wall 

 and the nature of the cell enclosures, carbohydrates are quan- 

 titatively the predominating ingredients of most plants, while 

 animal cells and tissues are chiefly proteid or fatty in character. 



2. ANIMAL TISSUES AND ORGANS 



78. Classification. Not only do the cells of higher animals 

 show extreme differentiation of form and function, but cells 

 having the same general nature and office are associated together 

 to form what are called tissues, such as nerve tissue, muscular 

 tissue, connective tissue and the like, each serving its own 

 specific purpose. These tissues, again, are grouped together 



