GROWTH OF ANIMALS; ANIMAL FOOD. 141 



in them, it is believed that they are quite uniform in 

 their value as nutrients. They are not only the most 

 important of the food compounds, but are indispensable, 

 as they are the sole source directly of the albuminoids 

 in the body of the plant-eating animal. 



Other nitrogenous compounds are also contained in 

 small quantities in most plants, though they are rela- 

 tively unimportant as sources of nourishment. 



Pat. — The fat or oil contained in plants agrees closely 

 in chemical composition with that contained in animals. 

 Fats contain a much larger proportion of carbon, and 

 less of oxygen, than the carbohydrates. Fat exists in 

 all plants, and in some seeds, as flax and cotton, in such 

 quantities as to make them of considerable commercial 

 importance as sources of oil. 



Carbohydrates. — These agree closely in composition. 

 They consist of carbon, hydrogen, and oxygen only, and 

 derive their name from the fact that the hydrogen and 

 oxygen in them always exist in the same proportions as 

 they exist in water. Cellulose, or woody fibre, and starch 

 are the most abundant of the carbohydrates, though the 

 sugars and gums are also well-known members of this 

 group. 



Cellulose is the substance which composes the cell walls 

 or woody part of the plant. It is seldom pure, except in 

 young plants. In trees where great strength is needed, 

 the cell walls become thick and hard, and joined with the 

 cellulose is a harder substance called "lignin." In ordi- 

 nary farm plants the cellulose exists in greater propor- 

 tions in the ripe straw, and in the stems and husks of the 

 various plants, than in the seeds. This fibrous material 



