FUNDAMENTAL CONSIDERATIONS 5 



living flesh. In botany, however, assimilation is generally 

 used in a more restricted sense, being limited to those 

 processes which result in the production of nutriment 

 only. Starches and sugars belong to a group of bodies 

 called carbohydrates, the simplest and most widely dis- 

 tributed of the food-stuffs. In plants they form the 

 starting-point of aU the other kinds of food. When we 

 recognize that the vegetation is the ultimate source of 

 food for the whole of the animal kingdom, and that the 

 starting-point of food-production in plants is carbo- 

 hydrate, we begin to realize the importance of that kiadly 

 light without which this earth would be a dead and un- 

 inhabited world. 



Classification of the Primary Food-Stuffs. — The constit- 

 uents of food are divided into three classes : 



1. Carbohydrates {e.g., starches and sugars). — These are 

 organic compounds of rather simple chemical composi- 

 tion, containing the elements carbon, hydrogen, and 

 oxygen. They owe their existence to the green cell of 

 the plant, which is the ultimate source of all the carbo- 

 hydrates in nature. 



2. Proteins.— These are substances of complex chemical 

 composition, and, in addition to carbon, hydrogen, and 

 oxygen, they contaia nitrogen and sulphur, and, iu some 

 cases, phosphorus. The source of the carbon is a carbo- 

 hydrate, while the nitrogen, sulphur, and phosphorus are 

 derived from mineral salts present in the soil and absorbed 

 by the roots in solution in water. The construction of 

 proteins is carried on chiefly in the leaf and at the points 

 of growth, and is not directly dependent upon light. 

 They are utilized in growth, and form the raw material 

 out of which the living protoplasm is made. Proteins 

 occur in all plant and animal tissues — e.g., as albumin in 

 white of egg, gluten in flour, casein in milk and cheese ; 

 lean meat is a mixture of proteins. 



3. Fats and Oils. — These contain carbon, hydrogen, 

 and oxygen, but the proportion of oxygen present is less 

 than in carbohydrates. They occur in plants chiefly as 

 food-reserves in fruits and seeds, and are very rarely 

 produced as the result of photosynthesis. 



Unlike plants, animals can make no food ; all that they 

 receive they obtain directly or indirectly from the vege- 

 table world. Green plants which absorb solar energy are 



