es 
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 all 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 kindly 
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 contain nitrogen and sulphur, and, in 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 
