When an animal eats a plant or part of one, he is eating for the sake of the sugar or sugar products which the plant has made. There is one sugar product that is useful as food for many animals, but not for man, except possibly to a very slight extent. That is the woody substance, cellulose, which is formed in plants mainly as a support to the delicate living protoplasm. Cotton fiber is nearly pure cellulose. Cellulose is very similar to starch chemically, and is an excellent fuel wherever it can be burned. The human digestive tract is unable to handle it in a manner to make it usable, although grazing animals do so quite efficiently. A good many plants make products that are either disagreeable in flavor or actually poisonous. Of course, in such cases the plants become useless as food unless a treatment can be devised that will remove the objectionable material or convert it into something harmless. The few dozen kinds of plants that we raise for food are those that are free from harmful substances and that yield large quantities of stored food materials, or in some cases that taste especially good, even though they may not have much food value. Tomatoes, lettuce, and the like, come in this latter class. The world has been pretty well ransacked for food grains, fruits, and herbs, but probably there are others yet to be found besides those we now have. CHAPTER IV THE USES OF FOOD We have had a good deal to say thus far about power development in living animals, and have talked about food in connection with its use as fuel for the purpose. While we are on the topic it may be as well to say something about other uses to which food is put in animals besides that of serving as fuel, and also something about what is done with the power that is developed by the burning of such food as is used for fuel. To begin with, it is evident that one use that is made of food is to build the body itself. The new-born infant usually weighs somewhere between 5 and 12 pounds. From birth until the body gets its growth there is an almost continuous gain in weight until a total which may range anywhere between 90 and 250 pounds is reached. Of course, every bit of this additional material came into the body in the form of food. The whole mass of the body divides itself, as has been said before, into living protoplasm and nonliving substance. We do not know accurately what proportion of the whole weight is made up by protoplasm; it has been estimated at about 60 per cent, but any estimate can be only very rough because about half of the nonliving substance consists of fat deposits which vary greatly in different people. In any case, that part of the food which goes to make gain in weight is passed over to the living [Illustration: DIAGRAM SHOWING THE RATE OF GROWTH IN MAN]