442 NATURE STUDY AND LIFE 



discussions of the three classes of foods : fats and oils, starches and 

 sugars, and, most important of all, proteids, or nitrogen-containing 

 foods, such as flesh, the white of egg, or the gluten or similar sub- 

 stances of wheat and other vegetable foods. This is the great essen- 

 tial food for all animal life. On no amount of fats or sugars can any 

 animal sustain its life for more than a few days. 



Without the help of the fungi we might, with a great deal of 

 labor, burn everything that died and thus return the elements to the air 

 and to the soil ; but in burning nitrogenous compounds we should return 

 their nitrogen to the air along with all the other gases, and the green 

 plants are not able to take nitrogen directly from the air. They 

 require nitrogen in some soluble form, as nitrates in the soil ; so that 

 the burning of nitrogenous compounds is a most wasteful process. In 

 fact, up to within a few years it used to be said that when a rifle is 

 fired a man is killed, whether the bullet strikes one or not. This 

 was thought to be true, because it was supposed that in burning the 

 soluble nitrate in the powder, thus returning the nitrogen to the air, 

 the nitrate could not be recovered and that eventually some one 

 would starve for the lack of it. How certain bacteria are able to 

 take free nitrogen from the air, and thus give food to plants and 

 everything that lives, we shall discuss in the next chapter. 



Another line along which this may be explained to the children in 

 a practical way has reference to their plant lessons. When the chil- 

 dren were given seeds and asked to see who could rear the best 

 plant, many of them immediately asked : " What will make a plant 

 grow best? What can I feed my plant to make it grow fast?" 

 The answer is nitrates, chiefly of potassium and sodium. These are 

 the main constituents of chemical fertilizers, now so commonly used. 

 No knowledge of chemical formulas is necessary to make this plain. 

 Simply get a little potassium nitrate, let the children see, handle, and 

 taste it, burn a little of it, and, finally, dissolve a teaspoonful in a quart 

 of water and treat a certain plant with it once a week. It would be 

 well to have two similar plants growing in two pots of rather poor 

 soil and give this solution to one and not to the other, to let the class 

 see how it makes the plant grow. This is a simple elementary les- 

 son in fertilization of the soil and will serve to show the r61e that 

 nitrates play in plant growth. 



