376 PECULIARITIES OF PLANT LIFE 
first in terms of their usefulness or harmfulness to us. As 
every animal in the world is dependent directly or indirectly 
upon plants for food, it becomes obvious to what a degree 
we are benefited by the ability of plants to make food out 
of the air and the soil. 
Man could live comfortably on what three plant families 
furnish, — the grasses, which include all the cereal foods 
and sugar; the pulse family, which furnishes most of our 
vegetable nitrogen; and the rose family, which includes the 
plants which furnish us our luxuries in the way of fruits. 
In eating animal products, man is still dependent upon the 
grass family to furnish food for the cattle from which he 
obtains meat, milk, cheese, and butter. For clothes, man 
depends indirectly upon plants for the leather and wool 
of the domestic animals, and directly for cotton and linen. 
Plants are the source* of many of the materials out of which 
houses are made and furnished. 
Some plants (bacteria) cause disease, while still others 
provide remedies with which to cure diseases. Plants 
please our eyes as we travel about. They keep up the 
supply of oxygen in the air; they rid the air of the carbon 
dioxide which we have cast off; they provide employment 
for millions of men who raise food plants, manufacture 
them into food, and distribute them throughout the world; 
and they employ other millions in the production of cotton 
plants and cotton cloth for our clothing. 
The farmer who raises plants has an interest in knowing 
what kind of soil and climate, how much water, air, and 
light each kind of plant needs to yield him the best results. 
To this end he has to know something about the habits of 
plants in general, and about their enemies and their dis¬ 
eases. He has learned by experience that some plants 
grow better when planted in hills; others in drills, and 
still Others sown broadcast. He is still trying to find the 
best kind of plant food for each plant, and the method of 
