THE ECONOMIC BIOLOGY OF PLANTS 447 



useful and that some disease germs are not bacteria at all, but are 

 protozoan animals. Other fungi also cause harm to man's crops 

 and foods; among these are the rusts, molds, smuts, and mildews, 

 which have also been studied before. Some plants are poisonous 

 and do a little harm in that way; among these may be mentioned 

 certain mushrooms, poison ivy, water hemlock, etc. In cultivated 

 land, many wild plants cause harm by interfering with crop growth. 

 We call these " weeds " and they demand much labor and expense 

 for their control. 



We shall now take up some of the economic applications of plant 

 biology in detail. - 



Oxygen Supply. The importance of plants as a source of oxygen 

 and in removal of carbon dioxide has been explained in Chapter 13 

 but cannot be over-emphasized. Without this action of plants, the 

 supply of oxygen would be exhausted and no animal life could exist. 



Nitrogen Fixation. The return of nitrogen compounds to the 

 soil by the action of certain bacteria has also been mentioned 

 (Chapter 17) and is one of the ways in which its fertility is main- 

 tained, while the natural decay of the plant tissue also aids in this 

 same process. 



Control of Drainage. The regulation of drainage is brought 

 about by the forests, which act like enormous sponges, soaking up 

 the rains and letting the water filter slowly through the soil, instead 

 of rushing off in floods, as it does when heavy rains fall on barren 

 regions. 



Foods. Cereal Grains. Of all plant parts used for food by man, 

 seeds are the most important, and among them the cereal grains 

 easily take first place. 



These cereals (whence the name?) are the fruits of various grasses 

 and include wheat, corn, rice, rye, barley, oats, etc. They con- 

 stitute the most important group of food stuffs used by man 

 or other animals. In their composition these grains contain but 

 little water, hence they keep well, and store considerable food in 

 a small bulk: they are all rich in starch. Wheat contains much 

 proteid (gluten)' and corn is well supplied with oil, of which the 

 other grains contain but little. 



