16 SOME REASONS FOR STUDYING BIOLOGY 



We do not always realize that if it were not for the green plants, 

 there would be no animals on the earth. Green plants furnish 

 food to animals. Even the meat-eating animals feed upon those 

 that feed upon plants. How the plants manufacture this food 

 and the relation they bear to animals will be discussed in later 

 chapters. Plants furnish man with the greater part of his food 

 in the form of grains and cereals, fruits and nuts, edible roots and 

 leaves; they provide his domesticated animals with food; they 

 give him timber for his houses and wood and coal for his fires; 

 they provide him with pulp wood, from which he makes his paper, 

 and oak galls, from which he may make ink. Much of man's cloth 

 ing and the thread with which it is sewed together come from 

 fiber-producing plants. Most medicines, beverages, flavoring ex 

 tracts, and spices are plant products, while plants are made use of 

 in hundreds of ways in the useful arts and trades, producing var 

 nishes, dyestuffs, rubber, and other products. 



Bacteria in their Relation to Man. In still another way, cer 

 tain plants vitally affect mankind. Tiny plants, called bacteria, 

 so small that millions can exist in a single drop of fluid, exist 

 almost everywhere about us, in water, soil, food, and the air. 

 They play a tremendous part in shaping the destiny of man on 

 the earth. They help him in that they act as scavengers, causing 

 things to decay ; thus they remove the dead bodies of plants and 

 animals from the surface of the earth, and turn this material back 

 to the ground ; they assist the tanner ; they help make cheese and 

 butter ; they improve the soil for crop growing ; so the farmer can 

 not do without them. But they likewise sometimes spoil our meat 

 and fish, and our vegetables and fruits; they sour our milk, and 

 may make our canned goods spoil. Worst of all, they cause dis 

 eases, among others tuberculosis, a disease so harmful as to be 

 called the " white plague." Fully one half of all yearly deaths are 

 caused by these plants. So important are the bacteria that a sub 

 division of biology, called bacteriology, has been named after them, 

 and hundreds of scientists are devoting their lives to the study of 

 bacteria and their control. The greatest of all bacteriologists, 

 Louis Pasteur, once said, " It is within the power of man to cause 

 all parasitic diseases (diseases mostly caused by bacteria) to disap- 



