IMPORTANCE OF PLANTS AND ANIMALS 3 



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

 tracts, and spices are plant products, while plants are used in hun- 

 dreds of ways in the arts and trades, yielding varnishes, 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 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 tliis 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 likewise they sometimes spoil our meat 

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

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

 diseases, such as typhoid, tuberculosis, pneumonia, and colds. 

 It is estimated that half the deaths that occur each year are caused 

 by these plants. So important are the bacteria that a subdivision 

 of biology, called hacterioV ogy , has been named for them, and 

 hundreds of scientists are devoting their lives to the study of bac- 

 teria and their control. The greatest of all bacteriologists, Louis 

 Pasteur (pas-ttir')/ once said, ''It is within the power of man to 

 cause all parasitic diseases [most of which are caused by bacteria] 

 to disappear from the world." His prophecy is gradually being 

 fulfilled. Each year sees some disease such as diphtheria, typhoid, 

 or scarlet fever conquered or brought under better control through 

 scientific knowledge, and it may be the lot of some boys or girls 

 who read this book to do their share in such work. 



The Harm done by Some Animals. — Animals also play an im- 

 portant part in the world in causing and in carrjdng diseases. Ani- 

 mals that cause disease are usually tiny and live in other animals 

 as parasites; that is, they get their living from the hosts on which 

 they feed and in so doing may cause disease or the death of the 

 hosts. Among the diseases caused by parasitic animals are ma- 

 laria, yellow fever, sleeping sickness, and the hookworm disease. 



1 The diacritic marks are those used in the Webster school dictionaries. 



