PLANTS WITHOUT CHLOROPHYLL 



151 



rotting of animal matter from the skeletons of sponges, and in the 

 process of tanning hides to make leather. 



Nitrogen-fixing Bacteria. — Still other bacteria, as we have 

 seen before, " change over " nitrogen in organic material in the 

 soil and even the free nitrogen of the air so that it can be used by 

 plants in the form of a compound of nitrogen. The bacteria 

 living in tubercles on the 

 roots of clover, beans, peas, 

 etc., have the power of 

 thus '^ fixing " the free 

 nitrogen in the air found 

 between particles of soil. 

 This fact is made use of by 

 farmers who rotate their 

 crops, growing first a crop 

 of clover or other plants 

 having root tubercles, 

 which produce the bac- 

 teria, then plowing these 

 in and planting another 

 crop, as wheat or corn, on 

 the same area. The latter 

 plants, making use of the 

 nitrogen compounds there, 

 produce a larger crop than 

 when grown in ground 

 containing less nitrogenous 

 material. 



Bacteria cause Disease. — The most harmful bacteria are those 

 which cause diseases of plants and animals. Certain diseases of 

 plants — blights, rots, and wilts — are of bacterial nature. These 

 do much annual damage to fruits and other parts of growing 

 plants useful to man as food. But by far the most important 

 are the bacteria which cause disease in man. They accomplish 

 this by becoming parasites in the human body. Millions upon 

 millions of bacteria exist in the human body at all times — in the 

 mouth, on the teeth, in the blood, and especially in the lower 



A field of alfalfa, a plant which harbors the 

 nitrogen-fixing bacteria. 



