70 DISEASES OF TROPICAL PLANTS 



CH. 



and known as bacteria, others long and rod -like are 

 known as bacilli, while others are 'more or less spiral- 

 shaped and known as spirilla. They may be separate 

 or bound together in filaments, planes, or masses by 

 means of a gelatinous sheath. 



When the conditions are favourable they reproduce 

 very rapidly by simple division ; each cell dividing into 

 two, and each of these dividing, etc., until a very 

 large number are formed. When the conditions are 

 unfavourable each cell may develop a thick wall and 

 become a spore, which in due time under favourable 

 conditions may give rise to a new colony. 



The physiological activities of different species of 

 bacteria are quite different. Some of them cause 

 diseases in animals, others in plants, others are useful 

 in certain industries, while still others hasten the decay 

 and removal of organic waste substances. Some require 

 air, while others live only where the air is inaccessible. 

 Some of them produce colouring substances, some are 

 phosphorescent, and still others produce heat. 



Among the most important in agriculture are the 

 nitrifying bacteria, which live in the soil and in the 

 nodules on the roots of legumes. Certain bacteria take 

 the free nitrogen of the air, which is not available for 

 plant food, and construct proteid compounds in the 

 nodules. In the decay of these nodules, the proteids 

 are eventually reduced to nitrates by the action of the 

 nitrifying bacteria, and thus become available food for 

 higher plants. This explains in brief why leguminous 

 plants are improvers of the soils. Many diseases of 

 plants are due to bacteria ; in some cases the disease 

 makes itself manifest in the breaking down of the 

 tissues, and later in the death of the plant, as in the 

 case of the bud rot of the coco-nut, banana, etc. 



Among other important bacterial diseases are those 

 caused by Bacillus gossypina, Sted., of the cotton ; B. 

 amylovorus, of the pear ; B. solanacearum, E. F. Smith, 

 of the tobacco, tomato, potato, egg-plant, and pepper 

 (pp. 210, 216, 221, 226); B. traclieiphilus, Smith, of 



