378 PRACTICAL BOTANY 



Therefore treatment with carbon disulphide is . said not only 

 to prevent phylloxera from living upon the roots of the grape, 

 but, by killing the amoeba-like animals, to make it possible for 

 the bacteria of nitrification to produce more ammonia com- 

 pounds within the soil than would be produced if the bac- 

 teria were themselves being preyed upon by their enemies. 



The student must clearly understand that there are many 

 kinds of soil bacteria. The above statement includes a few 

 facts from a very intricate and as yet incompletely under- 

 stood subject. 



344. Dependent relations of parasitic bacteria. In addition 

 to what was said in the chapter on bacteria, but a brief state- 

 ment need here be made regarding the dependent habit of 

 parasitic bacteria. Many kinds of bacteria may take their food 

 directly from other living things, and are wholly or partially 

 dependent upon living hosts for their food. 



The bacteria which cause " pear blight " may be taken as 

 an illustration of the nature and degree of dependence that 

 may exist. This is a disease which often seriously affects the 

 leaves, young twigs, and fruit of the pear and apple. The 

 bacteria cannot live under long exposure to direct sunlight or 

 to drying, but can endure low temperatures. During the winter 

 they live in the diseased twigs. In the early growing season 

 the leaves and young growth of the twigs become blackened 

 and soon wilt as a result of the internal growth of the parasite. 

 The bacteria secure nourishment from the cells of the host. 

 They may act so as partially or wholly to stop the cellular 

 passages of the host, and possibly are injurious in other ways. 



The question of how these bacteria are distributed to new 

 hosts is important. Even if they should be carried through the 

 air, and should withstand the consequent drying and sunshine, 

 and fall upon the surfaces of twigs, leaves, or fruit of the 

 proper host, it is said that they could not make their way into 

 the tissue. It is believed that the common means of infection 

 is through biting or stinging insects, or nectar-hunting insects 

 that visit the flowers and fruit. When a few bacteria are 



