THE BACTERIA 



211 



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 partially 

 or wholly stop the cellular pas- 

 sages of the host, and are possibly 

 injurious in other ways. 



The question of how these bac- 

 teria 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 should 

 fall upon the surface 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 

 method of infection is by means 

 of biting or stinging insects or of 

 nectar-hunting insects that visit 

 the flowers and fruit. When a 

 few bacteria are inserted into a 

 new twig, leaf, or floral structure, 

 the infection may spread several 

 inches, and soon the blighting 

 begins. When one flower is infected, insects may cany the 

 bacteria to almost every flower on the tree or on other trees 

 in the vicinity. Moreover, when the disease has developed far 

 enough for the characteristic gummy exudations to appear, 

 insects that bite into them may become loaded with the bac- 

 teria and may insert some of them into a new host. In pruning 



i From "Field Studies of the Crown Gall and the Hairy Root of the Apple 

 Tree," Bulletin 186, Bureau of Plant Industry, U.S. Dept. Agr., 1910. 



FIG. 170. A crown gall produced 



by bacteria (Bacillus tumifojciens) 



on young apple tree l 



A, the tissue of the apple plant which 

 grew around the infected area, thus 

 producing the knotted growth known 

 as the gall. One third natural size 



