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 Fie. 170. A crown gall produced 
the Howers and fruit. When a by bacteria (Bacillus tumifaciens) 
. ‘i on young apple tree! 
few bacteria are inserted into a ; 
i A, the tissue of the apple plant which 
new twig, leaf, or floral structure, grew around the infected area, thus 
the infection may spread several serene ng tir een ee 
inches, and soon the blighting 
begins. When one flower is infected, insects may carry 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 
1 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. 
