444 ' ELEMENTARY STUDIES IN BOTANY 



the parasite. It is easy to discover this trouble, for the rusty 

 patches of spores become abundant on the leaves and stem 

 of the host plant, but the cure seems hopeless, and preven- 

 tion is uncertain as yet. 



Crown gall. This is a very common bacterial disease of 

 trees and shrubs, and among cultivated plants it is note- 

 worthy in the various fruit trees and street trees. As the 

 name implies, the symptom of the disease is the develop- 

 ment of a gall-like growth (tumor) on the " crown " of the 



FIG. 93. Canker of bitter rot on apple twigs. After BURRILL. 



plant, which means the base of the stem where it joins the 

 root (Fig. 94). During the autumn and winter the gall 

 disintegrates and leaves an open wound. At the margin 

 of an old gall, new galls arise, and so the wounds are en- 

 larged from year to year. 



A most interesting fact has been discovered in connection 

 with these galls, and that is that they give rise to a disease- 

 carrying tissue (" infecting strands ") that penetrates to 

 other regions of the plant and gives rise to new galls. This 

 makes it impossible to remove the trouble by surgery, for 

 while galls may be removed and the wounds healed, the 

 " infecting strands " are spreading the trouble into other 

 regions. The whole trouble begins by some wounding or 



