POPLAR DISEASES ' 305 



limb-galls of various sizes (Fig. 57). No definite proof is 

 available whether or not these various galls are due to the 

 plant-cancer bacterium. Galls have been produced, however, 

 on poplar limbs artificially and the organism has been isolated 

 from galls on poplar occurring in nature. The galls are hard, 

 woody structures variable in size. The surface is rough and 

 covered with small knobs of tissue. The causal organism is 

 a species of bacterium. Little is known of its life history, so 

 far as its dissemination and resting stages are concerned outside 

 the host plant. The effect of the organism on the plant-tissue 

 has been made the subject of classical studies which have 

 revealed many interesting analogies to human cancer. 



References 



Smith, E. F., Brown, N. A., and Townsend, C. 0. Crown-gall of 



plants : its cause and remedy. U. S. Dept. Agr. Bur. PI. Ind. 



Bui. 213 : 3-215 ; pis. 1-36, flgs! 1-3. 1911. 

 Smith, E. F., Brown, N. A., and McCuUoeh, L. The structure and 



development of crown gall : a plant cancer. U. S. Dept. Agr. 



Bur. PL Ind. Bui. 255 : 3-60, pis. 1-109, flgs. 1-2. 1912. 



Common White Wood-Rot 



Caused by Fomes igniarius Fries 



White wood-rot caused by the false-tinder fungus (F. igni- 

 arius) is the most common wood-rot disease of deciduous 

 trees. It is the cause of enormous losses in hardwood forests 

 because of the destruction of the timber of the trees. Since 

 also the sapwood may be affected, it becomes an equally im- 

 portant disease of valuable individual trees. It does not 

 seem, however, to be very common outside of the forest areas 

 and does not equal in importance the brown checked wood-rot 

 in the relative damage caused to shade and ornamental trees. 

 The following kinds of trees are known to be affected by the 

 common white wood-rot : beech, poplar, willow, maple, birch, 



X 



