CAUSES OFlPLANT DISEASES 75 



of these is the Plasmodiophora brassicae, Wor. (p. 232) 

 which causes the well known " club root " upon cabbage, 

 turnip, and many other plants belonging to the family 

 Cruciferae. The galls which are so abundant on the 

 family Leguminoseae are caused by the nitrogen-fixing 

 bacteria (Bacterium radicicola), which are beneficial to 

 plant growth. B. tumefaciens (Smith and Townsend) 

 is the cause of the so-called crown-gall, which has 

 proved so destructive to many plants, especially the 

 family Kosaceae, in many parts of the world. Insects 

 are also the cause of gall-formations on the roots of 

 many plants. 



Upon cutting a nematode gall and examining the 

 cut surface with a hand-lens, a great number of very 

 small bodies will be found imbedded in the abnormal 

 mass of root tissue. A more critical examination with 

 a compound microscope will show that all the tissues of 

 the root have been deformed, and that these small bodies 

 contain one or more of the worms. It is probable that 

 the young worms usually attack the young roots, and 

 no doubt the variation in the character of the deformity 

 depends not only on the host plants but upon the 

 number of worms and the age of the roots at the time 

 of attack. The distortion of the roots, especially those 

 parts through which the food substances of the plant 

 pass, seriously interferes with the normal functions of 

 the plant. Furthermore, these diseased roots eventually 

 die and decay, even though the plant continues to live, 

 and form a most excellent source for infection by root- 

 inhabiting fungi. The upper parts of the plants also 

 become weakened and more susceptible to the attacks 

 of various fungi. 



The worms of H. radicicola are usually hatched 

 within the tissues of the old galls and work their way 

 between the cells or through the ducts to new parts of 

 the same plant, or to other plants. By the use of a 

 spear-like organ in the head they puncture the younger 

 portions of the root, often the root-hairs, and gain 

 entrance and become embedded in the tissues. This 



