124 PROTISTS AND DISEASE 



Its relationship to the Mycetozoa proper gives this group 

 a theoretical interest ; its power to cause disease in food- 

 plants a practical economic interest ; and in addition, the 

 likeness the trophic nuclei have to the " endogenous cells " 

 of cancer &c. gives these parasites a definite interest for 

 students of human pathology. 



As examples a member of genera 1, 4, and 5, will be 

 briefly described here. 



Plasmodiophora Brassicae (Woronin, 1877). This parasite 

 causes irregular lumpy growths, known as " finger and toe," 

 or " anbury," on the roots of several cruciferous plants ; 

 cabbage, turnip, etc. In certain localities it has done serious 

 damage, numbers of the infected plants being killed, and 

 others rendered useless for the table. The appearance of 

 the rootlets of a diseased cabbage is shown in Fig. 33 : 1, 

 a and b. The spores are said to remain dormant in the earth 

 during winter, and infect the next spring crop. The cortical 

 layers of the root are irregularly thickened and of an opaque 

 grey colour. The spore hatches out a zoospore, 3, which 

 penetrates a cell near the growing point of a rootlet. The 

 host-cell has a nucleus surrounded by a sheath of cytoplasm, 

 strands of which join that lining the cell- wall ; sap-vacuoles 

 occupy the intervening spaces. In this environment the 

 parasite becomes amoeboid, and in the earliest stage recog- 

 nisable by the microscope, it possesses two or more nuclei, 

 each with a nucleolus, round which is a clear space separated 

 by a sharp boundary from the cytoplasm, 1. These nuclei are 



