THE FUNGI : ARCHIMYCETES AND PHYCOMYCETES 195 



producing wide bands of thin-walled tissue (Fig. 185). This is manifested 

 externally by the swelling or hypertrophy of the root, which is the reason 

 for the popular name given to the disease. In these thin-walled cells lie the 





» * 

 •'*'^ 

 -,.^-. 









^^V 





4/^ -^'^ 



•u\ -^S ri,V 



Fig. 185. — Plasmodiophora brassicae. Transverse section 

 of hypertrophied root showing enlarged medullary- 

 rays and spore masses in cells of the ground tissue. 



myxamoebae, which are at first uninucleate. They increase in size, absorbing 

 the food material in the host cells, and they may divide, so that if division of 

 the host cell occurs both the daughter cells become infected. When the 

 tissue becomes more permanent the myxamoeba settles down and grows, 

 without dividing, into a multinucleate plasmodium (Fig. 186). When it 





** f 





If'.'ir'A* 



/^^^''^'^^f 



■■*¥^ ^: 



Early spore stage ^-""^^ - 



Plasmodium 



Fig. 186. — Plasmodiophora brassicae. Multinucleate 

 Plasmodia in tissue cells of host root. 



