THE FUNGI : ARCHIMYCETES AND PHYCOMYCETES 193 



Plasmodium may break up, and by passing from one cell to another during 

 cell division of the host plant it may become widely distributed in the host 

 tissues. The group is a small one represented by less than two dozen species. 

 They are mostly very rare, but two species, Plasmodiophora hrassicae, which 

 causes Club Root or Finger-and-Toe disease of Cruciferae, and Spongospora 



GERMINATION 



SWARM 

 CELL 



MEIOSIS 



GAMETES 



V ZYGOTE 



0j^ 

 (^COALESCENCE 



SPORANGIA 



PLASMODIUM 

 Fig. 182. — Life-cycle of a Myxotnycete. 



subterranea, which is responsible for Powdery Scab of Potatoes, are of con- 

 siderable economic importance. 



We shall take Plasmodiophora brassicae as a type of this order. 



Plasmodiophora brassicae (Finger-and-Toe Disease or Club Root 



of Cabbages) 



Club Root Disease is the name given to a characteristic swollen mal- 

 formation of the roots of cruciferous plants (Fig. 184). It is particularly 

 common on acid soils and frequently causes great loss to market gardeners. 

 The Fungus gains entry to the young root tissues through the root hairs 

 and stimulates great hypertrophy of the tissues. 



In a transverse section through an infected root it will be seen that the 

 normal arrangement of the tissues has been altered by the formation 

 of many additional cells, splitting up the original vascular system and 



