THE FUNGI : ARCHIMYCETES AND PHYCOMYCETES 203 



angium. Thus we see one way in which an ectoparasitic fungus may become 

 an endoparasitic one. 



The Hfe-cycle of Rhizophidium is so simple that it will not be necessary 

 to express it in the form of a diagram. 



Fig. 194. — Evolution of the subsporangial vesicle. A, Plilyctidium brez'ipes. 

 B, Rhisophidium cyclotellae. C, Chytridium schetukii. D, Phlycto- 

 chytrium vernale. E, Entophylyctis bulligera. (After Atkinson.) 



Synchytrium endobioticum (Wart Disease) 



This Fungus is a very important plant pathogen, causing the well-known 

 Wart Disease of potatoes, which may cause very great loss of the tubers 

 either in the ground or later in storage (Fig. 195). The disease is common 

 in most parts of the world where potatoes are grown commercially. In 

 comparativelv recent vears a number of varieties have been produced which 

 are immune to the disease, and such varieties should alone be planted in 

 infected soils. So far as the British Isles are concerned the West of England 

 is more liable to attacks of Wart Disease than either Scotland or the Eastern 

 Counties. 



The Fungus responsible for this disease has a relatively simple life-history, 

 though it is considerably more complex than that of Rhizophidium, in fact 

 Synchytrium endobioticum may be regarded as a member of a specialized side- 

 line from an evolutionary view-point. 



In describing the life-history it is desirable to start with the zoospore, 

 which enters a cell of the young potato tuber at or about soil level. Once 

 inside, this naked body passes to the base of the host cell and there forms a 

 wall around itself. It rounds off and enlarges greatly, and finally forms a 

 thick, two-layered wall, and is called the prosorus (Fig. 196). The nucleus 

 also increases ver\' greatly in size and gives off stainable chromatin material 

 into the cytoplasm. A pore now appears in the outer layer of the wall, and 

 the contents migrate into a sac formed by the extrusion of the inner wall 



