68 



THE PHYCOMYCETES 



In section of diseased tissues it may be noted that the outer 

 host cells are occupied by the pathogen in the vegetative phase. 

 It is an unwalled protoplast (plasmodium) that is able to pene- 

 trate host-cell walls, meanwhile increasing in volume. This 

 protoplast then forms a wall about itself and cleaves into a cluster 

 (sorus) of sporangia. These sporangia are of two kinds: thin- 



FiG. 18. A to C. Stages in liberation of zoospores by Macrochytrmm bo- 

 try dioides. (After von Mindcn.) 



walled ones that can grerminate at once, and thick-walled ones, 

 called resting sporangia, that can germinate only after a pe- 

 riod of dormancy. Under favorable conditions each kind of 

 sporangium organizes \\'ithin itself numerous uniciliate swarm 

 spores that escape into the soil. Swarm spores have an actively 

 amoeboid movement and behave either as s^^'arm spores or as 

 gametes. These single or mated swarm spores are able to initi- 

 ate new infections bv penetrating the "eves" of the young tubers. 

 Studies by Percival (1910) and Curtis (1921) contain detailed 

 accounts of the structure and developmental history of Synchy- 

 trhim evdobioticinn. 



