68 



THE FUNGI WHICH CAUSE PLANT DISEASE 



Key to Genera op-Olpidiaceae 



Vegetative body amoeboid 



Vegetative body of definite form 

 Sporangia free in the cells of the host 

 Sporangial membrane very delicate, 



evanescent 



Sporangial membrane firm, swarm 

 spores escaping by a definite open- 

 ing 

 Sporangium globular or ellipsoid 

 Sporangium with only one or two 

 openings 

 Swarm spores uniciliate 

 Vegetative cells globose or sub- 

 globose 

 Vegetative cells stellate 

 Swarm spores biciliate 

 Sporangium with several openings 



Sporangium elongate 



Sporangial membrane united to the wall of 

 the host cell 



1. Reessia. 



2. Sphserita. 



3. Olpidium, p. 68. 



4. Asterocystis, p. 69. 



5. Olpidiopsis. 



6. Pleotrachelus. 



7. Ectrogella. 



8. Pleolpidium. 



Olpidium A Braun 



In this genus a single swarm spore invades the cell of the host 



and develops in its pro- 

 toplasm. Later a cell 

 wall forms and the vege- 

 tative body changes into 

 a zoosporangium which 

 develops a neck. This 

 reaches to the outside 

 of the host even though 

 the fungus be developed 

 several cells below the 

 surface. The uniciliate 

 zoospores pass out 

 through this neck to 

 make their escape. Thick-walled resting spores are also formed. 



Fio. 40. — O. braaaicoe; right, 

 a cell; left, resting spores. 



three sporangia in 

 After Woronin. 



