146 Journal of Agricultural Research voi. xvi, No. 3 



immersion fluid. After the majority of the zoospores have escaped from 

 the zoosporangium, it is very often the case that the collapsing wall of the 

 endosporangium catches one or more of them within, where their motile 

 stage can be easily studied (PI. 13, /). A single zoospore has plenty of 

 room to swim around within the empty sporangium. 



ZOOSPORE GERMINATION AND HOST PENETRATION 



Within one to two days after the zoospores have come to rest, they 

 begin to germinate by sending out very fine, fibrous hyphge (PI. i3,fe),which 

 cease to grow after they have reached a few microns in length if they fail 

 to come in contact with the host plant. Few of them ever reach this 

 stage if they are kept in ordinary tap water, for they serve as a prey to 

 bacteria and numerous protozoa. They break down completely and 

 apparently disappear under adverse conditions. The small hyphae which 

 were produced by the germinating spores were made visible by applying 

 to the spore solution a few drops of 5 per cent potassium iodid with 

 enough iodin dissolved in it to give the solution a dark-brown color. 

 This gradually killed the zoospores and stained them slightly at the same 

 time. This method was used for staining both the cilia and hyphae of the 

 zoospores, which are so small that it requires a high power lens to make 

 them visible. In this case the immersion lens was used in the manner 

 previously described. 



If the zoospores are in contact with the host epidermis, the fibers con- 

 tinue to grow after germination and penetrate the epidermal cell walls, 

 thereby producing infection (PI. 14). Very delicate technic was 

 required in order to determine this point. The bud of a young corn 

 plant was unfolded until the very thin white tissue, which was free from 

 cholorphyl, was obtained. Sections somewhat smaller than a cover slip 

 were cut from this thin leaf tissue and placed on slides. Drops of a 

 zoospore suspension containing numerous spores were placed on these 

 thin sections. The slides were then placed in the moist chamber and 

 incubated at the same temperature required for resting spore germination. 

 After two days a drop of the iodin solution was placed in the drop of spore 

 suspension, a cover slip placed over the section, and an examination made. 

 It was necessary to use the immersion lens to see what was taking place. 

 Oil could not be used on a loose cover placed in this manner because it 

 was so viscous as to hold the cover slip while the slide was being moved, 

 and thereby disturbed the sections. A drop of water was used instead 

 of oil and was found to be fairly satisfactory. At the end of two days the 

 fine, fibrous hyphae had, in cases, penetrated the epidermis, and some had 

 produced the large swollen cells within the epidermal cells of the host 

 (PI. 14, c, e, /). The germinating zoospores were more commonly found 

 attached to the host near the dividing wall of two epidermal cells. In 

 some instances more than one hypha was seen passing through the host 

 cell wall from a single zoospore (PI. 14, a, h,d). 



