772 A Q UA TIC PH YCOM YCETES 



of Spongospora suhterranea on the potato. His results, although concern- 

 ed with a terrestrial parasite, are of significance here. The resting 

 spores only germinate in the presence of the potato roots and previous 

 drying stimulates the process. Resting-spore zoospores penetrate the 

 root hairs and root epidermal cells and form multinucleate plasmodia. 

 The plasmodial nuclei are of the wheel type and divide protomitot- 

 ically. Zoosporangia are formed by segmentation of the Plasmodium 

 into uninucleate or multinucleate units, which then undergo an akaryote 

 stage and give rise to zoospores after at least one mitotic division of the 

 nuclei. If free moisture is present, the biflagellate and heterocont zoo- 

 spores are discharged into the water, where they may fuse either in pairs 

 or in threes. No nuclear fusion, however, has been observed. 



In the early stages of infection myxamoebae and plasmodia are 

 present in the cells beneath the periderm of the host, whence they 

 proceed to infect the more deeply layered ones of the cortex. Infected 

 cells are stimulated to continuous cell division, producing wartlike out- 

 growths. During these divisions the fungus may be passively distributed 

 among the newly formed cells. As long as host-cell division continues 

 the fungus remains in the vegetative stage, the nuclei dividing by 

 protomitosis. When host-cell division ceases, the myxamoebae fuse to 

 form plasmodia whose nuclei undergo an akaryote stage. Subsequently 

 there are two or three or more consecutive mitotic nuclear divisions, after 

 which the plasmodia develop into the spore balls typical of the fungus. 



With respect to the parasites of aquatic fungi, the studies by Couch, 

 Leitner, and Whiffen (1939) on Octomyxa achlyae and of Goldie-Smith 

 (1951) on Sorodiscus cokeri are of interest. The first-named group of 

 investigators summarize their findings as follows: 



"The species is an obligate parasite on the hyphae of Achlya glomer- 

 ata, causing the formation of gall-like swellings on the hyphal ends. 

 Infection is by single spores which enter the hyphae without leaving a 

 cyst. The spore is carried to the hyphal tip where it develops into a 

 multinucleate Plasmodium. Vegetative nuclear divisions are of the 

 protomitotic type characteristic of the Plasmodiophoraceae. The early 

 formed plasmodia develop into sori of zoosporangia, which in turn 

 give rise to zoospores. The zoospores are biciliate with one long and 

 one short cilium. The later formed plasmodia form sori of resting 



