juneis, I9IS Spongospora Subterranea 275 



before it hardens. They float on the surface of the medium and are held 

 firmly in place when it hardens. The intimate contact thus obtained 

 seems to give better germination than when the spore balls are dusted 

 over the surface of the agar after it has hardened. Either method, how- 

 ever, will give good results. 



In water or on agar containing no organic food material, such as sugars 

 or proteids, germination takes place only after a considerable period of 

 time. Under such conditions the spores may remain inactive for a month 

 or longer, and even then a rather small percentage of them germinate, 

 while on nutrient agar abundant germination is usually obtained. The 

 spore balls in a given culture show considerable variation in the time 

 required for germination. On a favorable medium many of the spore 

 balls germinate within a few days, but in no case have all of the spore 

 balls germinated, even when left on the agar for as long as two months. 

 Why it is that the spores in certain of the spore balls fail to respond is a 

 question that remains to be solved. 



As the amoebae leave the mother spore ball they crawl out over the sur- 

 face of the agar. If the medium is allowed to become somewhat dry, 

 they round up, produce a thick, rough wall, and while thus encysted 

 are probably able to withstand various unfavorable conditions. The 

 thick wall suggests that they would be quite resistant to desiccation, 

 temperature variations, and toxic substances. They are generally uni- 

 nucleate, but occasionally a binucleate cyst may be found. Under favor- 

 able conditions the cysts germinate. Through some means a hole is 

 made in the thick wall and the amoeba crawls out, and again encysts as 

 soon as conditions become unfavorable. It seems that this process can 

 be repeated an indefinite number of times, the cysts or resting spores 

 furnishing a means by which the fungus may live over in the soil from 

 year to year. 



OBSERVATIONS ON PLASMODIA PRODUCED IN CULTURES OF GER- 

 MINATING SPORES 



Mention has already been made of finding plasmodia in the cultures of 

 germinating spore balls. The question that at once arises is whether 

 these Plasmodia are produced through the fusion of the amoebae of 5. 

 subterranea. The only way in which this problem can be definitely and 

 finally solved is through infection experiments. An effort has been 

 made to infect young potato tubers growing in a greenhouse by placing 

 one or more plasmodia on them, but this work has not yet yielded satis- 

 factory results. The method followed was to remove the soil from young 

 tubers without breaking them from the mother plant or otherwise injuring 

 them. The tubers are then washed in water, and a small piece of agar 

 covered with a plasmodium similar to that shown in Plate XLIII, figure 2, 

 is placed on each. In this way the plasmodium is brought into direct 

 contact with the skin of the tuber. The tubers are then covered with 



