PLASMODIOPHORALES 43 



favorable. Raper (1940) found that 20 to 24° C is the optimum 

 temperature and that pH S.S to 7.0 is a favorable reaction for D. 

 discoideum. Decreased relative humidity, increased temperature, 

 and change in light promote fruiting. 



In summary, the Acrasiales are distinct from the slime molds 

 proper in that the swarm cells lack flagella, are not known to 

 fuse, and hence do not form true plasmodia but form pseudo- 

 plasmodia instead. 



Labyrinthulales. The Labyrinthulales are an aberrant, little- 

 known group, mostly parasitic on algae of both fresh-water and 

 marine species, Cladophora and Vaucheria being common hosts. 

 They consist of naked, amoeboid cells connected by pseudopodial 

 processes to form chains or nets, long ago described by Zopf as 

 "net plasmodia." Each net plasmodium increases in size by the 

 formation of new cells. Spores arise by rounding up and encyst- 

 ment of the cells. On germination a single, naked, amoeboid cell 

 is liberated from each spore. These essential features were early 

 determined by Cienkowski (1867). 



Recently Dangeard (1932) described reproduction of another 

 kind, occurring by formation from the net plasmodium of a hol- 

 low sphere of cells aggregated around debris within the host cell. 

 From each cell of the sphere 4 to 8 naked swarm cells escaped by 

 rupture of the wall. This number of swarmers coming from 

 each cell indicates that sexuality and reductional division may be 

 involved in this reproductive process. 



Attention has been focused on the Labyrinthulales in recent 

 years because of the outbreak of a disease, called "wasting 

 disease," of Zostera inarma. This is a marine seed plant of 

 enormous importance as a source of food for wild ducks, geese, 

 and other water fow4 and for many marine animals. Associated 

 with diseased plants is LabyrintJnila 77jacrocystiSy which is re- 

 garded by some workers as the causal agent of the w^asting dis- 

 ease. The reports by Renn (1936) and Young (1938) will ac- 

 quaint the reader with evidences of its parasitism and with the 

 status of this problem. 



Plasmodiophorales. The members of this order, containing 

 approximately only a score of species, are all endoparasites of 

 vascular plants. They involve roots and underground stems and 

 cause the formation of excrescences or galls. Upon disintegra- 

 tion of the host tissues the spores are liberated in the soil. The 



