42 THE MYXOMYCETES 



sisting of a basal disk that supports a stalk which in turn is sur- 

 mounted by the spore mass. The construction of the sporocarp 

 is a communal enterprise. How this division of labor among so 

 many distinct entities is directed and controlled remains entirely- 

 unexplained. The sporocarp of D. discoideiim is always of the 

 same pattern. Similarly, whatever the pattern is for each other 

 species, it is always the same. 



Skupienski (1920) maintained that the amoeboid cells fuse in 

 pairs, a phenomenon that is known to occur among the true 

 slime molds. It seems highly probable that Skupienski is correct 

 as regards fusion among Acrasiales, but this point requires con- 

 firmation. 



The ability of pseudoplasmodia to maintain their specific iden- 

 tities is strikingly demonstrated by the experiments of Raper and 

 Thom (1941). They crushed and thoroughly mixed the pseudo- 

 plasmodia of Dictyosteliimi discoideiim and D. piirpiireiim, and 

 of D. discoideimi and Folysphojidyliinn piirpureum, and from 

 each pair of the mixture typical sorocarps of each species were 

 eventually organized. If these investigators withheld food (bac- 

 teria), the colonies resulting from a mixture of pseudoplasmodia 

 of the two species of Dictyostelium produced sorocarps in which 

 were combined the characters of each species. The combinations 

 were not hybrids, however, but physical mixtures. 



The body of the fructification or sorocarp consists of inert 

 material, and the living portion becomes encysted as multinu- 

 cleate, walled spores that accumulate in a globular body at the 

 tip of the mass. 



The Acrasiales were monographed by Olive (1902) and in- 

 clude three families, the Guttulinaceae, Acrasiaceae, and Dictyo- 

 steliaceae, differing mainly in structure of the sorocarps. The 

 best-known member is the Genus Dictyostelium, investigated by 

 Pinoy (1907), Skupienski (1920), and Raper (1937, 1940). 

 Raper grew Dictyosteliimi discoideum and D. ?mic oroides on a 

 variety of agar media in "pure-mixed culture" with various spe- 

 cies of bacteria, especially Escherichia colt and Serratia mar- 

 cescens. Cohen (1939) grew D. discoideimi in pure culture and 

 in "two-membered" cultures. These species commonly utilize 

 bacteria as food, as was first indicated by Pinoy (1907), and as 

 other slime molds are known to do. They continue to ingest bac- 

 teria so long as bacteria are available and environmental factors are 



