DIDYMIUM 117 



Sporangia gregarious, white, globose, slightly umbilicate, stipitate; 

 peridium thin, and nearly or quite colorless, frosted with crystals of 

 lime; stipe yellowish or yellowish brown, corneous, erect, subulate, 

 slender; hypothallus none; columella pale or white, turbinate, globose 

 or depressed-globose; capillitium of dull brown or colorless threads 

 more or less branched, always white at the tips; spores violaceous, 

 nearly smooth, 7-9 fj,. 



This seems to be the most common form in the United States. It is 

 distinguished from the preceding by the longer, more delicate, generally 

 orange-yellow stem with pale or white columella. N. A. F. 412 and 2089 

 are examples of D. xanthopus. The columella in blown-out specimens 

 is very striking, well confirming the diagnosis of Fries, "valde pro- 

 minens, globosa, stipitata, alba." Berkeley makes the color of the 

 capillitium diagnostic of D. proximum, but this feature is insufficient. 



As noted in the synonymy, in the Lister monograph this is regarded 

 as merely a variety of D. nigripes. This view is confirmed to some 

 extent by the studies of Miss Cayley (1929) who grew both forms in 

 artificial culture and found that the color of the stalks was inconstant, 

 varying with the amount of coloring matter in the substratum. She 

 states, however, that the columella in the xanthopus cultures was 

 always white, and suggests that some of the confusion is due to the 

 occurrence of hybridization between the two forms, and possibly 

 between them and D. eximium. Skupienski (1930) grew D. nigripes 

 in pure culture and found that when the plasmodium fruited at 8-10° C. 

 typical sporangia were produced, while when the same plasmodium 

 fruited at 18-20° C. the sporangia resembled those of D. xanthopus, 

 although both spores and capillitium were abnormal. Intermediate 

 forms undoubtedly occur, but as a rule the collections as brought in 

 from the field are easily distinguishable. The species may be recog- 

 nized tentatively pending fuller information. 



Cionium iridis Ditm., cited by Fries, apparently appeared before 

 C. xanthopus. If the synonymy should be confirmed, and the species 

 continue to be recognized, the name iridis takes precedence. 



Common in the eastern United States and the Mississippi valley, 

 also Washington, Nicaragua, Bolivia, Argentina; Europe, Africa, 

 Ceylon, Java, Japan. Generally distributed in temperate regions. 



18. Didymium vaccinum (Dur. &° Mont.) Buchet 



Bull. Soc. Myc. Fr. 36 : 110. 1920. 



1846. Diderma vaccinum Dur. & Mont., Expl. Sc. Alg. 407. 

 1875. Chondrioderma vaccinum (Dur. & Mont.) Rost., Mon. 180. 

 1898. Didymium trochus List., Jour. Bot. 36 : 164. 



