DIDERMA 121 



in the mature fructification, remote from the outer; columella generally 

 prominent. 



The genus Diderma is usually easy of recognition, by reason of its 

 double wall, the outer crustaceous and usually calcareous, and its 

 limits remain substantially as originally set by Persoon. His definition 

 is as follows: 



"Peridium ut plurimum duplex; exterius fragile; interius pellucens, 

 subdistans. Columella magna, subrotunda. Fila parca latentia." 

 (Syn. Meth. Fung. 164.) 



Rostafinski changed the name of the genus to Chondrioderma, 

 seemingly at de Bary's suggestion, and seems to have regarded Per- 

 soon's definition as applicable to those species in which the wall is not 

 only plainly double, but in which the two walls are as plainly remote 

 from each other. More especially he esteemed a new generic name 

 necessary, since he regarded several included species, e. g., D. spumari- 

 oides, D. hemisphericum, as monodermic. Persoon's Diderma, when 

 established, included D. floriforme. He caused some confusion in his 

 later work by admitting certain physarums. This induced Schrader 

 to throw all the didermas into his new genus, Didymium. 



KEY TO THE SPECIES OF DIDERMA 



Outer wall of sporangium calcareous, crustose, fragile; 



inner wall membranous, usually distant, rarely lacking I. Section Eudiderma 

 Outer wall of sporangium cartilaginous; inner wall often 



not distinct, or concrete with the outer II. Section Leangium 



I. EUDDDERMA 



o. More or less plasmodiocarpous or on a plasmodiocarp- 



like hypothallus ° 



a. Sporangiate or forming small separate plasmodiocarps d 



b. Broadly effused, flattened, varying to short plasmo- 

 diocarps massed in a larger network; occasionally of 

 separate sporangia; spores 7-10 \x I. D. ejfusum 



b. Plasmodiocarps smaller, spores larger c 



c. Plasmodiocarpous, or sporangiate on a plasmodiocarp- 



like hypothallus; spores about 12 n 2. D. alpinum 



c. Plasmodiocarpous to discoid, rarely with a short stalk; 



spores 10-15 yn 3. D. chondrioderma 



d. Spores marked with prominent but incomplete 



reticulations 4. D. subdictyospermum 



d. Spores warted, spiny or nearly smooth e 



e. Brown or brick-red, fading to brownish ochraceous. . 5. D. simplex 



e. White, grayish or pale flesh colored / 



/. Seated on a common hypothallus i 



f. Sporangia not on a common hypothallus * 



g. Outer wall not porcelain-like, adhering to inner wall . . 6. D. spumarioides 



