The North American Slime-Moulds / A Descriptive List of All Species of Myxomycetes Hitherto Reported from the Continent of North America, with Notes on Some Extra-Limital Species

by Thomas H. Macbride

The genus Diderma is usually easy of recognition, by reason of its double wall, the outer, crustaceous, 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., p. 168.

Rostafinski changed the name of the genus to Chondrioderma (chondri, cartilage), seemingly at De Bary's suggestion, and seems to have regarded Persoon's definition as applicable to those species only 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, as D. spumarioides, D. michelii, etc., monodermic.

Since it is doubtful whether any diderma is really monodermic, and since Persoon's definition in any case seems sufficiently elastic, we have seen no reason to discard the older name. Persoon's Diderma when established, l. c., included D. floriforme. He made some confusion in his later work by admitting some physarums. This induced Schrader to throw all the didermas into his new genus, Didymium.

According to the nature of the sporangial wall, the species fall rather naturally into two sections:—

A. Outer sporangial wall distinctly calcareous, fragile; species generally sessile				Diderma
B. Outer sporangial wall cartilaginous, the inner less distinct, or concrete with the outer; species oftener stipitate				Leangium
A. Sub-Genus DIDERMA
1. Fructification wholly plasmodiocarpous				1. D. effusum
2. Fructification of distinct sporangia.
	a. Sporangia on a common hypothallus.
		O Outer wall fragile, not widely remote from the inner		2. D. spumarioides
		OO Inner wall lacking		3. D. simplex
		OOO Outer wall crustaceous, porcelain-like.
			i. Spores 8–10	4. D. globosum
			ii. Spores 12–15	5. D. crustaceum
		OOOO Outer wall firm, not crustaceous		6. D. lyallii
	b. Sporangia isolated, or, at least, not on a common hypothallus, sessile.
		O Outer wall porcellanous, roseate		7. D. testaceum
		OO Outer wall white		8. D. niveum
		OOO Outer wall ashen		9. D. cinereum
	c. Sporangia stipitate			10. D. hemisphericum
B. Sub-Genus LEANGIUM
1. Sporangia generally sessile.
	a. Inner peridium distinct.
		O Membranous colorless, columella scant		11. D. sauteri
		OO Colorless, columella prominent, red		12. D. cor-rubrum
		OOO Outer ochraceous, inner yellow		13. D. ochraceum
	b. Peridial layers inseparable.
		O Peridium multifid; columella small or none		16. D. trevelyani
		OO Peridium breaking into but few irregular lobes; columella prominent.
			i. Peridium umber brown	14. D. roanense
			ii. Peridium ashen	15. D. radiatum
			iii. Peridium chocolate without, inside white	17. D. asteroides
2. Sporangia stipitate.
	a. Peridium pallid, smooth			18. D. floriforme
	b. Peridium white, rugulose			19. D. rugosum

1. Diderma effusum (Schw.) Morgan.

• 1831. Physarum effusum Schw., N. A. F., p. 257.
• 1896. Diderma effusum (Schw.) Morg., Jour. Cin. Soc., p. 71.
• 1899. Diderma effusum (Schw.) Morg., Macbr., N. A. S., p. 94.
• 1899. Diderma reticulatum Rost., Macbr., N. A. S., p. 95.
• 1911. Diderma effusum Morg., Lister, Mycetozoa, 2nd ed., p. 102.

Fructification plasmodiocarpous, reticulate, creeping, applanate and generally widely effused, white; the peridium thin, cinereous, covered by a delicate, white, calcareous crust; the columella simply the base of the plasmodiocarp, thin alutaceous; the capillitium pale, consisting of short threads somewhat branched toward their distal extremities; spores smooth, pale violaceous, 8–10 µ.

This is Physarum effusum Schw., vid. N. A. F., No. 2297. It is reported by Morgan from Ohio, and we have one specimen from eastern Nebraska, so that it is probably of general distribution in the eastern United States.