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. hemispheric.u.m_

_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 .

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.

This species was in the previous edition distinguished from the Rostafinskian _P. reticulatum_ with spores a little smaller, 6-8 , and with a much stronger tendency to the formation of definite sporangia, elongate indeed and branching but often globose or depressed globose.

This we may know as,

VAR. RETICULATUM Rost.

1875. _Chondrioderma reticulatum_ Rost., _Mon._, p. 170.

1894. _Diderma reticulatum_ (Rost.) Morg., _Jour. Cin. Soc._, p. 71.

Sporangia gregarious, generally rounded, not much depressed, flat, sometimes, especially toward the margin of a colony, elongate, venulose or somewhat plasmodiocarpous, dull white, the inner peridium ashen or bluish, remote from the calcareous crust, which is extremely fragile, easily sh.e.l.ling off; columella indistinguishable from the base of the sporangium, thin, alutaceous; capillitium of short, generally colorless, delicate, sparingly branching or anastomosing threads perpendicular to the columella; spores black in ma.s.s, by transmitted light violet-tinted, smooth, 6-8 .

Perhaps our most common form. Found in fall on dead twigs, leaves, etc.

Recognized by its rather large, white, depressed or flattened sporangia tending to form reticulations, and hence suggesting the name. The lines of fruiting tend to follow the venation of the supporting leaf; where the sporangium is round, the columella is a distinct rounded or cake-like body; where the fruit is venulose, the columella is less distinct.

By these rounded forms we pa.s.s easily, as by a gate, to _D.

hemispheric.u.m_, which, when wholly sessile, differs still in greater diameter of the sporangia and in having somewhat larger spores. Usually in such case the compared colony will show somewhere a very short and stout but very real stipe supporting the discoid fruit.

Rostafinski divided the genus _Chondrioderma_, i. e. _Diderma_, into three sections:--

_Monoderma_ to include those species in which the calcareous crust is less distinct or connate with the true peridium.

_Diderma_, in which the two structures were plainly separate.

_Leangium_, used as in the present work. In his first section Rostafinski placed _C. reticulatum_ and _C. michelii_; in the second, _C. difforme_ and _C. calcareum_.

Lister has examined Rostafinski's type of _C. reticulatum_ and declares that it has the usual didermic characters. Hence there is no doubt that our small-spored American specimens are covered by Rostafinski's description, No. 72. On the other hand, Lister makes _C. difforme_ (Pers.) Rost. a _Didymium_, by its crystalline coat. That species therefore is removed from consideration in this connection. _C.

calcareum_ remains as applicable to American forms having the spores 10-12 , but according to the author of the species the capillitium is abundant and definitive. Unhappily the type of _C. calcareum_ is lost (Lister, _Mon._, p. 95), so that there is no other means of verification than the description and Rostafinski's figure. Under these circ.u.mstances we consider the name _calcareum_ inapplicable to any American forms we have so far seen. See next species. As to the American species which have been distributed as _C. calcareum_ (Lk.) Rost., they are, so far as seen, referable to _D. reticulatum_ (Rost.), Morg. Here also belongs No.

1217, Ellis, _N. A. F._