Plasmodium pale, watery-white or gray; sporangia crowded, ovoid or cylindric, stipitate or sessile, blue-gray, often capped with white; stipe variable in length and structure, where well developed pure white, often flattened, expanded and diaphanous, connate with others through the irregular reticulate or sheet-like hypothallus; columella none; capillitium ample, the lime knots angular or rounded, white connected by hyaline threads; spores in mass black, by transmitted light dark violet, decidedly spinulose, 12-15 µ.
A very variable species in many particulars. The sporangia in the same cluster are stipitate and sessile, ovoid and spherical. Our description does not quite agree with that of Rostafinski. As may be seen from the plate, it is the _outer_ peridium that is with us white, burdened with lime, the inner is simpler and comparatively thin. The whiteness of the outer peridium is however, easily displaced. The colony may not show it at all, in which case the peridia remaining give to the fructification entire a pale lead color, very characteristic. The disposition of the lime in the capillitium is also notably variable. Specimens occur which in so far realize Rostafinski's _Crateriachea_; that is, the lime is massed as a snow-white pseudo-columella in the centre of each sporangium. In such cases the lime of the outer peridium is scant or limited in amount, never forming the calcareous cap shown in Fig. 1. The size of the spores is also variable. Rostafinski gives 12.5-14.2 µ; not infrequently a single spore reaches 16 µ, a very unusual range of variation.
The species is not common in the upper Mississippi valley, but can be obtained in quantity where once it appears, as the plasmodia are profuse.
Ohio, Carolinas, Tennessee, Iowa, South Dakota, Kansas. Especially to be looked for on the bark of fallen stems of _Populus_ and _Negundo_.
_Physarum lividum_ Rost., _Mon._, p. 95, is but a less calcareous form of this, as is evident even by the author's description. Professor Morgan thought _P. lividum_ a phase of _P. griseum_ Lk. Link, however, reckons _P. griseum_ the same as _P. cinereum_. Link, _Diss._, I., p. 27.
1809. _Physarum leucopus_ Link, _Diss._, I, p. 27.
Sporangia gregarious, stipitate, globose snow-white, with a didymium like covering of calcareous particles; stipe white, not long, conical or tapering rapidly upward, slightly sulcate, brittle, from an evanescent hypothallus; columella none or small; capillitium, consisting of rather long hyaline threads, connecting the usual calcareous nodes, which are large, angular, snow-white; spore-mass black; spores by transmitted light, violet-brown, distinctly warted, about 10 µ.