Key to Danish species of Physarum Pers.
| 1 | Fruitbodies predominantly sessile. Sporocarps, plasmodiocarps or pseudoaethalia, occasionally with a constricted basis, which may by drawn out to a lax stalk-like extension of the hypothallus. | 2 |
| 1' | Fruitbodies predominantly stipitate. Sporocarps, but plasmodiocarps and sessile sporocarps may occur, then mostly accompanied by distinctly stipitate sporocarps. | 17 |
| 2 (1) | Peridial lime white, occasionally absent. | 3 |
| 2' | Pridial lime yellow, orange, ochraceous or greyish rose. | 14 |
| 3 (2) | Fruitbodies laterally compressed. | 4 |
| 3' | Fruitbodies not laterally compressed. | 7 |
| 4 (3) | Peridium two-layered. | 5 |
| 4' | Peridum one-layered. | 6 |
| 5 (4) | Plasmodiocarps, often widely expanded and forming a net, more rarely sporocarps. Spores 8-10 µm in diam., pale violaceous brown in transmitted light, finely warted. | P. bivalve Pers. |
| 5' | Sporocarps and plasmodiocarps. Spores 10-13 µm in diam., medium brown in transmitted light, irregularly spinulose. | P. bitectum G. Lister |
| 6 (5) | Sporocarps or very short plasmodiocarps. Spores dark, 10-14 µm in diam. The capillitial lime granules small, rounded. | P. compressum Alb. & Schwein. |
| 6' | Plasmodiocarps forming rosettes or small nets. Spores 7-10 µm in diam. Capillitial lime granules large, elongated. | P. gyrosum Rostaf. |
| 7 (3) | Peridium two-layered. Spores dark brown in transmitted light. | 8 |
| 7' | Peridium one-layered. Spores light brown in transmitted light. | 10 |
| 8 (7) | Spores dark purple brown, with dark warts separated by pale bands. | P. straminipes A. Lister |
| 8' | Spores uniformly dark brown. | 9 |
| 9 (8) | Spores 10-12 µm in diam. Sporocarps ± spherical, always sessile. | P. diderma Rostaf. |
| 9' | Spores 12-15 µm in diam. Sporocarps obovoid to short cylindrical, mostly accompanied by stalked sporocarps. | P. didermoides (Pers.) Rostaf. |
| 10 (7) | Sporocarps always with a large pseudocolumella of white lime. | P. mutabile (Rostaf.) G. Lister |
| 10' | Pseudocolumella absent. | 11 |
| 11 (10) | Sporocarps with a constricted base, closely aggregated. Peridium often with little lime, sporocarp then of a bruish grey colour and occasionally with metallic lustre. | P. confertum T. Macbr. |
| 11' | Sporocarps and/or plsamodiocarps. Peridium mostly distinctly limy. | 12 |
| 12 (11) | Fruitbodies robust, up to 1 mm in diam. Sporocarps or very short plasmodiocarps and then mostly accompanied by stalked sporocarps. On dead wood or bark. | P. leucophaeum Fr. |
| 12' | Fruitbodies small, up to ca. 0.5 mm in diam. Plasmodiocarps, often accompanied by a few sporocarps. Most frequently found on leaves, herbs and compost. | 13 |
| 13 (12) | Spores dark brown to almost black. Mostly long plasmodiocarps. Peridium usually with much lime. | P. vernum Sommerf. |
| 13' | Spores brown. Mostly short plasmodiocarps and sporocarps. Peridium with scattered lime bodies. | P. cinereum (Batsch) Pers. |
| 14 (2) | Fruitbodies greyish rose to almost white plasmodiocarps, often forming rosettes or small nets. | P. gyrosum Rostaf. |
| 14' | Fruitbodies yellow to ochraceous sporocarps or small plasmodiocarps. | 15 |
| 15 (14) | Capillitial lime white to pale yellowish. Spores dark purple brown in transmitted light. | P. contextum (Pers.) Pers. |
| 15' | Capillitial lime yellow, ocasionally faded to light yellow. Spores light brown to medium brown in transmitted light. | 16 |
| 16 (15) | Sporocarps gathered in close clusters, often almost greenish yellow. Mostly on grass and leaves. | P. virescens Ditmar |
| 16' | Sporocarps or short plasmodiocarps, yellow to orange, often faded to yellowish white. Mostly on wood and bark. | P. auriscalpium Cooke |
| 17 (1) | Capllitium ± radiating from the sporocarp base, occasionally from a pseudocolumella. Capilltial lime bodies small and spindle-shaped. | 18 |
| 17' | Capillitium forming a ± isodiametric net. Lime bodies rounded, angular or ± branched. | 20 |
| 18 (17) | Peridial lime yellow, orange or ochraceous. | P. viride (Bull.) Pers. |
| 18' | Peridial lime white. | 19 |
| 19 (18) | Sporocarps ± flattened, mostly long-stalked and nodding. Pseudocapillitium absent. | P. album (Bull.) Chevall. |
| 19' | Sporocarps hemispherical to spherical, upright with a short stalk. Mostly with a distinct pseudocolumella of white lime. | P. robustum (Lister) Nann.-Bremek. |
| 20 (17) | Peridial lime coloured (yellow, ochraceous, beige, orange or brownish). | 21 |
| 20' | Peridial lime white, if the lime i sparse the fruitbody appear greyish to greyish brown due to the spore-colour. | 27 |
| 21 (20) | The stalk contains lime, brittle and opaque in transmitted light. | 22 |
| 21' | Stalk devoid of lime, ± translucent. | 24 |
| 22 (21) | Spores 10-13 µm is diam., almost black, clearly spinulose. Columella large, hemispherical. | P. schroeteri Rostaf. |
| 22' | Spores 8-10 µm in diam., brown, delicately warted. Columella at most a little conical extension of the stalk. | 23 |
| 23 (22) | Lime on peridium and stalk bright yellow. | P. citrinum Schumach. |
| 23' | Peridial lime pale rosy brown to greyish brown. | P. murinum Lister |
| 24 (21) | Columella an extension of the stalk, reaching almost to the apex of the sporocarp. | P. penetrale Rex |
| 24' | Columella absent or is small and conical. | 25 |
| 25 (24) | Peridium with metallic lustre and with small orange lime bodies. The stalk bright orange, translucent. | P. psittacinum Ditmar |
| 25' | Peridium without metallic lustre. | 26 |
| 26 (25) | Spores 8-9 µm in diam. Stalk short, as long as the sporocarp diameter, flesh-coloured. | P. carneum G. Lister & Sturgis |
| 26' | Spores 9-13 µm in diam. Stalk often long, 1-3 × the sporocarp diameter, reddih to greyish brown. | P. oblatum T. Macbr. |
| 27 (20) | Stalk meging gradually into the hypotallus, often lax and flattened. | 28 |
| 27' | Stalk clearly differentiated from the hypotallus. | 29 |
| 28 (27) | Peridium of two clearly separated layers. Spores uniformly dark puple brown. | P. didermoides (Pers.) Rostaf. |
| 28' | Peridal layers closely adjacent. Spores dark purple brown, with dark warts separated by pale lines. | P. straminipes A. Lister |
| 29 (27) | The sporocarp ± kidney-shaped, laterally compressed. | P. compressum Alb. & Schwein. |
| 29' | Sporocarp hemispherical to almost sperical. | 30 |
| 30 (29) | The stalk contains lime. Capillitial net widemeshed with large irregular lime bodies, which often form a pseudocolumella. | P. leucopus Link |
| 30' | Stalk devoid of lime. Capillitial lime bodies small, ± rounded or angular. | 31 |
| 31 (30) | Stalk reddish brown, translucent, often thin. The base of the peridium thickened to form a reddish brown disc. | P. pusillum (Berk. & M.A. Curtis) G. Lister |
| 31' | Stalk greyish to ochraceous, darker towards the base, opaque. Perial base not forming a disc. | P. leucophaeum Fr. |