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.