Summary:
The very large spores of Glomus macrocarpum measure 90-180 microns and are usually nearly round. They occur in clusters in soil or in fruitbodies up to 1.2cm wide. Fruitbodies are irregularly spherical to ellipsoid and beige to brown, with or without a white, cottony peridium that collapses when touched. Trappe(8) corrected the Latin ending to neuter, because Glomus is a neuter noun despite its ending. Gerdemann(1) examined "large-spored" collections of the type variety with spores often exceeding 150 microns in diameter (ranging up to 230 microns), and "small-pored" collections with spores ranging from 50-150 microns. Berch(2) recombined as separate Glomus species three of the synonyms given by Gerdemann(1): E. versiforme Karst. (here considered Diversispora versiformis, see SIMILAR), E. tenebrosa Thaxter, and E. australis Berk. As a result the spore size ranges only up to 140 microns. Gerdemann(1) also examined collections from Washington, Oregon, and California of var. geosporus, which is distinguished from the type variety "in not forming sporocarps, in its very dark spore walls, and in the long extension of wall thickening along the spore-bearing hyphae", (Gerdemann) but this has subsequently been raised to species status as Glomus geosporum (T.H. Nicolson & Gerd.) Walker. At least in the wide sense G. macrocarpum is abundant among false truffles in the Pacific Northwest and locally common worldwide, and forms arbuscular mycorrhizae with many plants, (Trappe(13)).

Interior:
in section, the spore mass contains soil debris and no cavity, and it may exude an oily latex, (Berch(2))

Microscopic:
chlamydospores 93-206(230) microns, usually round to nearly round, occasionally elliptic or oboval, smooth or appearing rough from debris, spore wall "up to 14 microns thick, light yellow-brown to brown, often becoming perforated at maturity"; spores borne in sporocarps or in open clusters, (Gerdemann), chlamydospores (90)120(140) x (70)110(130) microns in lectotype, but up to 180 microns in some collections, usually slightly longer than wide, nearly round to round, to irregular, spores wall of 2 distinct layers, the inner 6-12 microns thick and yellow, the outer 1-2 microns and colorless, (Berch(1)), chlamydospores (80)120(190) x (65)110(170) microns, outer spore wall smooth and colorless, swells in KOH, and is 1-4 microns thick, the inner spore wall is yellow or orange-brown, unaffected by KOH, 3-12 microns thick, and may be laminate; "gleba consists of brown chlamydospores embedded in loosely packed, highly branched, thin-walled, interwoven, light beige hyphae that are 5-15 microns wide", (Berch(2))

Notes:
The "large-spored" collections examined by Gerdemann(1) came from CA, and the "small-spored" from BC, WA, OR, CA; they said that the species is widely distributed over much of the world. Berch(1) examined collections in a narrow sense from OR, CA, PQ, ME, France, and the USSR.