COLOR: Cream (0-0-15-0) to dark orange-brown
(0-40-100-10), most orange-brown (0-30-80-0)

SHAPE: Globose, subglobose.

SIZE DISTRIBUTION: 160-260 µm,
mean = 213 µm (n = 116)

SPORE WALL: Consisting of three layers (L1, L2 and L3) in which L2 branches from the hypha subtending the "sporiferous saccule" (termed a pedicel), followed by synthesis of the outermost layer, L1 (time of synthesis easily pinpointed by sudden appearance of red coloration caused by reaction of L1 in Melzer's reagent--see second and third photos from left below).  L1 originates on the pedicel and forms continuously on the spore; it thickens only on the spore as it differentiates further.  In contrast, L2 thickens on both pedicel and spore as ontogenesis progresses.   L3 arises inside L2 and appears to grow from it (some evidence of association by emergence from the pedicel). 

Sequence (left to right) in the differentiation of a spore from a pedicel branching from the neck of a "sporiferous saccule".

L1: A friable layer with deep fissures in the surface that breaks apart easily with applied pressure, expanding somewhat in PVLG-based mountants; hyaline in youth, becoming pale orange-brown (0-20-40-0) with age; 6-15 µm thick when intact on the spore wall; degrading and/or sloughing with age; staining dark orange-brown (0-60-100-10) to red-brown (20-60-100-0) in Melzer's reagent. 

L2: Hyaline, continuous with the wall of the pedicel; 2.5-4 µm thick; consisting of sublayers that usually are adherent, but which sometimes separate slightly.   The entire layer is semi-flexible and may slough (with subtending pedicel) with L1 or one or more sublayers may remain with the spores.   

L3: Hyaline, rigid, 1.8-2.5 µm thick (most 2 µm), having slate-like properties when broken, forming numerous fracture lines and polygonal shards rather than a clean break (see photo at right).  This layer has birefringent properties in polarized light so that light is reflected brightly, especially at fracture edges.  It is the extensive breakup and concomitant light-scattering of this layer in field collected spores which made this species so hard to describe (see notes section below).

L4: Hyaline, semi-flexible, 3-7 µm thick (mean = 5.2 µm); consisting of multiple sublayers (see photos below) that can split into two groups (suggesting a bilayered organization); no reaction in Melzer's reagent. 

In PVLG				In 1:1 v/v PVLG and Melzer's reagent

Spores with outer two layers sloughed (photos below) are bright white and spores with all layers present (photos above) are pale to dark brown.

PEDICEL: So named because it is a feature divergent from most Acaulospora species, arising as a branch from the neck of the "sporiferous saccule", 200-280 µm from the "saccule". The pedicel is 65-90 µm in length (saccule hypha to spore), 16-28 µm wide at the base of the spore. Just prior to formation a distal thin septum forms that closes off contents of the pedicel and "saccule" from parent hyphae (see photos in developmental sequence above).  The septum is 1-2 µm thick and forms 38-55 µm from the saccule.

PEDICEL WALL: Two hyaline layers (L1 and L2). The outer layer (L1) is continous with L1 of the spore wall, 1.5-2 µm thick, staining dark orange-brown (0-60-80-0) in Melzer's reagent, sloughing except near the spore base as the spore differentiates.  The inner layer (L2) is permanent and continuous with L2 of the spore wall, 3-6 µm thick at the base of the spore when it is mature.

OCCLUSION A flat hyaline "septum" forms as an outgrowth of L2 of the spore wall and is 4-10 µm thick.

"SPORIFEROUS SACCULE"

COLOR: White in youth (see far left photo above) to hyaline at maturity.

SHAPE: Globose to subglobose (see far right photo above for unusual behavior in producing protrusions)

SIZE DISTRIBUTION: 220-300 µm, mean = 260 µm (n = 96)

WALL STRUCTURE: Hyaline and of variable thickness due to patchy sloughing of material so that the outer surface appears flaky; 3.5-7 µm thick.

SUBTENDING HYPHA: 28-56 µm wide at the saccule, with a hyaline wall 2.5-4 µm thick.

Not yet observed in in vitro assays.

Coiled and knobby hyphae, 4-9.0 µm in diameter, with finely branched arbuscules in cortical cells. No vesicles have been observed in the four accessions examined to date. The staining reaction of fungal structures in Melzer's reagent is highly variable, from invisible to a very faint pale blue that often is hard to detect under a stereomicroscope (mounted roots are better).  Contrast was digitally enhanced to view structural details that often are hard to see clearly otherwise. 

Stained roots with spores outside	Light-staining arbuscules and intraradical hyphae

The acaulosporoid spore morphotype of Archaeospora gerdemannii was first described as Glomus gerdemannii and this classification in Glomus was based on appearance of field-collected spores. The unusual properties of L3 of the spore wall when broken caused the spores to look so "messy" that interpretation of structure was difficult (see photo of holotype specimen at right).  Rose et al. (1979) did not appreciate at that time the extent to which spore morphology could be altered by senescence, parasitism, and unnumerable other variables in soil. 

Mode of saccule and spore formation, as well as spore size and subcellular organization, are identical to that of the acaulosporoid synanamorph of Ar. leptoticha. In fact, spores of the two species, with or without attachment to the "sporiferous saccule", are almost indistinguishable under a stereomicroscope. The sole difference in spore morphology resides in properties of the middle two layers of the spore wall. These layers are smooth, albeit with unusual crystalline properties, in spores of Ar. gerdemannii, whereas they have hemispherical depressions/protrusions in spores of Ar. leptoticha. While these differences seem small, this pattern of species level differences in divergent properties of the spore wall and its various layers appears to be universal in all genera of Glomales.