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WHOLE SPORES
COLOR: Dark orange-brown (0-60-100-10) to red-brown (40-80-100-0), most tending toward the latter at maturity.Immature spores are white to cream with a rose tint (0-10-40-0) under a dissecting microscope and in water (in Melzer's reagent they will turn almost black).
SHAPE: Subglobose to oblong.
SIZE DISTRIBUTION: 120-200 µm, mean = 159 µm (n = 95).
SPORE WALL: Three layers (L1, L2, and L3) with L1 and L2 of equal thickness at the start of spore differentiation, with L2 then undergoing further transformations (see sequence below). It is uncertain whether L3 forms concurrent with or following differentiation of L2 because it is so hard to resolve even in mature spores.
| Sequence in differentiation of spore wall layers (left to right): (i) Thin adherent L1 and L2; (ii) growth of L2; (iii) same stage, but in Melzer's reagent, (iv) pigmentation forming in in L2, and (v) transformation of L2 into rigid darkly pigmented laminae. |
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L1: An outer permanent rigid layer with tightly packed short rounded warts 1.0-2.5 µm high, pale brown (0-20-50-10) in color. Warts often become invisible in PVLG mountants after several months storage, although this effect is variable. They still are evident on the spore wall of holotype specimens.
L2: A layer consisting of fine orange-brown (20-80-80-0) to red-brown (20-80-100-0) sublayers (or laminae); 5.0-9.0 µm thick (mean = 7.4 m) in mature spores. This layer stains dark red-brown (20-80-70-10) in Melzer's reagent.
L3: A very thin hyaline flexible layer, < 1 µm thick, can be seen in vigorously crushed spores, usually only where it attaches to the spore wall near the occluding plug and the sporogenous cell region (not shown). A homologous layer was first detected in S. cerradensis and provided the impetus to search for a similar layer in other Scutellospora species.
GERMINAL WALLS: Two bilayered flexible hyaline inner walls (gw1 and gw2) that are synthesized consecutively after the spore wall has completed differentiation. They are formed completely separate from the spore wall. In many mature spores, both germinal walls tend to stay closely paired, even with applied pressure (see photos below).
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GW1: Two layers are formed (L1 and L2) that are tightly adherent. L1 is less than 0.5 µm thick; L2 is slightly thicker (0.8-1.5 µm). Both layers are thin enough that when tightly adherent, they appear as one layer. However, sometimes L1 separates in patches and resembles "warts" or undulations in the wall, much like that found in the only germinal wall of S. coralloidea, S. fulgida, S. gregaria, S. persica, and S. verrucosa.
GW2: Two layers are formed (L1 and L2) that are tightly adherent. L1 is 0.5-0.8 µm thick; L2 is 0.9-1.8 µm thick. Both of these layers also are thin enough to appear as one layer in PVLG. However, only L2 differentially stains a pinkish purple (0-60-30-0) to slightly darker purple (20-60-20-0) in Melzer's reagent.
SUBTENDING
HYPHA WIDTH OF SPOROGENOUS CELL: 24-28 µm (mean = 26.4 µm) (see photos above).
SPOROGENOUS CELL WALL STRUCTURE: Two layers (L1 and L2) probably are present (continuous with the two layers of the spore wall), but only L2 is readily discernible at the level of the compound microscope.
L2: Orange-brown in color (0-60-80-0), 1.8-2.2 µm thick near the spore and then thinning to 0.6-0.8 µm beyond the sporogenous cell.
OCCLUSION: : Closure by a plug concolorous with L2 of the spore wall.
GERMINATION
SHIELDCOLOR: Pale yellow-brown (0-30-70-10) to darker orange-brown (0-40-100-10).
SHAPE: Oblong, with length approximately 1.5 times that of the width. Margin of the shields is fairly smooth, with only a few folds and attendant paired germ holes. Shape of the shield resembles that of a violin. Position of the shield is on iw2.
AUXILIARY CELLS
Aggregate (1-10) cells borne on coiled brown (20-40-80-0) hyphae 4-6 µm in diameter; thin-walled (< 1 µm thick), with a light brown (0-20-70-10) wall in transmitted light; cells almost smooth with some having slight undulations on the surface.
Intraradical arbuscules and hyphae consistently stain darkly in roots treated with trypan blue. Arbuscules with many fine tips from a swollen trunk (see photos below). Hyphae often with knobs or projections, usually densely coiled near entry points.
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As the laminate layer of the spore wall (L2) becomes rigid, it then acquires orange and red pigmentation.
The first germinal wall was redescribed (Koske and Walker, 1985) as having two "membranous walls" with some undefined adhesive causing them to stick together. The spores studied were neither fresh nor healthy, so the two layers of each wall were not seen.
REFERENCES
Koske, R. E. and C. Walker. 1985. Species of Gigaspora (Endogonaceae) with roughened outer walls. Mycologia 77:702-720.