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WHOLE SPORES
COLOR:
Hyaline/white in most recently formed spores to
yellow-brown (0-5-40-0) in older spores (especially those from field soils).
SHAPE:
Globose, subglobose, often elliptical or strongly oblong.
SIZE
DISTRIBUTION: 120-240 µm, mean = 189 µm (n = 128).
SPORE WALL: Three layers (L1, L2 and L3) that are adherent that in juvenile spores are of equal thickness, with the laminate layer (L2) thickening as the spore wall is differentiated. Below (left to right) is a linear sequence in the differentiation of a spore wall and two flexible inner walls. It is uncertain when L3 appears developmentally.
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L1:
An outer permanent rigid hyaline layer with a smooth surface, 1.8-5.0 µm thick
and tightly adherent to L2. Easily distinguished from L2 when spores are placed
in Melzer's reagent, where L1 is nonreactive and L2 stains dark red-purple.
L2: A layer consisting of very fine adherent hyaline to pale
yellow (0-0-20-0) sublayers (or laminae), 3.0-8.8 µm thick (mean of 6.5 µm)
in mature spores; staining dark red-purple (40-80-40-0) to reddish-black (60-80-50-10)
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.
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In PVLG
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In PVLG & Melzer's reagent
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GERMINAL WALLS:
Two bi-layered hyaline
flexible inner walls (gw1 and gw2) are formed that readily separate from each
other and from the spore wall. Both walls form in sequence and begin synthesis
only after the spore wall has completed differentiation (see photos above)
GW1: Two layers (L1 and L2) that usually are adherent, but
sometimes L1 separates slightly and produces numerous fine folds. L1 is less
than 0.5 µm thick and L2 is 1.0-2.8 µm thick. Neither layer reacts in Melzer's
reagent.
GW2: Two layers (L1 and L2) that rarely separate. L1 is 2.0-4.8
µm thick and can produce a weak pink reaction (0-10-20-0) in Melzer's reagent
that is detected only when it separates from L2. L2 is hyaline and plastic enough
that it has been termed "amorphous". It is 4.0-18.0 µm thick (measured
in PVLG), depending on amount of pressure applied to it while breaking the spore;
staining red-purple (20-80-20-0) to dark red-purple (40-80-60-0) in Melzer's
reagent.
WIDTH OF SPOROGENOUS CELL:
32-45 µm (mean = 37.6 µm).
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: Hyaline to pale yellow (0-0-20-0), 2.5-3.2 µm thick near
the spore and then thinning to 0.8-1.2 µm beyond the sporogenous cell.
OCCLUSION:
Closure by a plug concolorous with the laminate layer of the spore wall.
COLOR:
Hyaline to pale yellow-brown
(0-10-30-10).
SHAPE: Ovoid,
with length approximately 1.5 times that of the width. Margins of shields generally
are smooth, with few folds (each with paired germ holes). The shield is sufficiently
robust to separate intact from the inner flexible walls when spores are broken.
Position of the shield is on iw2.
SIZE: Not
measured
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Aggregate (1-12) cells borne on coiled brown (20-40-80-0) hyphae, thin-walled (< 1 µm thick), pale yellow-brown (0-20-50-10) in transmitted light, each cell with shallow swellings 0.5-2 µm m high and 7-10 µm wide.
Intraradical arbuscules and hyphae consistently stain darkly in roots treated with trypan blue. Arbuscules with many fine tips from a swollen trunk. Hyphae often with knobs or projections, usually densely coiled near entry points. Auxiliary cells and external hyphae often amass around the root, with the former most abundant in pot cultures just prior to spore formation and declining thereafter.
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Older spores resemble those of S. calospora under a dissecting microscope, except they are more oblong and larger in size range. The original description of S. pellucida (Koske and Walker, 1986) illustrates the problems in interpreting what constituted "walls" and "wall groups" (Walker, 1983).
REFERENCES
Koske, R. E. and C. Walker. 1986.
Species of Scutellospora (Endogonaceae) with smooth-walled spores from
maritime sand dunes: Two new species and a redescription of the spores of Scutellospora
pellucida and Scutellospora calospora. Mycotaxon 27:219-235.
Walker, C. 1983. Taxonomic concepts in the Endogonaceae: spore wall concepts
in species descriptions. Mycotaxon 18: 443-455.