The spore wall in the smut fungus Entorrhiza has been investigated, wi
th particular reference to layer 3. The wall is stratified into four l
ayers, numbered 1-4 from the outside to inside of the wall. Layer 3 ha
s a lamellated or striated organization, depending on the type of spec
imen preparation used for examination. In this study, thin sectioning
and freeze-etching methods were used in transmission electron microsco
py. Layer 3 is approximately 50 nm thick and is the narrowest layer of
the wall. Thin sections viewed at high magnification show a lamellate
d organization, consisting of alternate electron dense and translucent
spaces. Usually between 16-20 lamellae form the layer, with a lamella
having a thickness of about 1-2 nm. At high resolution, each electron
dense lamella is resolved as a row of closely packed subunits, approx
imately 1. 5 nm in diameter. The electron translucent lamellae probabl
y represent mainly lipoidal material, which is extracted during specim
en preparation. In freeze-etch preparations layer 3 is termed the ''st
riated layer''. Fracturing exposes face views of the layer, which at l
ow magnification has a wrinkled appearance. At high magnification, lay
er 3 has a structure consisting of an irregular mosaic of striations.
The striations in each area of the mosaic are arranged parallel, and r
egularly spaced 11-13 nm apart. Each area of the mosaic is separated f
rom an adjacent area by a small step; this represents where the plane
of fracture changed to a different level within layer 3. Fracturing pr
obably occurs in the lipoidal region of the layer, corresponding to th
e electron translucent lamellae seen in thin section. At high magnific
ation, the striations are resolved into subunit-like particles, approx
imately 1-2 nm in diameter. Layer 3 is believed to form an impervious
region in the spore wall. Layer 3 in Entorrhiza closely resembles the
''partition layer'' reported in spores of other Tilletiaceae. This dem
onstration of a common wall layer strengthens the relationship between
Entorrhiza and the rest of the Tilletiaceae. Entorrhiza is the only s
mut that forms galls on host roots.