114
THE ESSEX NATURALIST.
Alexander has gathered it repeatedly on dead elm leaves in the
grounds of St. George's College, Weybridge, Surrey.
When the Leytonstone growth was in vigour, some of the
straw was brought indoors and kept moist under a piece of wet
pasteboard; a month later a number of sporangia, some very
minute, others forming long slender or ring-shaped plasmodio-
carps, had formed on the under surface of the pasteboard;
all the spores examined had the characteristic rough markings.
The following is a description of Didymium trachysporum:—
Plasmodium colourless; sporangia more or less scattered, sessile,
white or cream coloured, either hemispherical, 0.2 to 0.6 mm.
diam., or forming slender curved simple or branched plasmodio-
carps; the outer sporangium-wall a smooth and egg-shell-
like or wrinkled crust of closely compacted crystals of carbonate
of lime; the inner wall membraneous, colourless; the floor of
the sporangium pale yellow, membraneous with a thickened
margin, usually containing scanty deposits of lime-crystals,
occasionally these are sufficiently abundant to form a small
convex columella ; capillitium rather scanty, variable in char-
acter even in adjacent sporangia, consisting of threads which are
either colourless or dark, stout or slender, nearly simple or
branched and anastomosing, sometimes expanded to form
vesicles containing lime-crystals: spores clear brownish-purple,
9 to 10μ diameter, marked with short spines which are either
scattered or grouped in clusters, rarely marked with an im-
perfect reticulation; the spore-wall is often traversed by a low
ridge.
The present species shows affinity with several other members
of the genus. D. quitense Torrend, of which only two specimens
have been recorded, from Ecuador and from Colorado, re-
sembles L. trachysporum in general appearance, but has large
and very dark brown spores 13 to 14μ diam., marked with a
close imperfect reticulation. From D. vaccinum the new species
is distinguished by the sessile and often depressed character
of the sporangia, whose external crust breaks away in fragments,
instead of falling away as a whole, and also by the absence of a
prominent columella. From D. dubium Rost. it differs in the
sporangia not being solitary, in the more scanty capillitium
threads which are often stouter at the base, and in the rougher
spores. From sessile forms of the protean species D. squamulo-