113
ON A NEW SPECIES OF DIDYMIUM OCCURRING
IN ESSEX.
BY G. LISTER, F.L.S.
(With one Plate.)
[Read 25th November, 1922.]
THE subject of this note was first obtained by Mr. James
Saunders, A.L.S., in the summer of 1897, when hunting
for Mycetozoa in prolific heaps of old straw near Barton, Bed-
fordshire. The small sessile white sporangia resembled those
of Didymium difforme Duby, with which they were often
associated, but even in the field they could be generally dis-
tinguished by the external crust of lime-crystals being not quite
so smooth and egg-shell-like as in the latter species; when
examined microscopically the spores were found to be rough with
scattered spines, not smooth or marked only with a low branch-
ing ridge, as in D. difforme.
In the same straw heap the fragile top-shaped sporangia of
Didymium vaccinum (Dur. & Mont.) Buchet1 were also abundant,
a species then new to us; so similar are the two forms in some
respects, it seemed possible that the smaller sessile form might
be a variety of D. vaccinum. Experience has shown, however,
that this is not the case; the sessile form proves to be a widely
distributed and constant species, retaining its characters in a
number of different habitats. I propose to name it Didymium
trachysporum, in reference to the rough spores. It has been
obtained from the following localities:—from near Luton, Bed-
fordshire, on old straw, by Mr. Saunders in 1906; from Lesmoir,
Aberdeenshire, on dead grass, by the Rev. W. Cran, in 1913;
repeatedly, in cultures, on old pellets of rabbit and deer, and old
bean stalks, Berlin, by Dr. Jahn; from near Vienna, also on
deer-pellets, by Christian Lippert, in about 1895, and from
Pornic, Loire Inferieure, on old straw, by M. S. Buchet, in 1912.
In the summer of 1917 it was abundant on old straw manure in
our garden at Leytonstone, Essex, and also on straw near Theydon
Bois. During the present summer and autumn the Rev. P. J.
1 This name replaces Didymium Trochus Lister, M. Samuel Buchet having found that the
specimen from Algeria, on old Opuntia stem, described as Diderma vaccinum Durieu Montagne
(published in Exploration Scientifique de l'Algerie, 1846, p 407 (22 bis, fig 1 a to h) is the same
species; the specific name Trochus is thus antedated ; M. Buchet has published the com-
bination Didymium vaccinum in Bulletin de la Societe Mycologique de France, xxxvi.,
p. 110 (1920).
H