The Collemaceae of Ohio 
41 
and we have not attempted to differentiate. Loosely interwoven hyphse 
with much elongated cells we have designated mycelial structure. The 
mycelial structure is probably present in the medullse of all somatic 
tracts within the group, though a plectenchymatoid appearance is some¬ 
times seen in the medulla in instances in which the hyphse are so densely 
interwoven and so interspersed with algal-host cells as to obscure the 
structure. The hypothecia and the exciples may be composed of ordinary 
long-celled, but closely interwoven hyphse (Fig. 9), or they may be 
plectenchymatous (Fig. 10). The hyphse of the medullse extend in various 
directions (Fig. 1), and the number that lie perpendicular to the surfaces 
of the thallus is usually largest about the apothecia and toward the 
margins of the thalli. This distribution gives strength to the thalli. 
Perhaps no part of our work presented greater difficulty than de¬ 
termining whether certain hypothecia, exciples, and medullse, or certain 
portions of these structures, are plectenchymatous or whether they are 
composed of closely interwoven hyphse which have elongated cells. 
Transitional conditions occur, and one may easily mistake interwoven 
hyphse with elongated cells for a plectenchyma when the hyphse are 
closely packed. We found a plectenchymatoid appearance in the medullse 
of certain specimens of Leptogium tremelloides (Fig. 2) and Leptogium 
cliloromelum, and even in the same section, certain portions may show a 
plainly mycelial structure while others, in which the hyphse are densely 
packed, may appear like a plectenchyma. Careful maceration failed to 
show a true plectenchyma in any instance in our specimens of these two 
species. Other species are said by certain workers to be plectenchy¬ 
matous throughout, but it may well be doubted whether this is true. We 
examined Leptogium tcnuissinium, which is figured in Engler and Prantl 
as plectenchymatous throughout, but found a plainly but densely mycelial 
structure toward the lower surface. Leptogium plectenchymiim, de¬ 
scribed in this paper, varies greatly with respect to position and amount 
of plectenchyma, but we found no specimens which were wholly 
plectenchymatous in the vegetative areas. Certain hypothecia and 
exciples present greater difficulty than any of the medullje that we have 
examined, and only the use of high powers, the oil-immersion lens, and 
careful maceration will solve the problem in many instances. We do 
not wish to leave the impression that such obstacles arise in the study 
of all the species of the Collemaceae. On the other hand, the typical 
plectenchyma (Fig. 3) and the typical mycelial structure (Fig. 1) are 
readily recognizable. 
