42 
Ohio Biological Survey 
The apothecia of all of the Collemaceae studied possess a more 
or less strongly developed exciple. Surrounding the exciple and often 
extending above it is a thalloid margin, composed of elements of the 
lichen thallus, with which algal-host filaments are intermingled. The 
thalloid margin is sometimes absent or overgrown, when the exciple is 
naked as in many other ascomycetes. In some of the species, one must 
resort to the oil-immersion lens to demonstrate the exciple. This struc¬ 
ture extends below into the hypothecium, which is much thicker as a 
rule and is always easily demonstrable. It needs to be made clear that 
the thalloid margin is not a part of the apothecium, but belongs rather 
to the somatic tract of the lichen, and is intermingled with elements of 
the algal-host colony, which, in this family, obscure the real thalloid 
margin. 
The Collemaceae form a very distinct group of ascomycetes, especially 
peculiar with respect to conditions of parasitism and resulting thallus 
disposition and structure. In the lower members of the family, the 
thallus is a mycelium pure and simple as found in Syneclioblastiis and 
Collema. In certain other genera as Leptogium and Mallotiiini, the 
mycelial structure gives way in part to a cortical plectenchyma. In rare 
instances, the plectenchyma extends throughout a large portion of the 
thallus, which, as noted above, is said by some students to be wholly 
plectenchymatous in some species. A plectenchyma sometimes occurs in 
the hypothecium and in the exciple, even in the lower members of the 
family, in which the vegetative areas are wholly mycelial. Hence, it 
seems safe to assume that the most primitive plectenchyma is that found 
in the fruit rather than that found in the somatic areas. About and 
below the apothecia of Lcptogla, the plectenchyma is usually several 
layers of cells in thickness (Fig. 3), and it is similarly thickened below 
the apothecia in the material of Mallotium at hand for examination. 
An interesting series of transitional conditions which indicates how 
a cortical plectenchyma may have arisen came to light in our studies of 
Synechohlastus Haccidus (Figs. 6, 7, and 8). Short cells at the upper 
and the lower ends of erect hyphse often coalesce over limited areas of 
the thallus and form either a typical plectenchyma, or a structure strongly 
suggestive of a cortical plectenchyma. In some instances, the cells which 
coalesce are elongated and form a palisade which dififers considerably 
from a true plectenchyma (Fig. 6). Over other areas the short end- 
cells are formed but do not cohere (Fig. 7). On account of these 
transitional conditions, this species is often placed in the genus Leptogium 
in herbaria. In Leptogium chloromclum, the plectenchymatous cortex is 
