PARASITIC ON PELLIA EPIPHYLLA. 103 
the margin of the diseased area small white downy patches (Pl. 6. 
fig. 1) strongly resembling those of Penicillium when grown on 
nutrient gelatine for class-work in the laboratory. One of these 
patches was carefully teased, and showed a dense mat of sep- 
tate mycelium from which very numerous aerial hyphe radiated 
outwards. Later these patches became bluish, again recalling 
Penicillium; but they then passed through a blue-green colour 
to a distinct sap-green. A teased specimen (Pl. 7. fig. 26) now 
showed numerous round greenish spores borne terminally in 
clusters on the aerial hyphze, which are therefore the conidiophores 
arising from the septate mycelium. These spore-bearing patches 
occurred chiefly, though not exclusively, at the margin of the 
diseased thallus, though I never found them at that corner of 
the pan where the disease first appeared. 
My next step was to determine whether or not a similar 
mycelium was present in the thallus ; and, if present, whether it 
was intracellular or intercellular. 
The structure of Pellia thallus is easily made out from a 
transverse section. The extreme edge is usually only a single 
cell in thickness, though occasionally a double layer may be 
found. Proceeding towards the ** midrib,” the thickness increases 
to three layers of cells, of which the central layer is of large cells, 
covered above and below by the smaller, flatter, superficial cells 
that contain a larger number of chlorophyll corpuscles. At the 
midrib there may be as many as fourteen layers of cells, all con- 
taining a certain number of chloroplastids, which, however, are 
specially abundant in the upper superficial cells and those imme- 
diately below. At this region many of the cells of the lower 
superficial layer are prolonged into brown rhizoids. 
The progress of the disease could be easily traced in each piece 
of thallus. The oldest part was brown, but towards the apex 
the normal green colour of the thallus persisted. So general 
was this appearance and so suggestive of the entrance of the 
fungus by the older part of the thallus, that I could not help 
Wondering as to its precise significance. Does the fungus live 
as a saprophyte on the dead thallus, and thence extend as a 
parasite into the living tissues ? or is this appearance associated 
With the fact that rhizoids are not yet developed at the apical 
parts, and is it by them that the fungus enters ? Such questions 
I hoped to be able to auswer as the investigation proceeded. 
