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Pellia epiphylla , (Z.) Cor da. 
immediately beneath. No mycelium bearing conidia like those described 
by Ellis have been observed on the Pellia plants which have been used for 
the present paper ; moreover, infected material remains quite green and 
appears perfectly healthy. It is therefore clear that Ellis was dealing with 
a fungus quite different from the one which forms the subject of this paper. 
He did not notice our fungus apparently. 
H. E. Greenwood (1911), in a paper, ‘ Some Stages in the Development 
of Pellia epiphylla \ described in great detail the structure and life-history 
of the plant, but no mention of the presence of any fungus was made. 
Distribution of the Fungus. 
A. In the Gametophyte. 
Transverse sections of the thallus indicate that the fungus occurs only 
in the lower portion of the thickened central region. 
The fungus is most easily seen in longitudinal sections of the thallus 
through the 4 midrib which, in this species, is normally from ten to twelve 
cells in thickness. The number of fungal hyphae present does not vary 
according to the habitat of the Pellia , but rather according to the degree to 
which the infection of the thallus has proceeded. As the plant is perennial 
this depends mainly on the season of the year. 
In new growths branching from the thallus, only the rhizoids and 
lower epidermis contain the fungus ; in older branches the* two or three 
layers of cells adjoining the ventral surface as well as the rhizoids are 
infected, and in mature branches eight or nine layers of cells are inhabited 
by the fungus ; the upper two or three layers of cells, including the upper 
epidermis, remain free from hyphae. In one or two cases only the upper 
epidermis remained absolutely free of fungal hyphae — all the other cells were 
infected. 
The hyphae apparently enter the thallus through the rhizoids. Two 
or more hyphae may pass up one rhizoid ; in a few cases these show ladder- 
like fusions (cf. Nemec, Calypogea trichomanes ), but more often remain 
quite separate. Cross-walls occur at rather long and irregular intervals. 
The fungus branches as soon as it reaches the thallus, but sometimes 
not until it enters the layer of cells immediately internal to the lower 
epidermis. In the cells of the thallus the fungus branches freely, ramifies 
through the thallus, and extends to within about 2 mm. of the growing- 
point (Fig. 1 ). 
In some cells the ends of the hyphae become considerably swollen. 
These swellings occasionally occur also in intercalary positions, but they 
are usually terminal. They possess very granular contents, are thin- 
walled, and are probably merely swellings of the vegetative mycelium, 
which may be regarded as storage organs (Fig. 1 ). Large spherical and in 
some cases oval-shaped bodies also occur on the hyphae (Fig. 1 ). These 
