42 Transactions British Mycological Society. 
A transverse section of the thallus taken more than a centi- 
metre from the end of a branch exhibits a dorsiventral structure 
consisting of the following differentiated layers, (a) upper cortical 
layer, (b) gonidial layer, (c) medulla, and (d) lower cortical layer, 
but a section across one of the extreme points of a branch shows 
an approach to a radial arrangement of the above. 
The hyphae of the upper cortical layer are welded together 
so completely that their identity is almost entirely lost, but in 
the part adjoining the gonidial layer it is possible to trace the 
cut ends of the hyphae after treatment with potassium hydrate. 
The gonidial layer consists of bright green spherical cells 
interspersed among hyphae that have moderately thin walls. 
A feature of this layer, especially when there is active growth, 
is that the gonidia are grouped in spherical masses (fig. I), a 
condition that suggests the common origin of each set from a 
single mother cell. The gonidia composing a group are approxi- 
mately of the same size, those of one group may have a diameter 
of three microns while those of another measure as much as 
ten microns across, but groups of intermediate sizes are always 
present. 
The medullary layer extends through one-half of the thickness 
of the section; it consists of a loose meshwork of branched 
hyphae with few septa. The average width of a hypha is 
four microns. 
The mature gonidium of Evernia prunastri is a spherical cell 
with a diameter of from 10 to 15 (fig. 3), except when it is 
subjected to the considerable pressure caused by the rapid 
multiplication of cells during the periods of active growth 
(fig. 1). The colourless cell-wall has a thickness of 1-5. The 
addition of chlorzinc-iodine produces a blue-purple colour reac- 
tion but the walls of the hyphae that are present remain colour- 
less. Each gonidium contains a single bright green parietal 
chloroplast, minutely irregular on the outer surface, surrounding 
a large central nucleus the diameter of which equals one-third 
that of the cell (fig. 2). The cytoplasm is finely reticulated; 
this may account for the statement by a recent writer that a 
network of hyphae, devoid of a cell wall, sometimes surrounds 
the chloroplast. One or more minute peripheral bodies stain 
deeply; they are always surrounded by a light area. The signifi- 
cance of these bodies we have not yet been able to ascertain. 
Most writers regard the central body as a pyrenoid but a 
close examination of it has led to the conclusion that it is the 
nucleus, for when seen with critical illumination and a magnifi- 
cation of 1000 diameters it appears distinctly granular(7) and 
when removing superfluous stain (Heidenhain’s iron-alum-hae- 
matoxylin), by means of the mordant, the colour is reduced 
