Tur NoRTHERN MIcROscoPIst. 
ING, 5. MAY. 1881. 
AN INTRODUCTION TO THE STUDY OF 
LICHENS, 
By THE Rev. W. JOHNSON, 
Concluded from p. 90. 
‘lag stratified cellular layers of.the Lichen proper are called :— 
1. Zhe cortical layer, (1 & 4 Fig. 13). This is the outer covering 
of the thallus; and is a somewhat tough, transparent, cellular mem- 
brane. In Peltigera horizontalis, we found it to be three or four cells 
deep; these cells in the horizontal thallus, are generally angular from 
lateral pressure ; but in fruticulose forms as in Ramalina, they are 
tubular and elongate. 2. Zhe gonidial layer, (2 Fig. 13). This 
lies immediately beneath the cortical stratum, and is composed 
of globular, free, bright green, yellowish, or bluish-green cells. 
The gonidia in some species, instead of being simple cells filled with 
green granular matter, are found to consist of clusters of two, three, 
or more roundish granules, with no distinct cell-membrane. These 
are termed, gvanula gonima. 3. The medullary layer, (3 Fig. 13), 
This is a mass of cylindrical, interlacing, articulate, colourless cells ; 
which are so woven together, as often to make a milk-white or flesh 
coloured spongy mass. ‘They enclose the gonidia on the under- 
side of the thallus, and constitute the base of the plant. These 
three cellular layers are quite distinct from each other. So clearly, 
that in some of the foliaceous Lichens, they may be recognised by 
the naked eye; yet, notwithstanding their distinctness, they per- 
fectly cohere and unitedly fulfil the vegetative functions of the 
plant. Intermediate between all the cells of the thallus, as well as 
the germinal organs, is a transparent gummy substance called 
lichenine, which binds the whole together, and gives elasticity and 
comparative toughness to the several parts of the thallus when 
moist. This gummy matter evidently, has a close connection with 
the development of the spores ; hence, it concentrates and lodges 
in large quantity in the Aymenium of the apothecia, which when 
