272 FOUNDATIONS OF BOTANY 
330. Examination of the Thallus. — The thallus of Physcia as seen 
in cross-section will be found to consist of four layers, the upper 
cortical, gonidial, medullary, and the lower cortical. The cortical 
layers will be seen to serve for protection, answering the purpose of 
an epidermis or bark. The cells which compose them make. what 
is called a false parenchyma, — resembling parenchyma in form but 
— as to origin being trans- 
RES ee TAOS 
c aie GI SOdWY ae fe 
s() Give VIOLA ote the orm © the 
BIS es hyphe composing the 
g 
A 
medullary layer. Are 
there any cross-parti- 
tions? Do any cells 
appear circular, and if 
so, what is the explana- 
tion? The upper por- 
tion of the cortical 
layer, having green 
cells intermixed, con- 
stitutes the gonidial 
layer. Why should the © 
green cells be at the 
upper part of the med- 
ullary layer? Can you 
detect any connection 
between the green cells 
Fig. 200. — Transverse Section through Thallus and ne hyphs : De 
of a Lichen (Sticta fuliginosa). (x 500.) these green cells re- 
c, cortical or epidermal layer ; g, gonidia ; h, hyphe. semble any cells pre- 
viously studied ? 
Make a diagram to show the structure of the thallus. 
What arrangement of layers would you expect to find in a lichen 
thallus, upright or suspended? Compare the arrangement in the 
fruit-body (apothecium), describe, and sketch. How does the layer 
of cells beneath the spore-sacs resemble the cortical layer? All but 
these two layers may be considered as part of the thallus. To make 
out the details of the fruit, the section must be very thin. 
