497 
liberated. This could, however, not take place, if the epidermis 
and the upper and lower cortical layers were unbroken or without 
intercellular spaces. I shall now discuss some of the structural 
adaptations to meet this requirement. 
On the closer examination of a thin vertical section of the 
thallus, it is found that in many of the foliaceous lichens numerous 
intercellular canals (intercellular spaces) pass from the algal layer 
through the cortical and epidermal layers. These canals, which 
resemble the stomata in their physiological function, facilitate the 
interchange of the gases resulting from the activities of chloro- 
phyllian assimilation. The canals (breathing pores) are much 
branched and occur most frequently in the thin areas of the upper 
cortical layer. Asa rule, they do not pass to the exterior in a 
vertical direction. In the epidermal layer they take almost a hort- 
zontal course; this is because the cells of the epidermis are elon- 
gated horizontally. 
In the dry state these canals are practically closed, thus reducing 
the loss of moisture to a minimum. In the moist state they en- 
large considerably, thus enabling assimilation, which is dependent 
upon the presence of CO, and sunlight, to take place. These 
breathing pores are especially numerous in Mephromium, Solorina 
and Parmelia. 
Other lichen genera do not have the breathing pores just 
Mentioned. For example they are not noticable in the genera 
Sticta and Stictina. In these two genera the epidermal layer is 
distinct, as has been stated. The upper cortical layer is of uniform 
thickness and consists of very compact, rather small hyphal cells. 
The lower cortical layer is comparatively thin and its cells are less 
closely united; yet they are sufficiently compact that no inter- 
cellular spaces can be detected. The question now arises, how are 
the enclosed algae * supplied with the necessary atmosphere? Inmy 
opinion this is done by means of the so-called cyphellae which 
Occur on the lower surface of the thallus. These structures have 
long been known to lichenologists. Haller (1776) was perhaps 
the first author who called special attention to them. He de- 
nf! have purposely substituted the term “algae” for the term “ gonidia.” Go 
nidia and related terms as “ gonimia,” “ gonidimia,” etc., are meaningless in modern 
lichenology, and should therefore be rejected. 
