496 
blance to the cuticle of higher plants. We find a similar cuticular 
covering in the related genera, Mallotium, Hydrothyria and Poly- 
chidium. This structure not only serves the function of an epi- 
dermal layer, that is, prevents excessive evaporation, but also a 
mechanical function, giving greater firmness to the thallus. In 
other words, the single layer of cells in the above genera of the 
Collemaceae serves a function similar to the many-layered epider- 
mis and cortical structure in S¢tcfa, Stictina and other genera. 
The epidermal layer is generally colored, usually a yellowish 
brown or dark brown. This is no doubt for the purpose of tem- 
pering the influence of sunlight. We may safely assume that the 
epidermal layer serves three functions. Its primary function is to 
prevent the excessive evaporation of moisture; its secondary func- 
tions are to reduce the injurious effect that direct isolation would 
have upon the symbiotic algae and to aid in protecting the under- 
lying tissues as well as to give additional firmness to the thallus.* 
Below the epidermis is found the cortical layer, which is usually of 
considerable thickness in the majority of foliaceous lichens. I will 
not dwell upon its anatomical structure, as that is sufficiently well 
known. Its primary function is mechanical. It also serves a 
function similar to that of the epidermis; it prevents the evapora- . 
tion of moisture. 
According to the principles of mechanics and for the purpose 
of protection the cortical layer should occur near the upper surface 
of the thallus. But for physiological reasons the algal layer 
should also be near the upper surface, that the algae may be acted 
upon by the sunlight and that they may more readily come in_ 
contact with the CO, of the atmosphere and that they may readily 
give up the O which is liberated as the result of assimilation. We 
actually find such an adaptive relationship between the algal 
layer and the cortical layer. In many of the foliaceous lichens 
we find that the algae extend almost to the upper surface of cer- 
tain circumscribed areas; that is the cortical layer is not of 
uniform thickness. Such an arrangement enables the algae 
to take up CO, from the atmosphere in exchange for the O 
*It should be borne in mind that there is no living structure whose function is 
purely mechanical or purely physiological. «Dead ” structures may have only a me- 
chanical function. 
