134 
IMycologia 
tlialli have cracks, through which air may readily reach the 
medulla. Soralia, cyphellae and empty spermagones furnish 
avenues, through which air may enter. Isidioid branchlets have 
thin cortices at their summits, through which air may gain 
entrance. Hollow cylinders in the center of certain fruticose 
thalli serve as air chambers, and these are sometimes in direct 
communication with the exterior through dying away of the 
basal portions of the branch. 
The most extended studies are those of Zukal (153). This 
worker thought that the hyphae of the medulla of lichens, on 
account of their branching and elasticity, might be well adapted 
for retaining air in the interior of the thallus, and for giving it up 
to the tissues of the lichen and the algal host as needed. The 
cortex of many lichens becomes pure white after the surface has 
been moistened, the coloration being due to the air retained within 
the plectenchyma. When the cortex is brought into water under 
cover glass on a slide, treatment with alcohol is necessary to drive 
out the air, the numerous air bubbles disappearing rather slowly. 
Since the plectenchyma of the cortex admits air with great diffi- 
culty, Zukal supposed that the hyphae of the medulla must pass 
the air admitted to the interior of the thallus upward to the algal 
host cells. The difficulty with this is that it makes aeration de- 
pendent upon such crude makeshifts for getting the air into the 
interior of the thalli as have been enumerated above, and these do 
not seem sufficient for adequate exchange of gases for the algal 
hosts. 
Zukal found that when he placed a crustose lichen in glycerine or 
clove oil and observed with high power hand lens, he saw black, 
glistening air bubbles under the liquid at numerous points where 
the atmosphere seemed to be in easy communication with the 
medulla. This he was able to demonstrate for hypophloeodal and 
for hypolithic crustose lichens as well as for those above the 
substratum. He also found that if thin sections through the 
thallus of a lichen that is parasitic on a gelatinous alga be dried 
and brought into water-free glycerine, a large number of black, 
air-containing lines and dots appear in the transparent mass. 
These black lines and dots pierce through the sheaths of the 
gelatinous Nostoc or Gloeocapsa colonies and often follow the 
