22 CONTRIBUTIONS FROM THE NATIONAL HERBARIUM. 
CYPHELLA. 
These structures are small pits found on the lower side of certain 
thalli. They are common in the majority of Stictas (pl. 26, facing 
p.154). As lenticels commonly appear at points corresponding to the 
positions of stomata in seed plants, so the cyphelle appear at points 
on the lower surface of the lichen thallus corresponding to the position 
of breathing pores. (See next section.) In their development a 
circular break first appears in the lower cortex, over the breathing 
pore. As the opening in the cortex increases in size, the adjacent 
hyphe of the medullary layer give off numerous short branches, which 
fill the bottom of the opening so that the cyphella appears as a minute 
cup-shaped or saucer-shaped depression, whose bottom and sides are 
covered over with a dense coating of short hyphal branches. Indeed, 
in all of the Minnesota Stictas that contain cyphelle these branches 
completely fill the cavity of the organ. The position of the cyphelle 
over the breathing pores would seem to indicate that they serve to 
admit air to the interior of the thallus. 
BREATHING PORES. 
In lichens having a thallus well developed on both sides, it is evident 
that some provision for the admission of air to the interior will be 
advantageous. This is especially true where the cellular cortex is 
quite thick. The breathing pores of the lower cortex in Stictas have 
just been mentioned. Such structures are found in the lower cortex 
of many other lichens, but are even more common in the upper cel- 
lular cortex. They consist of more or less branched pores extending 
from the surface of the algal layer upward through the cortex in a 
somewhat irregular course and having no proper wall of their own 
but forming simply intercellular canals. They resemble stomata 
in that they can be closed, and also, as in the case of stomata, it is 
doubtful whether they are really of as much use for the exchange of 
gases as for some other purpose. They are found closed when the 
thallus becomes dry, and this doubtless helps to prevent the transpi- 
ration of moisture. Similar openings between the hyphe of pseu- 
docortices may sometimes be made out, especially when these cortices 
are quite thick. However, whether the cortex is cellular or not, 
these pores are to be looked for, for the most part, in the thinnest 
portions of the cortex, especially over areas where the algal cells are 
numerous. 
GROWTH OF THALLI. 
Since as a rule it is the fungus or the thallus proper which deter- 
mines the form, the study of growth may naturally begin with this, 
though its growth is accompanied by an increase in the number of 
algal cells. In crustose and foliose lichens the growth is for the most 
part horizontal and mainly at or just behind the margin of the thallus. 
