16 CONTRIBUTIONS FROM THE NATIONAL HERBARIUM. 
they appear only in sections. They were formerly thought to be of 
considerable value in the determination of species. Their structure 
and functions will be further considered in the section on reproduc- 
tion. The soredia are small, powdery masses, usually whitish in 
color, and scattered over the surface of the thallus as in Pyzxine 
sorediata (pl. 48, facing p. 230) and many other lichens. They will 
be further considered in succeeding sections. Cephalodia are wart- 
like bodies found on the upper surface of the thallus as in Peltigera 
aphthosa (pl. 30, facing p. 160), or within the thallus, as in some other 
lichens. Cyphelle are small pits or depressions in the lower surface 
of some foliose thalli as in some Stictas (pl. 26, facing p. 154). Cepha- 
lodia and Cyphelle will be further considered under minute mor- 
phology, as their structure can be made out only with the microscope. 
Finally, in some lich- 
ens the so-called hypo- 
c 4 thallus is conspicuous 
f to the unaided eye or 
a . 
seen with the lens. 
> This is true of some 
members of the genus 
Pannaria. For in- 
stance, in Pannaria 
Fig. 3.—G@yrophora cylindrica. a, Median section through a sper- nigra thi s structure 
magonium; 6, sterigmata and spermatia; c, sterigmata and sper- . ; . 
matia ofa Cladonia. a, Enlarged 90 diameters; 6, 390 diameters; &PPears as a bluish 
¢, 450 diameters. a and b from DeBary; ¢ from Tulasne. black ring all around 
the thallus. Its nature is not well understood, but possibly it is : 
remnant of some lichen that the Pannaria has overgrown. Some of 
the older authors considered the rhizoids a portion of the hypo- 
thallus. In the present work there has seldom been oceasion to refer 
to this little-understood structure. 
MINUTE MORPHOLOGY. 
FOLIOSE THALLI. 
In the foliose type of thallus there are the following layers: An 
upper dermis, an upper cellular cortex, an algal layer,? a medullary 
layer, and a lower cellular cortex (fig. 4, p. 17). The dermis, how- 
ever, is scarcely distinguishable in many foliose lichens, and there are 
several other variations from the structure just outlined, For in- 
stance, in Collema (pl. 21, facing p. 136) there is no cortex, and the 
alge are not arranged in a definite layer; in some of the Physcias 
the cellular cortex is replaced by layers of densely interwoven hyphae; 
in Peltigera the upper cellular cortex is present, but there is none 
below; a few small foliose thalli, as in some Acarosporas (pl. 32, 
4 The alge, however, are not strictly a part of the thallus but are inclosed within it. 
