FINK—THE LICHENS OF MINNESOTA. 21 
continuations of certain hyphx of the medullary layer. Plainly 
such a cellular tissue on the lower side of the thallus must serve for 
support, and it becomes very thick in those large foliose thalli which 
are attached by an umbilicus, as Gyrophora, Umbilicaria, and Der- 
matocarpon. But, since it lies between the rhizoids and the tissue 
above, this cortical tissue must permit the passage of any crude or 
elaborated materials taken from the substratum by the rhizoids, for 
which reason it remains thinner than might otherwise be expected. 
Also, the lower cortex is usually a looser tissue than the upper cortex 
and is usually darker colored. Peltigera is a genus of large foliose 
lichens in which there is a well-developed upper cortex and no lower 
cortex, while in Collema (pl. 21, facing p. 136) and Synechoblastus, 
genera containing large foliose forms, there is neither upper nor 
lower cortex. 
SOREDIA AND SPERMAGONIA. 
A soredium consists of a tangled mass of algal cells and fungal 
hyphe which has pushed upward from the algal layer and appears 
at the surface in a small spot where the upper cortex is broken. 
The soredia function for reproduction. The spermagonia (fig. 3, 
a, p. 16) appear as little spherical or cup-shaped bodies, sunken in 
the surface of the thallus. Within each spermagonium are borne 
the spermatia on stalks called sterigmata (fig. 3, 6). The sper- 
matium is a small, slender body, commonly colorless, and straight or 
slightly curved (fig. 3,¢). The spermatia are usually regarded as male 
reproductive bodies. 
CEPHALODIA. 
These bodies sometimes appear upon the surface of the thallus and 
sy other instances oceur within it. Those situated on the upper or 
lower surface are called octotrophic, and those that are found within 
the thallus endotrophie cephalodia. There is much difference of 
opinion as to the frequency of the appearance of cephalodia, due 
partly to lack of careful observation and partly also to difference of 
view as to their nature. A cephalodium is an abnormal development 
upon or within the lichen thallus, containing some other algal sym- 
biont than the one found in other portions of the thallus. The 
cephalodia of our Peltigera a phthosa (pl. 29, facing p. 159) are ecto- 
trophic and plainly visible. The foreign algze found in the cephalodia 
of this lichen are of the genus Rivularia, and they are supposed to be 
brought to the thallus in some way and, being held by the tricho- 
matic hyphe, to multiply and become surrounded by a true cortex. 
Ectotrophic cephalodia also occur in Stereocaulons, and the alge 
contained are not always the same. But these are minute structures 
and seldom noticed. Endotrophic cephalodia are said to occur 
sometimes in Sticta amplissima, 
