FINK—THE LICHENS OF MINNESOTA. 17 
facing p. 170), are cellular throughout. Other instances of similar 
modifications in structure will be met in the descriptions of the 
genera of Minnesota lichens. 
CRUSTOSE THALLI. 
In those crustose forms that are hypophlewodal or hypolithic, there 
is simply a tangled layer of interwoven algal cells and fungal hyphe, 
without any differentiation into layers. In the epiphlaodal and epi- 
lithic species the structure may be quite as rudimentary, or there may 
be a more or less evident upper 
pseudocortex of interwoven hyphe — @ _ 258 
(pl. 4, fie. 4,4, facing p. 67). In. Qe 
instances of the latter kind there 6 0 
is frequently also a more or [ess nei 
distinct algal layer below the pseu- 083 
docortex (pl. 4, fig. 4, 6) and some 4 
representation of a medulla below Oy 
the algal layer (pl. 4, fig. 4, ©). zy 
Whether these superficial forms Ee 
are thus differentiated or not, 
the modified hyphze known as a 
hyphal rhizoids may always be 
looked for penetrating the substra- 
tum. It is, however, very difficult 
to obtain them in sections of these 
. re sue . oyu 
thalli. With each description of a 
erustose genus will be found a | met 
statement regarding the amount YH) 
of differentiation. Mie ee 
FRUTICOSE THALLI. 
The fruticose type of lichen — F's. 4. -Sticta amplissima, section of the thallus. 
. . - . . a, The dermis; 6, the uppér cortex; c, the 
thallus is peculiar, and is for this algal layer; d, the medullary layer; e, the 
reason treated after the crustose lower cortex. Enlarged 400 diameters. From 
~ . . Schneider. 
type. Fruticose thalli are usually, 
though as noted below not always, more or less cylindrical in form. 
The outer layer of the cylinder or other form is a pseudocortex of 
densely interwoven hyphex, extending either in the direction of the 
axis or at right angles to it. Within this outer layer is the algal 
layer, which, like the outer layer of hyphe, is commonly more or 
less nearly circular in transverse section (pl. 11, fig. 4, @, b, facing p. 
107). Sometimes, as in Alectoria, there is within the algal layer a 
well-developed medullary layer filling the remainder of the space 
and making the cylinder a solid one. In other instances the medulla 
is poorly or searcely at all developed. Within it,swhen only par- 
