18 CONTRIBUTIONS FROM THE NATIONAL HERBARIUM. 
tially developed or within the algal layer when the medulla is want- 
ing, is frequently found a solid. or a hollow cylinder of hyphe extend- 
ing in a longitudinal direction (pl. 18, fig. 3, b, ¢, facing p. 131). If 
this cylinder is hollow there are usually strengthening bundles of 
hyphe to be found traversing the hollow central portion of the thallus 
in the transverse direction. 
The outer pseudocortex is usually composed of hyphe that are 
more or less gelatinized, so that the structure is very difficult to 
make out (pl. 11, fig. 4, a, facing p. 107). This gelatinization no doubt 
increases the protective function of the pseudocortex. The inner 
cylinder of hyphx functions principally for conduction like a stele 
and the walls of the hyphx show much less gelatinization. The algal 
layer is seldom a complete cylinder, the alge more often appearing 
in clusters incompletely filling the space. The pseudocortex is sel- 
dom even approximately of the same thickness throughout and is 
sometimes scarcely at all developed except over the algal clusters. 
This arrangement brings the algal layer into closer contact with the 
atmosphere without completely exposing the alge. The hyphe are 
much less frequently branched in fruticose thalli than in foliose or 
crustose forms. In connection with this fact it is to be noted that 
fruticose thalli are not always even approximately cylindrical in 
form. Of the Minnesota species, Ramalina calicaris Jraxinea departs 
most widely from the cylindrical form and appears much more like 
a foliose thallus growing away from the substratum, to which it is 
attached at a single point. However, when we section this thallus 
the structure is seen to be essentially that of the fruticose type. 
Indeed, in outward form, there is every gradation between the fru- 
ticose and the foliose thallus, and in a few so-called fruticose thalli 
there is found the cellular cortex characteristic of a foliose thallus. 
THE DERMIS. 
This structure consists of a few layers of flattened cells, lying upon 
the cortex and derived from it. The dermis aids in protection against 
excessive evaporation of moisture, and from its surface there some- 
times arises a dense covering of short hyphe which also functions in 
the same way. These are the trichomatic hypha of certain Pel- 
tigeras, and they also serve in Peltigera aphthosa to help in retaining 
the soredia which develop into cephalodia. The dermis is rudimen- 
tary or wanting in most lichens except Peltigera and Sticta (fig. 4, a, 
p. 17). The inner layers of cells are less flattened and gradually pass 
into those of the cortex from which they are developed. 
THE UPPER CORTEX. 
This structure is cellular (pl. 9, fig. 4, d, facing p. 101), consisting 
usually of several layers. The cells of these layers may be as distinct 
