FINK—THE LICHENS OF MINNESOTA. 19 
as those of any ordinary parenchyma or the walls may be gelatinized 
and swollen to such an extent that the cellular nature is not made out. 
This layer may appear quite hyaline in section or it may contain 
more or less of some coloring matter. In the majority of the foliose 
lichens such a structure is developed both above and below, and the 
lower cortex is more frequently of a dark color. However, as 
already stated, there are a number of foliose genera in which part 
or all of the species lack such a cortex on one or both sides. And 
it may be added here that in Leptogium (fig. 5) the cortex usually 
consists of a single 
layer of cells. 
The cortex is con- 
stantly being built 
up from the hyphe 
of the algal layer be- 
low and is gradually 
transformed above 
into a dermis. In 
this gradual upward 
passage of tissues 
dead algal cells be- 
come entangled and — Fia. 5.—Leptogium scoltinum, showing single layer of cells in the cortex 
l | ied and the intermingled fungal hyphe and algal filaments within. 
are at engt 1 CarT1ec Enlarged 550 diameters. From Sachs. 
off by the abrasion 
of the upper surface. The lower cortex is usually thinner and is 
more often absent than the upper; but in thalli in which this lower 
cortex is especially needed for mechanical support it is often better 
developed and thicker than the upper cortex, 
THE ALGAL LAYER. 
In some instances, as in Collema (fig. 1, ¢, p. 9), the fungal hyphe 
are simply in contact with the alge and in others they are attached 
to them by haustoria. In some cases the haustoria penetrate the 
walls of the algal cells and they are then said to be intracellular. 
These naturally secure the closest union. Every gradation between 
intracellular and extracellular haustoria may be looked for in certain 
species, but it is rarely the case that the haustorium attains a full 
development, i. e., is divided into a network of hyphe, within the 
algal cells. Indeed, intracellular haustoria are either very rare or 
not often distinguished as such. The extracellular haustoria usually 
consist of a network of short, thin-walled hyphe growing over a 
large portion of the outer surface of the wall of each algal cell. 
Where these are present the food must pass through the wall of the 
haustorium and also through that of the alga, but in instances where 
the haustoria become intracellular the passage is through the wall of 
