20 CONTRIBUTIONS FROM THE NATIONAL HERBARIUM. 
the haustorium only, The algal cells of the algal layer are numerous 
in vigorous thalli, but in old and dying thalli few of them occur in 
the living state. The hyphe of this layer give rise to the outer or 
upper cortex, whether a true cortex or a pseudocortex, and are also 
continuous with the hyphe of the medullary layer. The food assimi- 
lated by the living alge, together with that which the fungus may be 
able to take from the substratum through the rhizoids, the umbilicus, 
or the hyphal rhizoids, serves both for the production of new algal cells 
by division and for the growth of various portions of the thallus. 
As the algal cells die and pass outward new ones are constantly being 
formed toward the lower or inner portions of the algal layer, so that 
the thallus is always possessed of an abundance of this assimilative 
tissue. The growth and division of the algal cells is of course most 
active in young and vigorous thalli and in the younger and more 
active portions of older thalli. 
THE MEDULLARY LAYER. 
The medullary layer lies below the algal layer in horizontal thalli 
(pl. 35, fig. 3, ¢, facing p. 191) and within it in fruticose thalli (pl. 11, 
fig. 4, c, facing p. 107). This layer consists of loosely interwoven 
fungal hyphe and is especially adapted to give strength by connecting 
the layers above with those below or those without. with those 
within. It serves also as a medium for the free passage of gases 
much after the manner of the spongy parenchyma of a leaf. The 
differentiation into algal layer and medullary layer is not always 
perfect even in the best developed thalli, and algal cells may oceur 
in small numbers in the latter layer. The medulla is quite commonly 
the thickest of all the layers of the thallus, and the seetion in this 
portion of the thallus is more constantly transparent or hyaline than 
in either of the cortical layers. As the hyphe of this layer serve for 
giving strength, the walls are scarcely ever perceptibly gelatinized. 
Aside from the functions already named, the hyphx of the medulla 
must carry up to the alge any materials taken in a crude state from 
the substratum by the attaching organs, whether rhizoids, an um- 
bilicus, or hyphal rhizoids. Also the foods assimilated by the alge 
must in part pass downward or inward by means of these hyphe 
for the nourishment of certain portions of the thalli. Tt is also sup- 
posed that fats and other food materials are often stored in the 
hyphe of the medulla. 
THE LOWER CORTEX. 
In most respects the lower cortex is very similar to the upper, but, 
as already stated, it is neither so constantly present nor is it as a rule so 
thick. When the lower cortex is present the rhizoids extend from it 
into the substratum, but when it is absent rhizoids are usually present as 
