26 CONTRIBUTIONS FROM THE NATIONAL HERBARIUM, 
THE HYPOTHECIUM. 
This area lies immediately below the hymenium (fig. 9, b, p. 62) and 
varies considerably both in thickness and structure. Sometimes it 
exceeds the hymenium in thickness, but in most instances it Is con- 
siderably thinner in vertical section. In structure it may be com- 
posed entirely of hyphe (pl. 3, fig. 3, b, facing p. 63) or entirely of 
cells (pl. 11, fig. 3, 6; facing p. 107) similar to those of a cellular cortex, 
or it may be partly cellular and partly hyphal. In some of the 
higher lichens, even when the structure is hyphal throughout, the 
hypothecium is more or less plainly differentiated into two layers, 
with the hyphe extending in a general vertical direction in the 
upper layer and more nearly horizontally in the lower layer (pl. 40, 
fig. 3, b, ¢, facing p. 204). This differentiation may sometimes be 
made out in some of the lower lichens, and in some instances where 
the structure of the hypothecium is cellular throughout there is a 
similar differentiation into an upper and a lower layer. The cells 
and hyphex of the hypothecium are smaller than the similar structures 
of the thallus, and the walls are more inclined to become gelatinized 
so that the structure is obscured; and the same statement applies to 
the proper exciple and the perithecium, which when present are con- 
tinuations of the hypothecium. The color of the hypothecium, like 
that of the exciple, varies from the palest shades to black, and like- 
wise the sections may appear perfectly hyaline. Both exciple and 
hypothecium often become darker with age, so that there may be a 
considerable amount of variation in color in the same species. 
THE AMPHITHECIUM. 
In those lichens in which the proper exciple is produced into a 
perithecium there is sometimes a dark layer outside and a lighter 
and often hyaline layer within between the dark outer covering and 
the hymenium. This inner layer is known as the amphithecium. 
In some pyrenocarpic lichens, in which the apothecia are immersed 
in the thallus, as in Dermatocarpon (fig. 18, p. 243), the dark, outer 
protective layer is not needed, and the whole of the tissue surround- 
ing the hymenium is here hyaline or colorless, and this also is known 
as an amphithecium. 
THE PARAPHYSES. 
These structures are specialized hyphx which arise from the tissues 
of the hypothecium. They are commonly cylindrical in form and 
divided by transverse walls into a number of cells. They appear 
at a hasty examination to be uniformly simple in most lichen spe- 
cies, but more careful observation usually brings to light some that 
are branched in nearly every hymenuim. The branching may be 
