14 CONTRIBUTIONS FROM THE NATIONAL HERBARIUM. 
the same as that of the thallus. The surface of the disk, further, may 
be pruimose. usually with a' white powder, concealing the essential 
color. 
THE EXCIPLE. 
Below the disk is the hymenium (pl. 3, fig. 3, a, facing p. 63), which 
may easily be seen in sections with the hand lens. This structure is 
usually lighter in color than the disk and is composed of paraphyses 
and asci, (See under minute morphology.) Below the hymenium 
is the hypothecium (pl. 3, fig. 8, 6, facing p. 63), often darker in color 
than the hymenium above it, so that the line of demarcation between 
the two structures may easily be made out with the hand lens. The 
hymenium and hypothecium are mentioned here mainly that another 
structure, the exciple, may be located with reference to them. The 
exciple is a saucer-shaped or cup-shaped rim around the hymenium, 
consisting primarily of a continuation of the hypothecium upward on 
all sides. Such is the proper exciple (fig. 8, 6, p. 60); but there is some- 
times outside of this, or more often replacing it, what is known as a 
thalloid exciple (fig. 16, p. 232). This is similar to the thallus in struc- 
ture, and usually of the same color, which is never true of the proper 
exciple, this usually approaching the disk in color. Either of the 
exciples may be entirely absent, and either or both may be quite 
evanescent and seen only in young apothecia; but usually one of 
them is present and either permanent or only tardily disappearing. 
It may be seen readily with the unaided eye or by the aid of the 
lens, and its nature and degree of development and permanence are 
points of considerable value in the classification of lichens, even to 
the determination of species. The perithecium, already mentioned, 
is simply a produced exciple found in some lichens, growing com- 
pletely around the upper part of the hymenium but for the small 
opening or ostiole at the summit (fig. 17, p. 239). The margin of a 
proper exciple is usually about at the level of the outer margin of 
the disk, or it may be somewhat raised above the disk. This margin 
is almost always quite entire, while the margin of a thalloid exciple 
is frequently crenulate or crenate, or variously branched, ciliate, or 
irregular. 
POSITION OF THE APOTHECIA. 
Sometimes the apothecium is raised on a slender upward extension 
of the thallus, a short stalk or pedicel, quite different in form from 
the stipe and podetium soon to be described. This is most frequently 
met in the larger foliose lichens. The stalk may be absent and the 
apothecium attached to the thallus at the center of its lower side, 
in which case the apothecium is said to be sessile. Again, the apothe- 
clum may be more closely attached to the thallus by the whole of its 
lower side, when it is said to be adnate. Finally, the apothecium 
