FINK—THE LICHENS OF MINNESOTA. 25 
an exciple is directly continuous with the upper layer of the thallus, 
whether a true cellular cortex or a pseudocortex. This cortex of the 
exciple is often thicker than that of the thallus, to serve both for pro- 
tection and support to the structures within and to aid in the disper- 
sion of the spores. Within the cortex of the thalloid exciple there 
may be an algal layer and within this a medullary layer, but these 
two areas are by no means always clearly differentiated. There is 
no layer in any thalloid exciple that corresponds to the lower cortex 
found in so many foliose lichens. Occasionally the alge may disap- 
pear from the thalloid exciple with age, but such a structure is still 
a thalloid exciple. The structure of the thalloid exciple is perfectly 
plain in instances where the apothecia are adnate, sessile, or pedicel- 
late, but in instances where they are immersed it is by no means so 
easy to determine whether the portion of thallus surrounding should 
be regarded as a thalloid exciple. In some instances, also, where the 
thallus itself is of a rudimentary character and devoid of distinct 
differentiation into layers the thalloid exciple may be present in a 
similarly rudimentary condition, often simply as a thin veil, which 
may be quite evanescent. 
THE PROPER EXCIPLE. 
As the name indicates, the proper exciple is in reality a portion of 
the apothecium, being merely an upward extension of the hypothe- 
cium (fig. 11, b, p. 107) and, like it, either cellular in structure or com- 
posed of closely interwoven hyphe. Both hyphal and cellular areas 
are found in a thalloid exciple, so that it is not always easy to distin- 
guish between the two kinds of exciples by microscopic structure 
alone. The fundamental distinction is that if an exciple can be 
traced to the hypothecium (see below) it is to be regarded as a proper 
exciple, and if it can be traced to other portions of the thallus it is 
thalloid. But the thallus may be evanescent and disappear and a 
thalloid exciple still be present. In such instances the algal cells are 
usually present and the structure, further, does not appear to be con- 
tinuous with the hypothecium. The hypothecium is always present, 
and the proper exciple may always be traced back to it in vertical 
sections through the center of the apothecium. Some authors seem 
to consider any exciple that does not contain algal cells a proper ex- 
ciple, but such a disposition disregards morphological relationships 
and leads to confusion in the study of species. The proper exciple 
may be pale in section, or varying from this condition to a black 
color. It may be permanent or quite evanescent, and thin or very 
thick and conspicuous. It is a protective covering about the hyme- 
nium and is often produced into a perithecium, which completely 
surrounds the hymenium, except for the ostiole at the summit. The 
proper exciple also aids in the dispersion of spores. 
7920— 10——3 
