FINK—THE LICHENS OF MINNESOTA. 13 
The tendency of thalli, as of other lichen structures, is to darken with 
age, and the variations of color in a species may usually be traced to 
peculiar conditions of growth. 
THE APOTHECIUM. 
Likewise in the fruit, or apothecium, the main features of gross 
morphology are size, form, and color. The apothecia are usually 
superficial and large enough to be seen easily with the unaided eye 
(pl.8, A, facing p. 100). In some instances, however, they are so small 
that they can be made out only with difficulty with the hand lens; 
or they may be immersed in the thallus and indicated at the surface 
by a slight elevation or depression as a disk or an ostiole (pl. 49, A, 
facing p. 235); or, when immersed, they may be scarcely discernible 
in any way except in sections of the thallus. From 0.1 to 5 mm. 
is well within the range for diameters of apothecia. 
The apothecia are most commonly saucer-shaped, or some slight 
modification of this form, as when the disk is flat or somewhat con- 
vex instead of concave. In some instances the disk becomes very 
concave, the apothecium at last becoming cup-shaped. In other cases 
it is strongly convex, giving the apothecium at maturity a spheroidal 
form. In all of these forms the outline of a transverse section of the 
apothecium when young would usually be very nearly a perfect circle; 
but the form may become irregular as growth proceeds, so that at 
maturity this outline is quite irregular. In other lichens the apothe- 
cia are of some other form from the beginning. Thus, there are the 
elongated and often branched forms, such as are found in Graphis 
(pl. 2, A, facing p. 54), and the difform or variously irregular forms, 
as in Arthonia (pl. 2, B). Again, some apothecia are produced into 
a well-developed perithecium, and these usually approximate a 
spherical form. 
THE DISK. 
In those lichens in which the exciple (see below) is not produced 
into a perithecium the upper surface of the apothecium is naked, 
except for a very thin film of thallus which may persist as an epithe- 
cium, a structure not mentioned in the descriptions of species. This 
upper and essentially naked surface, whether flat or more or less 
strongly concave or convex, forms the disk. The outline of the disk, 
then, may be circular or variously elongated or irregular, varying in 
this respect with the form of the apothecium as a whole. In color 
the disk varies considerably even in the same species. It is usually 
light-colored in its early development and commonly becomes darker 
as it reaches maturity. The final color may be a light or darker 
flesh color or a light or darker shade of yellow, orange, red, brown, 
chestnut, olive, or even black. Whatever the color, it is very seldom 
