FINK—THE LICHENS OF MINNESOTA. 11 
entire, and it may be either ciliate or devoid of cilia. The most ob- 
trusive differences among the fruticose thalli consist in their being 
branched or unbranched and, when the former, in the manner of 
their branching. Secondary differences are found in the character 
of the surface, particularly in various small outgrowths other than 
the branches, known as phyllocladia. The crustose thalli appear 
usually as a more or less conspicuous layer spread over the sub- 
stratum or sometimes really lying wholly or partly in it and indicated 
at the surface often only by a change in color. These crustose thalli 
will be found to be irregular in outline or more or less plainly orbicular 
and to form a continuous or a more or less broken and scattered 
stratum (pl. 8, A, facing p. 100). In some species the tendency is 
toward orbicular forms and in others toward irregularity in form, but 
in any case the peculiarities of the surface of bark, dead wood, or 
rock forming the substratum will determine the form of the particular 
thallus to a large extent. 
Lichens are of late evolution, and the forms are still quite plastic. 
Nevertheless, the forms, sizes, and colors of lichen species are quite as 
constant as those of many other organisms, whether plant or animal. 
Indeed, in many lichens the morphological characters, whether gross 
or minute, are quite as constant as are those of most flowering 
plants, and it may well be doubted whether even the Cladonias are 
more plastic than the members of the genus Crataegus, including 
our common hawthorns. 
SIZES OF THALLI. 
In northern Minnesota the fruticose thalli (pl. 42, B, facing p. 206) of 
Usnea longissima, which grows in tangled masses hanging over the 
branches of trees, frequently reach 1.5 meters in length, while the 
foliose thalli of Gyrophora dillenit, in the same portion of the State, 
sometimes reach 35 cm. in their longer diameter. Both fruticose and 
foliose thalli may vary from these large forms to minute ones not 
more than 0.2 mm. in height or diameter. In the crustose thalli the 
spread over or within the substratum may vary greatly, but is seldom 
more than 10 cm. In these and the foliose forms the thickness is to 
be taken into account. In the descriptions, however, actual meas- 
urements of thickness are very seldom given, though comparative 
statements are often resorted to. In the descriptions of the fruticose 
forms the diameters of the thalli or of their branches are usually given; 
and here again a considerable amount of variation is found, though 
very much less than in length. 
SURFACES OF THALLI. 
In the foliose thalli the upper surface is comparatively smooth, 
wrinkled, corrugated, or pustulate, and it may bear cilia, soredia, 
