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General Forms of Thalli. 
Here we may consider such characters of the three types of thalli as may 
be readily seen. Beginning with the foliose forms, which the student will be 
likely to observe first, it will be readily noticed in comparing a number of 
them that they are variously lobed, or that some are quite entire at the 
margin. In instances where the lobing is evident, the lobes may be more or 
less imbricated. In both lobed and unlobed forms the margin may be wavy 
or crenate instead of entire, and it may be ciliate or devoid of cilia. 
Passing to the fruticose thalli, which are quite as likely to attract atten- 
tion, one would notice first of all whether branched or unbranched, and the 
manner of branching. Then attention would be attracted to the surface, and 
one would readily observe that in some there are small outgrowths from the 
main axes, other than the branches. These are flat expanses in the Clado- 
nias , and called squamules. In the Stereocalons, these outgrowths are 
more irregular in form, and are known as phyllocladia. 
In the crustose thalli, one would note with the eye, as a rule, simply a 
more or less conspicuous crust spread over the substratum, or sometimes 
really lying wholly or partly in the substratum, and indicated at the surface 
often only by a change in color. These crustose thalli will be found irregular 
in outline or more or less plainly orbicular, and to form a continuous or more 
or less broken and scattered crust. In some species the tendency is toward 
more orbicular forms, and in others more 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 a late evolution, and the forms are still quite plastic. 
Nevertheless, the forms, sizes and colors of lichen species are quite as con- 
stant as in many undoubted autonomies, 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 very much more plastic than the 
members of the genus Craetegus , including our common hawthorns. 
Sizes of Thalli. 
Having disposed of the matter of forms and positions of lichen thalli, 
some words are in order regarding sizes. The measurements are all given 
in this paper in units of the metric system, and fruticose thalli of Usnea 
longissima frequently reach 1.5 metres in length, while the foliose thalli of 
Gyrophora Dillenii sometimes reach .35 of a meter in diameter. To simplify 
somewhat, strands of the Usnea five feet long have been carefully picked 
out of the tangled masses hanging over the branches of trees, and specimens 
of the Gyrophora and another species of the genus have been measured 
which surpassed one foot across the longer way of the thallus. Both fruti- 
cose and foliose thalli may vary in size from these large forms to minute 
ones not more than .2 mm. in height or diameter. In the crustose thalli, we 
most naturally think of the spread over or within the substratum, and this 
may vary greatly, though the spread is seldom more than 10 cm. In these 
