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and the fruticose forms, the thickness is to be taken into account. But in 
the descriptions, actual measurements of thickness are very seldom given, 
though comparative statements are often resorted to. In the descriptions of 
the fruticose forms the diameter of the thallus, or branches of it are often 
given; and here again is a considerable amount of variation found, though 
very much less than that of length or distance across the thalli. 
The Surfaces of Thalli. 
After noting the size and form of the thallus the observer would naturally 
turn to the surface and note its general character. First, in the foliose 
thalli, he would note whether the upper surface is comparatively smooth 
or wrinkled, corrugate or pustulate; whether it bears cilia or the minute 
growths known as isidioid branchlets, and whether it is sorediate or not. 
Also, now, if not before, he must notice whether the margin of the thallus is 
closely attached to the substratum, or more or less ascendant. Then turning 
to the lower surface, it will commonly be found that it is more or less covered 
with the attaching organs known as rhizoids. It must be noticed whether 
these are large or small, whether numerous or few, and whether evenly 
scattered or collected into rows or in groups or other forms. Then, too, the 
lower surface is sometimes quite smooth, except for these rhizoids, but in 
other instances it will be found to be variously wrinkled or pitted, or in Gyro- 
p koras, bearing vertical plates which gives' strength. 
In the fruticose thalli, one will find the surface smooth or more or less 
pitted, and in some instances it is somewhat tomentose. Then, in the 
Stereocaulons, one will find the peculiar structure known as phyllocladia, and 
in the Cladonias , the squamules. The form, size, frequency of occurrence 
and distribution of these organs must be noted carefully. And in the Cla- 
donias especially, it is necessary to note whether the cortex of the podetium 
is entire or more or less broken so that it becomes areolate or even disap- 
pears over some portion of the podetium. And in this same genus, careful 
observation with a lens is necessary to ascertain whether any part of the 
fruticose portion of the thallus is sorediate or not. 
Finally, turning to the crustose thalli, they are also smooth or variously 
roughened. Those that are hypophloeodal or hypolithic simply take the con- 
tour of the surface of the substratum as do also some thin and smooth forms 
that are in part or wholly epiphloeodal or epilithic. Others are scurfy or 
granular, and these are usually rather poorly developed and thin. In 
thicker forms we are likely to find the warty or verrucose condition, and here 
and there may occur minute chinks, so that the thallus is said to be 
rimose or chinky, or finally the chinks may become numerous and divide 
the thallus into minute or small several sided areas known as areoles. Such 
a thallus is said to be areolate. 
Colors of Thalli. 
As compared with size and form, color is usually regarded as a rather 
more variable and therefore less reliable taxonomic character. Yet the colors 
of thalli play quite an important part in determining lichens, and though 
often quite variable, they must be carefully noted. Colors in lichen thalli 
