12 CONTRIBUTIONS FROM THE NATIONAL HERBARIUM. 
or the minute growths known as isidioid branchlets. The mar- 
gin of the thallus may be closely attached to the substratum (pl. 36, 
A, facing p. 194), or more or less ascending (pl. 30, facing p. 160). 
The lower surface is more or less covered with the attaching organs 
known as rhizoids. These may be large or small, numerous or few, 
and evenly scattered or collected into rows or into groups of different 
forms. The lower surface is sometimes quite smooth except for these 
rhizoids, but in other instances it is variously wrinkled or pitted, 
or in Gyrophora, provided with vertical plates which give strength. 
In the fruticose thalli the surface is either smooth or more or less 
pitted, and in some instances it is somewhat tomentose. The Clado- 
nias put forth as superficial outgrowths the flat expansions known as 
squamules. The Stereocaulons bear the peculiar structures more ir- 
regular in form called phyllocladia. The form, size, frequency of 
occurrence, and distribution of these organs must be noted carefully. 
In the Cladonias, especially, it is necessary to observe whether the 
cortex of the podetium is entire or more or less broken, so that it be- 
comes areolate or even disappears over some portion of the organ. 
In this same genus 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 hypophleodal or hypolithic 
simply take the contour 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 is found the warty or 
verrucose condition; sometimes there occur here and there 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 (pl. 8, B, facing p. 100), 
in which case the 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 charac- 
ter. Yet the colors of thalli play an important part in determining 
lichens and, though often quite variable, they must be carefully noted. 
Colors in lichen thalli vary all the way from a white to a black, but 
the most common is a greenish gray, in this paper designated as sea 
green. Some other colors which occur are ashy, olivaceous, brown, 
and straw color, together with various intermediate shades, as brown- 
ish black, olive brown, ete. The thallus, further, is often more or less 
variegated, while the lower surface is frequently of a different color 
from the upper. Also, in the fruticose forms the basal portion is fre- 
quently of a different color from the distal portions, usually darker. 
