PLANT ASSOCIATIONS AND HABITATS. 25 
rock-inhabiting lichens of Tumamoc Hill every species having a light- 
colored thallus shows a more or less evident development of black lines 
or spots on the upper surface. These lines or spots are so numerous 
on older portions of some of the thalli as to darken, more or less, the 
otherwise light-colored surface. Zukal, in his discussion of the protec- 
tive significance of colors in lichens, speaks of such lines of black as occur- 
ring on younger or injured portions of thalli to protect the algal cells 
from the intense rays of the sun in hot regions; but the writer has found 
the lines and spots better developed over older parts and could not detect 
their unusual development in connection with cracks of the thalli. In 
the unsettled state of our knowledge regarding protective coloration, it is 
very desirable that careful observation should be made of the relation 
of development of the black areas to the position of the particular plant 
on the rocks with reference to the sun’s rays. The study of the relation 
of development of coloration at the tips of the branchlets of Parmelia 
conspersa to the orientation of these branchlets to the sun’s rays would 
be specially instructive. 
The 15 genera thus far collected have been subjected to careful exam- 
ination with reference to the structure of the thallus and its relation to 
xerophytic conditions. The Acarosporas, which form a large proportion 
of the rock-inhabiting species, are cellular throughout, so that the algal 
cells are unusually well protected against too intense light or too much 
transpiration of moisture, or both. The same is true of the Endocarpis- 
cums. The Lecanoras have either an upper cellular cortex or a pseudo- 
cortex of entangled hyphz, and the Placodiums show similar structure. 
The one Parmelia is a very closely adnate species, which is found to 
possess a stronger cortex than the closely related Parmelia caperata, which 
usually grows in less xerophytic conditions. The Dermatocarpons are 
all well protected by strong cortices, and Dermatocarpon miniatum, the 
only species not closely adnate, is attached to the rocks by a very strong 
umbilicus, while the lower cortex is so strongly developed that no ordi- 
nary wind can tear the plants from the rocks. Not to enumerate the 
remaining genera, most of which can not be ranked as important floral 
elements, the general statement. may be made that structurally the lichens 
of the Laboratory domain are well protected both against the danger 
of excessive transpiration and that of being torn away by high winds. 
The lichens sent for study were collected from 7 stations. They are 
most numerous on the north side of the dark-colored basaltic rocks which 
compose so large a part of Tumamoc Hill, and are very poorly represented 
on southern exposures and on the tuff which has been quarried in a few 
places for building purposes. At station III, which is a very steep slope 
on the south side of Tumamoc Hill, facing directly south, at an altitude 
of 2,700 feet, the lichen habitat consists of loose blocks of tuff and basalt, 
where nothing in the way of seed plants exists but a few creosote-bushes. 
Only 7 species of lichens were collected here, 5 of which belong to the 
genus Acarospora. The frequent occurrence of members of this genus on 
southward-facing riprap and their very rare occurrence on the northward- 
facing riprap a few feet away has been noted elsewhere by Fink (1904, 
p. 278) and, with the data obtained at station III, establish beyond doubt 
that the Acarosporas, with their strong, protective cortices and their 
cellular structure throughout, are the most characteristic xerophytes of 
all our American lichens hitherto studied from the ecological point of 
view. Tuff is a drier rock than basalt, not holding water so well, and, 
