Recent Works on Lichens. A. Lorrain Smith. 203 
Parmelia ambigua, Cetraria saepincola etc. and (3) finally a tree 
formation (pines) with Parmeliae, Everniae, Usneae etc. and 
with Lecanora coilocarpa on all the small branches. 
There is still another paper from these northern climes: Du 
Rietz (1921) has published an important contribution to general 
ecology with a special section on lichens. He makes much use 
of the term “‘constant.”’ In order to determine the “constants”’ 
of any given associations, these were divided up into areas, and 
the plants determined for each area. Only a limited number of 
species appeared in each area, but as a result of the tabulation 
it was found that each association presented a characteristic 
basis of “constants” or dominant species. On these lines he 
describes various associations either of mixed vegetation such 
as the Pinus silvestris-Calluna vulgaris-Lichen ass. in which 
Cladonia rangiferina and Cl. sylvatica are constants, the former 
being the dominant species; or again a licheno-nanolignosa or 
Empetrum nigrum-Stereocaulon paschale association with Stereo- 
caulon as a constant. Other associations include only lichens 
or lichens and mosses, as for instance a Parmelia omphalodes 
ass. which consists of two mosses and about sixty different 
lichens, widely represented on Scandinavian sea-coasts on the 
vertical faces of the rocks. It is impossible to do more than 
indicate the character of Du Rietz’s work; the results are given 
in tabulated form with the numbers and percentages of the 
species in the different associations. 
Going to more distant lands we learn from Shirley (1919) that 
in Australia Parmeliae inhabit stones, rocks, fences and trees 
in situations where there are strong contrasts of temperature 
and rainfall. Also that Stictae, so abundant in the southern 
hemisphere, are to be found in scrubs and on bushes where shade 
and rain are plentiful. 
Plitt (1921), after collectingin Jamaica, concludes that ‘‘ lichens 
are rather the children of light than the children of moisture; 
they are found on the dry barren hillsides, in the intensest sun- 
light, but rarely in the deeper shadow of the moist jungles.’’ 
The competition for place between the different forms of vege- 
tation is very keen, but lichens, he found, held their own in the 
struggle. Plitt very definitely indicates associations as for 
instance in his account of those that favour special trees: on a 
rose-bush he found Usnea florida, U. hirta, Lecanora varia, 
L. subfusca, Ramalina sp., Parmelia sp., Lecidea sp., Graphis sp., 
Pertusaria sp., Collema sp. and Haematomma puniceum. He 
found marked differences between the lichens on Junipers and 
those on Cryptomeria and he notes that though bamboos were 
singularly free from lichens, yet on nearly all of them grew 
Opegrapha filicina. 
