180 
IOWA ACADEMY OF SCIENCE 
feet upon shrubs is not easily diagnosed. In the park near 103d St. 
and Yates Ave., the privet ( Ligustrum vulgar e L.) is seriously af- 
fected. But in another park at 79th St., it is noticed that the stag 
horn sumac ( Rhus typhina L.) and nine bark ( Physocarpus opulifolius 
(L) Maxim are at least able to withstand a small amount of smoke. 
Belt (5) occupies a greater area in the restricted survey than any of 
the previous ones. The Stony Island region is the conspicuous outstand- 
ing example. In this particular region another species is added, namely 
the burr oak ( Quercus macrocarpa Michx). In addition any of the 
species represented in the preceding zones may be present. As the 
oak association is the climax forest as far as this region is concerned, 
an interesting situation presents itself. This area allows the presence 
of the burr oak, but it is present only in a pathological condition. An 
examination of the growth and a comparison with rings noted from 
sections made of normal trees of the same species show a surprising dif- 
ference. Even a small tree possesses a large number of growth rings. 
In many instances these rings are so narrow as to be extremely hard to 
separate out when counting. The trees also possess peculiarly twisted 
branches. The bark is also noted as being extremely thick. In the 
vicinity of the Wisconsin Steel Co., a similar situation is met with. A 
further study of the situation at Des Moines, where again the climax 
forest is of the oak type, shows a similar result. These observa- 
tions would immediately lead one to conclude, that where the oak 
type of forest is the climax, that the burr oak forms a good ‘ ‘ indicator ’ r 
of a smoke region. 
The next area (belt 6) is in the smoke zone, yet the injury does not 
extend to the point where the common deciduous trees are affected. At 
times there may be noticeable effects, but from a practical point of 
view conditions are not looked upon as being serious even if conifers 
are not able to thrive. 
Belt (7) represents a step in advance of the former in that it per- 
mits conifers such as the pines, to have a normal development. 
Since the Des Moines survey another belt in advance of the previous 
one has been marked out. This zone is conveniently designated as the 
Pleurococcus belt. The idea of using Pleurococcus as a possible index 
to smoke injury was brought to my attention by Dr. William Crocker 
of the University of Chicago. He noted that trees in the vicinity of the 
University and in Jackson and Washington Parks did not have any 
Pleurococcus upon their trunks. Even trees possessing a sufficient 
amount of shade and having more than sufficient moisture were without 
