IOWA ACADEMY OF SCIENCE 
181 
the green material upon the north side. The territory studied at Chi- 
cago did not. offer conditions which would permit of a mapping out of a 
Pleurococcus 'belt. That it too is an index for a small amount of smoke 
injury was noted in the Des Moines survey. On account of great 
amount of rainfall in the fall this was readily done. For instance, 
when the residence portion of Des Moines on the west side begins at 
12th St. the pines or the 6th zone will be marked out from 18th St. 
to 32d St. At this point Pleurococcus is noted for the first time. Sim- 
ilar conditions are noted on the east, north and south sides of the city. 
It may be w r ell to point that the Licheus arc even more susceptible to 
smokes and gases than Pleurococcus. 
The results of the restricted survey show : that smoke and gases pres- 
ent in industrial cities have a detrimental effect upon the vegetation in 
the vicinity; that different belts represented by outstanding types are 
of special interest in a survey; that certain forms are • very susceptible 
to smoke and in this way act as “indicators.” 
LABORATORY EXPERIMENTS. 
As has been pointed out in the earlier part of this: discussion Pleuroc- 
occus offers a means by which this plant may be used as an “indicator,” 
for smoke where lichens are not normally found. It was then thought 
advisable to obtain Pleurococcus and grow it upon the bark or upon 
Knop’s solution and then treat it with various quantities of common 
gases, in order to ascertain the exact physiological actions encountered. 
A culture of the green alga was subjected to acetylene gas (2-6750) 
for four days. At that time the cells showed a marked plasmolysis. 
In a culture exposed for a longer period cellular disintegration took 
place. Where acetylene (2-7750) for four days a similar effect was 
shown. Illuminating gas in like proportions showed an action similar 
to the acetylene. At a longer exposure cells became dark and ex- 
amination revealed a complete disintegration. 
In connection with the Des Moines Survey S0 2 was generated from 
CS 2 by burning with alcohol as was done by Haywood*. In compar- 
ing this with acetylene, it was found that S0 2 was slightly more toxic 
than the acetylene. Another experiment consisting in taking an amount 
of S0 2 and acetylene together and making the two gases, equal in vol- 
ume to where one gas is used. 
This experiment was run in connection with one having acetylene 
alone, and another with S0 2 alone. Where there was a mixture even 
*Bull. 89 Bur. of Chem. U. S. D. of Agr. 
