182 
IOWA ACADEMY OF SCIENCE 
if the proportion of the two gases was same as the one gas or even 
slightly less, the mixture revealed a greater toxic action. Pleurococcus 
after having been in the two gases for four days had lost practically 
all its color, while in the jars with S0 2 and acetylene a brownish color 
was present. On further examination microscopically, the mixture 
presented considerably more plasmolysis, when precipitated with ferric 
chloride. This injury was further identified with the amount of tannin 
present in the interior of the cells, when precipitated with ferric chloride. 
The next series of experiments made in the laboratory utilized the 
pine ( Finns resinosa ) taken from Washington Park. As the stomata 
of the pines are deeply sunken they are easily made the resort places 
for the tarry compounds emitted in smoke. For instance it was 
found that ten needles from Pinus resinosa had material that was re- 
moved by washing in ether equal to .0185 grams. In using a small 
limb six feet long taken from a tree in Washington would have 2.96 
grams. At this rate it would not be long before an amount equal to a 
pound is reached. By taking the same number of the pine needles of 
the same species having the same area from Pinus resinosa at Ames, la., 
during the month of November a much smaller amount of tarry mater- 
ial was found present. If the time element was the same in both cases 
the result w T ould doubtless be more marked. 
It has been noted that certain trees are more resistant than others. 
The cottonwood and willow have been shown to be less susceptible from 
injury by smokes than others. This is due to the hard cutinized layer 
that these species possess along with their compact texture. On ac- 
count of the tomentose character of the under surface of the leaf of the 
white poplar ( Popnlus alba L.) and its cutinized upper surface, the 
tree is very resistant. 
The results of the laboratory experiments then call attention to the 
need of perfect combustion in the burning of coal, for it is shown that 
when there is a mixture of two gases that the action is more toxic than 
where one gas alone is given out. The experiments also show that there 
is a direct relation between the amount of injury and the amount of 
tannin present ; that the tarry material emitted in smoke is of sufficient 
amount to at least- partially clog the stomata and at the same time inter- 
fere with the assimilatory processes; that the resistance of the cotton- 
wood and the silver poplar is due to the anatomical structure of the 
epidermal cells. 
