IOWA ACADEMY OF SCIENCE 
183 
PHYSIOLOGICAL FACTORS. 
In noting the various physiological conditions, the first outstanding 
one is that of the light intensity. An atmosphere laden with smoke 
does not offer as good conditions for photosynthesis as one that does 
not contain any impurities. 
When a leaf is coated with a tarry compound, there is an interference 
with the assimilation process. Many of the investigators on the sub- 
ject have claimed that the stomata do not enter in. In case of the 
stomata of the conifers this opinion will not hold. In cross sections 
made of conifers of a smoke region, the stomata were found to be at 
least partially filled with a tarry compound. Cohen and Ruston have 
also emphasized this point in their study of the silver fir at Leeds. 
Such a deposit will interfere with the assimilation of C0 2 and 0 2 , and 
with the transpiration stream. * 
Another means by which assimilation is diminished, is: by cutting 
down of the leaf area. This is accomplished (1) by a loss of leaflets 
when a plant with a compound leaf is in question, (2) by curling or by 
taking on abnormal shapes, (3) by the formation of spots or lesions. 
The loss of the leaflets has been frequently observed by the writer 
for the honey locust ( Gleditsia triaccmtlios L.) and the common lo- 
cust {Rdbinia Pseudo- Acacia L.) at Bessemer Park at South Chicago. 
The common locust at Des Moines in the smoke district, also shows a 
decrease in leaf surface by this method. 
Trees in a district where the fumes of S0 2 are prevalent often have 
their tips browned or curved. As a result there is naturally a de- 
crease in the leaf area. In addition a portion of the leaf will be twisted 
from its normal position and in this way the leaf area that under nor- 
may conditions is exposed to the light will be cut down. This feature 
is well marked out among some sycamores that have been planted 
along the cinder path, on the way to the city of Ames, from the College. 
The trees near the heating plant, with its low smoke stack, are either 
in a state as mentioned above or have lost their leaves entirely. 
Where there is a quantity of S0 2 present, in the smoke and where 
the injury is intermittent, brown localized spots or lesions are pro- 
duced. When there is an accumulation of these spots a considerable 
reduction in leaf area takes place. 
The effect upon cells themselves is well noticed in the cells of Pleuroc- 
occus of which an account has been given. There is a resulting plas- 
molysis and associated with the extent of the plasmolysis is the amount 
