IOWA ACADEMY OF SCIENCE 
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thinks that carbon monoxide is the toxic agent. He is of the opinion 
that tobacco smoke has a marked effect upon micro organisms. Gaten 
(9) in his long paper on the effect of tarred roads npon vegetation calls 
attention to two central ideas; (1), that injury may be due to various 
gases; (2)1 that injury may be due to the action of the dust. The vapors 
of the various gases emitted from tar have been determined to be at 
least twenty-seven in number. A large number of these products are 
known to be toxic. In- the case of the dusts, the vapors may be more 
directly applied. The effects produced may according to Gaten be sum- 
marized accordingly: (1), there is a decrease in leaf area due to a drop- 
ping of the leaves, and also to a diminution of local leaf areas; (2), there 
are no noticeable anatomical differences; (3), there is a difference in the 
form, size, and arrangement of the cells; (4), there is in a great many 
instances an accumulation of cork; (5), there is not a, sufficient amount 
of storage food in the tissues; (6), there is a reduction in the width of 
the annual rings; (7), there are certain species that are more susceptible 
than others. P. Brizi (15) has made a very extensive study of the 
effect of gases and fumes upon cultivated plants. The fumes came from 
gas works, chemical establishments, smelters and foundries. Although 
S0 2 is considered for the most part, yet, the fumes from HOI, and other 
substances, such as zinc and arsenic are discussed. The general effect 
produced upon the cells themselves is plasmolysis, and later disorgani- 
zation. Richards & McDougal (16) have shown that carbon monoxide 
although not as effective as illuminating gas yet is considered as being 
very toxic to plants. 
Crowthers & Ruston (9) have carried on a number of interesting 
experiments in and about the city of Leeds, England. They established 
throughout this industrial center a number of stations and then compared 
records. The results arrived at, show that there is considerable differ- 
ence in the amount of suspended matter, in the amount of free acids, 
in the amount of sulphur dioxide of rains at Station I, situated in the 
heart of the city than at Station II, situated in the residential portion. 
These various products were proven to have a direct injurious effect upon 
vegetation in the main by causing a reduction of the intensity of the 
sunlight and by causing a decrease in assimilation of C0 2 . The follow- 
ing figures taken from their experiments performed according tio the 
method used by Brown & Escomb* in their classical experiments upon 
the C0 2 intake, show a marked difference. 
*Phil. Trans. 193: 298. 1900. 
