28 
C. A. LUDWIG 
It is hardly possible at this time to state definitely just what 
causes the inhibiting action of illuminating gas. Some checks run 
with hydrogen, carbon dioxide, and air washed in pyrogallol indicated 
rather strongly that a good part of it is due to the lack of oxygen, 
even with the facultative anaerobic species. Not all of the results 
can be so accounted for, however, as it does not explain the after- 
effects, nor why one gas in a given concentration should produce a 
greater effect than another, as, for instance, why a mixture of 25 
percent carbon monoxide and 75 percent air should produce almost 
as great a retarding effect on the growth of B. suhtilis as a mixture of 
50 percent illuminating gas and 50 percent air. Certainly no one 
component of the illuminating gas has toxic properties sufficient to 
account for the results . Ethylene and methane are relatively innocuous, 
and in addition ethylene is present in only small quantities in illumi- 
nating gas. There is also to be considered the possibility that some 
of these compounds, especially methane, may serve as food material 
for the organisms. In this connection the work of Miinz (12) is 
suggestive since it shows that some bacteria are capable of assimilating 
methane. Carbon monoxide proved to be more toxic than the 
illuminating gas in some cases, but it also is present in only small 
quantities. What appears to be the most reasonable hypothesis for 
the present is that the results are the sum of a relatively large effect 
due to the dilution of the oxygen plus a smaller effect due to the 
weakly poisonous properties of some of the component gases, the most 
important apparently being carbon monoxide. 
In view of the very great toxicity which illuminating gas and 
ethylene show toward many phanerogams, it was a distinct surprise 
to the writer to find his cultures showing uniformly such a high degree 
of tolerance to these gases. The comparative degrees of tolerance 
can perhaps be better realized by a brief consideration of the general 
results for the two kinds of plants. One of the recent studies on 
the effect of illuminating gas and its constituents on some phanerogams 
and higher green cryptogams (Doubt, 3) included a large number of 
species in the plants tested. The concentration of gas at which 
reactions first occurred in some of the most sensitive species was 25 
parts per million (0.0025 percent), while the only species not affected 
at 60,000 parts per million (6 percent) were species of Polypodium, 
Aspidium, and Asplenium, although some others which were affected 
could live at that concentration. In all of the phanerogams tested 
