Ig11] SHULL—OXYGEN MINIMUM AND GERMINATION 467 
pared with the two fluctuating experiments in the same table, 
show this to be true. The decreased growth in the fluctuating 
temperature in this case is possibly due partially to the decreased 
oxygen pressure in the germinators as the vapor pressure increases 
with the rise in temperature. 
PRESSURE AND THE OXYGEN MINIMUM 
The method employed in these experiments has been briefly 
described, and the results are recorded in table III. Parallel 
experiments at reduced pressure were run along with the hydrogen 
tests, the germinators being kept in the same running water at a 
pressure of 85 mm. The oxygen pressure in the hydrogen and 
reduced pressure experiments would have been the same but for 
the aqueous pressure in the latter. The correction for water vapor 
reduces the O, pressure from 17.79 mm. to 14.39 mm., a large 
enough difference to make the results not directly comparable. 
The first three tests were with X. pennsylvanicum, the remainder 
with X. glabratum. 
The first of these hydrogen tests is of little value in determin- 
ing the effect of pressure on the oxygen minimum, for the oxygen 
content was 35.25 mm., or more than double that used in any of 
the reduced atmospheres. Only a slight reduction of the growth 
is brought about, which would indicate that this amount is prob- 
ably somewhat below the optimum oxygen pressure for Xanthium. 
Nagpoxicu has shown that the atmosphere contains considerably 
more than the optimum oxygen supply for growth in higher 
plants. 
On comparing the results of the remaining hydrogen tests with 
those at reduced pressures, it is seen at once that there is a higher 
percentage of germination, and a greater average growth in the 
hydrogen than in the reduced atmosphere. For instance, in hydro- 
gen 96.6 per cent of the lowers, and 43.3 per cent of the uppers of 
X. pennsyloanicum germinated, as compared with 26.6 per cent of 
the lowers and 23.3 per cent of the uppers in the 85 mm. atmosphere. 
X. glabratum shows a similar behavior, 56.6 per cent of the lowers, 
and 23.3 per cent of the uppers germinating in the reduced atmos- 
phere. Is this difference due to a difference In oxygen pressure 
