344 H. W. Turpin 
growth of the millet would resemble in general that for the oat crops. 
The general tendency of the curve after August 25 was to fall as the plants 
advanced toward maturity, as in the case of the oat crops. It will be 
noticed from figure 59 that the actual amount. of carbon dioxide produced 
on the soil cropped to millet was much the same as that produced on the 
. oat soil. The maxima for the two oat crops of 1917 and 1918 were, 
respectively, 2.031 per cent and 3.343 per cent, while the corresponding 
figures for the miilet crops in series 1 and 2 were 3.345 per cent and 2.715 
per cent. It must be remembered, however, that there were but fifteen 
oat plants as compared with thirty millet plants; so that it may be con- 
cluded that an individual oat plant causes the production of about twice 
as much carbon dioxide as is produced by a millet plant. 
‘ Summary of ceperiment 2 
From the results of the second experiment it may be concluded that 
a soil cropped to millet causes about the same fluctuations in carbon- 
dioxide production as are found in a soil growing an oat crop. In general, 
however, the oat crop gives the greatest production of carbon dioxide 
previous to heading, while the millet has its most marked effect a week 
or two after heading. It would seem also that an individual millet plant 
causes the production of approximately half as much carbon dioxide 
as an individual oat plant. From the close agreement between the two 
curves shown in figures 56, 57, and 58, for series 1 and 2, it may be assumed 
that in spite of slight differences in the previous treatment of the soil 
the excess carbon dioxide due to the crop was fairly similar where the 
crops growing showed no apparent differences in vigor. ‘This is indicated 
also in figures 52 to 55, which show the two crops at an early and at 
a later stage of growth, the crop on the soil previously cropped twice to 
oats being designated as a high-carbon-dioxide crop and that on the soil 
that was previously bare being called a low-carbon-dioxide crop. } 
EXPERIMENT 3 
As is pointed out in the review of literature, it is not clear whether or 
not the increased amount of carbon dioxide observed in a cropped soil 
is due to the excretion of carbon dioxide by plant roots (plant activity) 
or to the decay of root particles from the growing crop (bacterial activity). 
Data obtained in experiment 3 seem to throw a little light on this question. 
In this experiment, cans 1, 2, 3, and 4, which had previously grown two 
