July, 1923] RIGG AND OTHERS — INFLUENCE OF PLANTS ON AIR 
385 
wind, which creates more or less draft through the room. Under the 
second head come (a) the presence of people, producing carbon-dioxide by 
their respiration, (b) the green plants likewise producing carbon-dioxide 
whether in light or in darkness as the result of respiration, and ( c ) the 
micro-organisms also producing carbon-dioxide at all times. 
Table i. 
Parts per Ten Thousand of Carbon Dioxide in Air of a Greenhouse at Various 
Times of Day * 
Time 
Parts CO 2 
in 10,000 
1921 
July 12 4:10 P.M. 10 
13 8:30 A.M. 6 
13 3:10 P.M. 8 
14 3:00 P.M. 5 
15 10:30 A.M. 9 
15 5:20 P.M. 5 
16 12:50 P.M. 6 
18 6:45 A.M. 4 
18 7:00 P.M. 4 
19 7:05 A.M. 5 
19 7:00 P.M. 6 
20 2:45 P.M. 4 
21 6:45 P.M. 4 
21 8:50 P.M. 3 
22 7:00 A.M. 3 
22 10:50 A.M. 4 
22 6:15 P.M. 5 
22 9:50 P.M. 5 
23 1:30 P.M. 6 
24 1:00 P.M. 5 
24 7:45 P.M. 6 
25 1:45 P.M. 6 
* Calculated for dry air at standard conditions. 
It might be expected that during the day photosynthesis, using carbon 
dioxide, and respiration, liberating it, would approximately neutralize 
each other, and that thus the plants would have little effect on the net 
amount of carbon dioxide in the air; while at night, photosynthesis being 
stopped and respiration continuing, the carbon-dioxide content of the 
air would be increased. On this basis, one should expect a greater percent¬ 
age of this gas in the morning when the plants have been several hours in 
darkness than in the afternoon when they have been in light for some hours. 
The tests, however, show just the reverse of this. The average of the 
seven carbon-dioxide determinations made in the forenoon was between 
4 and 5 parts per ten thousand, and that of the twenty in the afternoon, 
between 5 and 6 parts per ten thousand. 
It does not seem probable that the activities of the soil organisms could 
account for this difference, though it is known, of course, that the rate 
of respiration in many plants is increased by intense illumination. Ven¬ 
tilation might either increa.se or decrease the carbon-dioxide content of 
the air in the greenhouse, but, since it was at the time of all tests higher 
than that of normal air, it is evident that it never increased but always 
28 
