1918] NOYES, TROST, & YODER—ROOT VARIATIONS 365 
ti) Ne ecological bearing of these facts is manifest. Although deficiency in 
aération has frequently been suggested as an agricultural difficulty, or as the. 
reason why certain species do not grow upon soils of heavy texture, it does not 
appear that this suggestion has had any exact experimental basis, nor does it 
seem to have been appreciated that different species may have great differences 
in the oxygen requirement of their roots and widely variant responses to differ- 
ences in soil aération, responses which appear to be quite as specific and 
significant as the responses to temperature and to available water which forms 
the present basis of ecological classification. 
One of the writers’ reported 2 preliminary experiments with Zea 
Mays and Lycopersicum esculentum. Flower pots containing these 
species were kept surrounded by an atmosphere of carbon dioxide. 
Practically all the aérial portions of the plants were in normal 
atmosphere. The plants responded differently to the gas during: 
and subsequent to the 2 weeks’ treatment given. 
This paper is a report of experiments in which carbon dioxide 
gas was introduced subterraneously into soil in Wagner pots. 
Experiments will be reported following up the work of CANNON and 
FREE, in which the plants will be grown in soil sealed away from 
the air, so that there is no chance for the oxygen of the air to diffuse 
down into the soil. Studies on the effects of aération on bacterial 
activities have convinced the writers that unless the soil worked 
with was sterile (which would be unnatural) or contained known 
organisms of known antagonisms and activities, the responses 
to changed conditions of aération might be due to a cessation of 
certain necessary biological activities, or to the occurrence of 
certain detrimental biological activities. Adding carbon dioxide 
gas to the soil was expected to change the biochemical activities 
of the soil, but by having the atmosphere come in direct contact 
with the surface, it was believed that necessary biochemical activ- 
ities could exist, although perhaps closer to the surface than 
normally. The surface of the soil of all pots was left normal 
(dust mulch), so that all conditions might more nearly approximate 
those present when the carbon dioxide content of the soil was 
increased by natural means. Differences in amount, nature, and 
type of root growth were thus to be attributed to the carbon 
5 Noyes, H. A., The effect on plant growth of saturating a soil with carbon dioxide. 
Science N.S. 40: 1914. . 
