78 PHOTOSYNTHESIS 



in question. This applies more particularly to the conclusions of the latter 

 authors as to the influence of the distance between openings. Brown 

 and Escombe consider that when the small apertures are more than ten 

 diameters apart they influence each other very little, while Jeffreys con- 

 cludes that the stomata in a leaf must close to a diameter %o of that 

 of their full aperture before they act independently. There exists thus 

 such a wide divergence between the conclusions of Jeffreys and what has 

 been held for a long time regarding the function of stomata that a reinves- 

 tigation of the subject seems highly desirable. 



b. The Sources of Carbon Dioxide. 



The question has repeatedly arisen whether the atmosphere is the 

 only source of carbon dioxide or is a sufficient supply of this gas for 

 the photosynthetic activity of the plant. From the time of Senebier who 

 maintained that plants absorb the carbon dioxide through the roots this 

 question has been a topic of much controversy. The careful and numer- 

 ous analyses of Benedict -° show a very constant per cent of carbon 

 dioxide in the atmosphere, 0.031 per cent. Samples of air collected over 

 the ocean, at different times of the year and on the top of Pike's Peak 

 gave essentially the same results. An appreciable increase in the car- 

 bon dioxide-content of the atmosphere is detectable only where there 

 is not perfectly free movement of the air, in cities and industrial centers 

 where the liberation of enormous quantities of carbon dioxide is taking 

 place. 



The soil air may vary greatly from the normal, and under circum- 

 stances, as for instance when the soil is dunged or in pasture land, the 

 carbon dioxide in the soil may rise considerably. When the air supply 

 is cut off the carbon dioxide-content of the soil may rise to very high 

 percentages.-^ Thus, in India under monsoon conditions Leather found 

 that the carbon dioxide rose to 16-20 per cent. 



Lundegardh '- has made an extensive study of the carbon dioxide 

 in the soil with a specially devised apparatus. At a depth of 15-20 cm. 

 the carbon dioxide-content of the soil atmosphere ranges from about 

 2.5 to 0.12 per cent depending upon the type of soil, kind of fertilizer, 

 season, etc. Through the action of the bacteria in the soil large quanti- 

 ties of carbon dioxide are constantly being formed in the soil. Lunde- 

 gardh points out that the amount of carbon dioxide formed in the soil 

 approaches that which is absorbed by i^lants in photosynthesis per unit 

 area. As a consequence of the formation of carbon dioxide in the soil 

 the concentration of this gas immediately above the ground is often con- 

 siderably higher than that usually reported. Thus, for example, Lunde- 

 gardh found in a well fertilized field of beets in October the carbon 



"Benedict, Carnegie Institution of Washington, Pub. No. 166 (1912). 

 '^ Russell,^ E. J., Soil Conditions and Plant Grozi'fh, p. 229 (1921). 

 ""Lundegardh, Der Kreislanf dcr Kohlcns'dnre (1924), p. 144. Literature cita- 

 tions. 



