72 CARNEGIE INSTITUTION OF WASHINGTON. 



will of course be necessary that we have some quantitative measure 

 of the environmental feature here emphasized, and that we secure 

 some knowledge as to just how this supplying power varies from 

 time to time and how it differs from place to place in nature. Also, 

 whenever it shall have become possible to study ordinary plants under 

 artificially controlled conditions, so that true experimentation with 

 these can finally begin, it will be essential to measure and control the 

 carbon-dioxide-supplying power of the air in the culture chambers and 

 to know how this compares with natural conditions. 



With such considerations as these in mind, some preliminary deter- 

 minations of this dynamic condition of plant environment were made 

 at Tucson in the summer of 1921. The method employed was very 

 simple, following the principles on which the evaporating power of 

 the air, the water-supplying power of the soil, etc., have been studied. 

 A carbon-dioxide-absorbing surface with adequate absorbing power is 

 exposed to the air, and the amount of the gas actually absorbed is 

 taken as a measure of the average supplying power for the period of 

 exposure. Small glass cylinders exposing about 24 sq. cm. of absorb- 

 ing surface (of a solution of sodium hydroxide) were used, the alka- 

 linity of the absorber solution being determined by titrations before 

 and after each exposure. The results, which are to be regarded as 

 only preliminary, indicate that the carbon-dioxide-supplying power of 

 the air has values ranging from a minimum of about 1.4 grams to a 

 maximum of about 1.4 grams of carbon dioxide per hour and per square 

 meter of absorbing surface. The minimum represents a day-time 

 exposure on the soil surface under tall, dense grass in the experiment 

 grounds of the Desert Laboratory. The maximum represents a night 

 exposure indoors, wioh rotating table and electric fan. 



As in the case of the evaporating power of the air, the carbon-dioxide- 

 supplying power depends upon two component conditions; the par- 

 tial pressure of the gas (corresponding to the vapor-pressure deficit 

 for evaporation) and the velocity of air-movement over the standard 

 surface. With low average wind velocities the supplying-power 

 values are generally low, while periods of high wind give relatively 

 high values. 



Comparative Rates oj Water Evaporation from Different Kinds of Surfaces, by 



Burton E. Livirigston. 



Earlier studies in atmometry and measurements of plant transpira- 

 tion have made it empirically clear, as it was already clear on a priori 

 grounds, that the rate at which water is vaporized and removed from a 

 surface of liquid water (whether free or held in a solid matrix, such as 

 porous porcelain, cellulose, etc.) is influenced as much by the kind 

 of surface as by the conditions of the surroundings. As has been 

 emphasized in a previous report, the power of the aerial surroundings, 



