TIME COURSE OF PHOTOSYNTHESIS FOR A 

 HIGHER PLANT 



By E. D. McALISTER 



Division of Radiation and Organisms, Smithsonian Institution 

 (With Two Plates) 



A spectrographic (infrared) method of carbon dioxide determina- 

 tion of unique speed and sensitivity has been developed. It has the 

 additional merits of being independent of water vapor and of having 

 small pressure and temperature corrections. At low concentrations 

 the method will detect one part of carbon dioxide gas to a million 

 parts of air. Without a great loss in sensitivity these measurements 

 may be made in a fraction of a second. By changing the wave length 

 of radiation used, other gases may be similarly observed. For ex- 

 ample, by using radiation of wave lengths absorbed by water vapor 

 the transpiration of a plant may be studied with equal success. Because 

 of its speed, the method is particularly useful in following the time 

 course of a gaseous exchange. In this connection it could be arranged 

 to measure the metabolic ratio for each breath taken by an animal. 



The purpose of this paper is to report results obtained by the appli- 

 cation of this method to measurements of the carbon dioxide exchange 

 between a higher plant (wheat, variety Marquis) and its surroundings. 



The data obtained by Hoover, Johnston, and Brackett (1933) on 

 the carbon dioxide assimilation of young wheat plants (variety Mar- 

 quis) as a function of light intensity and carbon dioxide concentra- 

 tion are the most accurate and self-consistent so far reported. Figure i 

 is one of their families of assimilation curves. The precision and 

 reproducibility here shown justify their concluding sentence : " These 

 experiments indicate that a wide range of critical experiments upon 



photosynthesis may be carried out with higher plants " The 



present experiments on the same organism bear out this conclusion 

 and bring forth much new information. 



It is a pleasure for the writer to acknowledge his indebtedness to 

 Dr. C. G. Abbot for his constant interest and enthusiastic support ; 

 to Dr. Earl S. Johnston for many helpful suggestions on the plant 

 physiological side of this work ; and to Dr. Dean Burk for many stimu- 

 lating discussions. 



Smithsonian Miscellaneous Collections, Vol.95, No. 24 



