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CARBON DIOXIDE ASSIMILATION IN A HIGHER PLANT 



By W. H. hoover, EARL S. JOHNSTON, and F. S. BRACKETT, 

 Division of Radiation and Organisms, Smithsonian Institution 



(With Two Plates) 



INTRODUCTION 



Carbon dioxide assimilation of young wheat plants grown under 

 controlled environmental conditions has been determined for a wide 

 range of radiation intensities and carbon dioxide concentrations. 

 Special growth chamber and control equipment have been developed 

 for determinations using the entire plant. The purpose of this in- 

 vestigation is to determine the possibilities and limitations inherent 

 in such technique for the investigation of photosynthesis. 



The use of excised leaves common in such investigations, while 

 offering the advantage of more ready isolation and control, and lend- 

 ing itself to better radiation distribution, raises many questions regard- 

 ing the possible influence of accumulated end products and abnormal 

 growth conditions. Work with algae has many advantages, chief among 

 them being (i) avoidance of shielding, (2) the maintenance of defi- 

 nite temperature through immediate contact with water (owing to 

 the high heat capacity of water, the actual temperature of the algae 

 structure cannot differ perceptibly from the observed temperature of 

 the surroundings), (3) because of their small size, unicellular algae 

 offer the possibility of greater simplification and wider latitude in 

 methods of illumination. Disadvantages common to the algae work 

 however are (i) problems of dift'usion of gas through the nutrient 

 solution, and (2) the difficulties of obtaining suitable buffer solutions 

 which can maintain algae continuously in a healthy condition over a 

 long period. A great deal of work in this field has been done in such 

 solutions that algae can exist only for a few hours. This again raises 

 questions as to the influence of physiological factors upon the reactions. 

 These difficulties common to most of the earlier work have been to a 

 considerable extent overcome by Van den Honert.^ Our observations 



^ Van den Honert, T. H., Carbon dioxide assimilation and limiting factors. 

 Rec. trav. hot. neerl., vol. 27, pp. 149-286, 1930. 



Smithsonian Miscellaneous Collections, Vol. 87, No. 16 



