THE NATURE OF PHOTOSYNTHESIS 127 



Lundegardh's results would indicate that with light intensity %o 

 that of direct sunlight an increase in carbon dioxide-concentration above 

 the normal (0.03 per cent) results in an increased photosynthetic rate. 

 Also, with increasing carbon dioxide-concentration the augmentation of 

 the photosynthetic rate is high at first and gradually becomes less. With 

 higher light intensities, i.e., illumination approximating conditions exist- 

 ing in nature, the photosynthetic rates are raised considerably by an in- 

 crease in carbon dioxide-concentration. In absolute measure these latter 

 increases are high, though relatively they are not as high as with low 

 illumination. This point is taken up under the discussion of the effect 

 of light intensity. 



From what has been said of the effect on photosynthetic activity of 

 increasing the carbon dioxide concentration, it may be concluded that the 

 normal carbon dioxide-concentration of 0.03 per cent in the atmosphere 

 represents a condition in which this factor is ordinarily to be expressed 

 as the "limiting one" in terms of the Blackman conception or as being 

 "in minimum" in the terms of Harder and Lundegardh. From the prac- 

 tical \newpoint of agriculture the question of photosynthetic rate is 

 naturally a very important one and the problem of how to increase this 

 rate has occupied plant physiologists for a long time, for on the rate of 

 the photosynthetic process depends more or less directly the amount of the 

 plant product, the crop. The question then arises, can an increase in 

 crop yield be attained by raising the carbon dioxide-concentration in the 

 atmosphere surrounding the plants? 



The question has received much attention especially during the past 

 few years. A full discussion of all the aspects and developments of the 

 problem would reach beyond the purpose of this monograph. The prob- 

 lem has been widely discussed in Gennany where flue gases from the 

 blast furnaces have been used as a source of carbon dioxide."^ Many 

 technical problems have arisen which still require much study. It is 

 becoming evident that increasing growth and dry material is not simply 

 a matter of increasing the photosynthetic rate. There are questions of 

 water relations, and migration of the material formed which play a very 

 important role. Different species exhibit very different behavior toward 

 air enriched in carbon dioxide. Finally, growth, the formation of new 

 tissue, and the laying down of storage material, are in part only depend- 

 ent upon the photosynthetic process. There are many other factors which 

 also come into play. As a result of the neglect of due consideration of 

 this interrelation of various factors in the development of plants, many 

 contradictory observations have been made and erroneous conclusions 

 drawn. This is probably inevitable when it is attempted to apply intricate 



"'Reinau, "Kohlensaure and Pflanzen," Halle, 1920. Bornemann, "Kohlensaure 

 und Pflanzenwachstum." Berlin, 1923. Classen, H., Chem. Zeitq.. 44, 585 (1920). 

 Cerighelli, R., Ann. Sci. Awon., 38, 68 (1921). Fischer, H., "Pflanzenbau und 

 Kohlensaeure," Stuttgart, 1921. Wagner, H.. Die Kohlensaeure als Wachstums- 

 faktor. Die Umschau., 27, 758 (1923). Lundegardh, "Der Kreislauf der Kohlen- 

 saeure in der Natur.," Jena., 1924. Jess, Jour. agr. pract., N. S. 35, 250 (1921). 



