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ANNUAL REPORT SMITHSONIAN INSTITUTION, 1936 



Smithsonian Institution, as well as those performed elsewhere, clearly 

 show that plants under normal atmospheric conditions grow better 

 and better as the light intensity is increased up to a certain value. 

 Beyond this value there is no further increase. The excess radiant 

 energy is apparently wasted so far as the process of photosynthesis is 

 concerned. One naturally wonders why it is impossible to "push" 

 the plant in its manufacture of sugar and starch. What holds back 

 this all-important work of the plant? The answer is simple enough 

 when the factors of photosynthesis are examined. Some idea of what 



FiGL'HB 4. — Diagram representing the excliange of oxygen and carbon dioxide which takes 

 place in darkness (left) and light (right). 



takes place in the plant during photosynthesis may be expressed in 

 the shorthand of chemistry : 



6C02+6H20+light-^C«Hi206+602 



(carbon (water) (sugar) (oxygen) 



dioxide) 



The raw products that are used up in this process are carbon di- 

 oxide and water. Normal air contains about 0.035 percent of carbon 

 dioxide. Thus one can understand that as the process of manufactur- 

 ing sugar is speeded up by increasing the light intensity there will 

 com© a point at which the rate is slowed down by a lack of carbon 

 dioxide, which at this low concentration flows into the plant at a 

 limited rate. If, however, the level of the reservoir of carbon dioxide 

 be raised by increasing its concentration in the air surrounding the 

 plant the work done by the plant should be increased as the light 

 intensity is further increased. This is exactly what was done during 

 the past two summers in experiments with wheat (Johnston, 1935 b). 



Three plots of wheat plants were employed in the first experi- 

 ment (pi. 3, fig. 1). The one in the foreground was open to normal 



