56 PLANT PHYSIOLOGY 



chlorophyll. If, however, catalase obtained from the liver of a 

 sheep was added to the gelatin-chlorophyll film, the chlorophyll 

 was still green at the end of the day. They explained this by as- 

 suming that one of the products formed is hydrogen peroxide 

 which causes the bleaching of the chlorophyll. If catalase is 

 present, which is the case in the cells of the plant, the peroxide 

 is broken down into water and oxygen and the chlorophyll is not 

 decomposed. An excess of formaldehyde may poison the catalase 

 so that the peroxide is not broken down, in which case the peroxide 

 bleaches or decomposes the chlorophyll. 



Schryver (1910) and others have criticized and supplemented 

 the work of Usher and Priestley in some particulars. Schryver 

 found that small amounts of formaldehyde were produced in the 

 sunlight in an atmosphere free from carbon dioxide, which would 

 indicate that at least some of the formaldehyde produced is not 

 related to photosynthesis. This may, however, be a part of the 

 mechanism to regulate the amount of aldehyde present. If more 

 is produced at any one time than can be condensed into glucose, 

 it unites with the chlorophyll and is later freed at a time when it 

 can be converted into sugar. 



The work of Klein and Werner (1926) was construed for some 

 time as giving decided support to the formaldehyde hypothesis. 

 Using the very sensitive dimedon test for aldehyde, they seemed 

 to have found that formaldehyde was actually formed in the cell 

 during photosynthesis; but Barton- Wright and Pratt (1930) 

 showed that this was due to the action of light on bicarbonates 

 and carbon dioxide and was independent of the photosynthetic 

 mechanism. 



In summing up our present knowledge of this question, all we 

 can say is that the formaldehyde theory has not been proved. 

 But neither has it been disproved nor has any very serious objec- 

 tion been raised to it. Although toxic, as stated above, it is 

 probably polymerized to sugars as soon as formed, so that little is 

 ever present in the cell. This hypothesis agrees best with the 

 known facts of photosynthesis and is, therefore, becoming more 

 firmly entrenched as the years go by. 



Further Steps in the Process. — After the chemistry of chloro- 

 phyll and the carotinoids had been worked out, Ewart (1918) 

 and many others tried to make use of these formulae to construct 

 equations which might show more fully what goes on in photo- 



