26 CHEMISTRY OF PLANT LIFE 



however, later showed conclusively that under the conditions 

 described by these investigators, formaldehyde can be obtained 

 even if no carbon dioxide is present, being apparently produced by 

 the action of water upon the organic sensitizer which was used. 



These conflicting reports led Usher and Priestley, in a series of 

 studies reported between 1906 and 1911, to submit the whole 

 matter to a critical review. Briefly, these investigators showed 

 that the photolysis of carbon dioxide and water results in the for- 

 mation of formaldehyde and hydrogen peroxide, as represented 

 by the equation 



CO 2 +3H 2 = GH 2 0+2H 2 O 2 . 



The formaldehyde is then condensed by the protoplasm into 

 sugars, while the hydrogen peroxide is decomposed, by an enzyme 

 in the plant cell, into water and oxygen. If the formaldehyde is 

 not used up rapidly enough by the protoplasm, it kills the .enzyme 

 and the undecomposed hydrogen peroxide destroys the chloro- 

 phyll, which stops the whole photosynthetic process. Usher and 

 Priestley were able to cause the photolysis of carbon dioxide and 

 water into formaldehyde outside of a green plant, in the presence 

 of a suitable catalyzing agent which continually destroys the 

 hydrogen peroxide as fast as it is formed; to show the actual 

 bleaching effect of an excess of hydrogen peroxide in plant tissues 

 which had been treated in such a way as to prevent the enzyme 

 from decomposing it; and, finally, to demonstrate the condensa- 

 tion of formaldehyde into starch by the action of protoplasm which 

 contained no chlorophyll. 



In the meantime, Fenton, in 1907, found that in the presence 

 of magnesium as a catalyst (it will be shown in Chapter VIII that 

 magnesium is a constituent of the chlorophyll molecule) formalde- 

 hyde may be obtained from a solution of carbon dioxide in water, 

 especially if weak bases are present. 



Further, Usher and Priestley's later results showed that 

 radium emanations, acting upon a solution of carbon dioxide in 

 water, produce hydrogen peroxide and formaldehyde, and the 

 latter polymerizes but not up to the point represented by the 

 hexose sugars; also, that the ultra-violet rays from a mercury 

 vapor lamp are very effective in bringing about the production of 

 hydrogen peroxide and formaldehyde from a saturated aqueous 



