104 



Messrs. F. L. Usher and J. H. Priestley. [Apr. 13, 



The above are given as a specimen of a large number of experiments of a 

 similar nature, the results of which leave no doubt in our minds that the 

 bleaching of chlorophyll in sunlight, whether carbon dioxide is present or 

 not, is due to the formation of hydrogen peroxide. The direct oxidation 

 of chlorophyll, in the absence of water, gives rise to a brown and scorched 

 appearance, very different from the true bleaching observed when water is 

 present. As regards the production of formaldehyde, the experiments are 

 equally conclusive in showing that it is only detected by Schiffs reagent 

 when carbon dioxide is present. Great care must be taken to remove any 

 carbon dioxide dissolved in the films used in control experiments. 



(b) In the Plant. — Generally the same results have been obtained with 

 green tissues, although in this case the observations are not quite so 

 uniformly consistent as in the film experiments, owing to the greater 

 difficulty in controlling experimental conditions. None of these experiments 

 are recorded, since their evidential value is certainly inferior to that of the 

 more easily controlled extra-cellular experiments. 



It may be as well to state here that, although no results have been 

 obtained which require any substantial alteration in the theory originally 

 advanced, the following modifications of the conclusions given in Parts I 

 and II have been found necessary : — (1) The statement that the " catalase 

 enzyme is exclusively localised in chloroplasts and amyloplasts must be 

 abandoned. It appears to have been based on a careless observation, and 

 subsequent experiments have merely indicated a greater concentration of 

 the enzyme in the chloroplasts, for when the green juice obtained by 

 pounding fresh leaves in a mortar is filtered, the green residue containing 

 the chloroplasts decomposes hydrogen peroxide vigorously, whereas the 

 filtered juice is relatively inactive. (2) The bleaching of chlorophyll, 

 whether in or outside of the plant, does not require the presence of carbon 

 dioxide ; there is, however, now even more reason to believe that the process 

 is dependent on the formation of hydrogen peroxide. 



The whole problem of the production of formaldehyde, both in the plant 

 and in artificial arrangements, can be more satisfactorily dealt with in the 

 way described and experimentally illustrated by Schryver,* and the 

 method may be employed to yield quantitative results. All the experi- 

 ments recorded in this paper in which Schiffs reagent was used to detect 

 the aldehyde were performed before the work of Schryver was published. 



* Loc. ext. 



