124 PHYSIOLOGY 



formation of sugar from the formaldehyde had taken place in the solu- 

 tion and that the plants were using up the sugar rather than the for- 

 maldehyde as the source of their starch. 



One must assume, with Tirniriazefi, that the function of chlorophyll 

 in the process of assimilation is that of a sensitiser. Just as the addition 

 of eosin to the emulsion used for coating photographic plates will 

 render these sensitive to the red and green parts of the spectrum, i.e. 

 will excite change in the silver salt when light from these parts of the 

 spectrum falls upon it, so the chlorophyll serves as a means by which 

 the absorbed solar energy can be utilised for the production of chemical 

 change in the chloroplast. Attempts have been made to imitate this 

 process outside the plant. Thus Bach passed a stream of carbon 

 dioxide through a 1-5 per cent, solution of a fluorescent substance, 

 uranium acetate, in sunlight. As a result there was a precipitate 

 of uranium oxide and peroxide, with the formation of traces of for- 

 maldehyde. Usher and Priestley, on treating a solution of carbon 

 dioxide with 1-5 per cent, uranium acetate or sulphate in bright sun- 

 light, obtained uranium peroxide and formic acid, but no formaldehyde. 

 The formation of peroxides in these conditions suggests that the first 

 change in the chloroplast may be as follows : 



C0 + 3H0 == 2H0 + CH0 



22 



Such a reaction must be regarded as reversible since the hydrogen 

 peroxide first formed would tend to oxidise the formaldehyde again. 

 Moreover it would have a destructive influence on the chlorophyll 

 itself, which is easily oxidised. In order, therefore, that the reaction 

 should go on in one direction only, i.e. that of assimilation, means 

 must be present in the chlorophyll corpuscles for the removal of 

 both hydrogen peroxide and formaldehyde as soon as they are formed. 

 The removal of the hydrogen peroxide can be effected by a catalase, 

 which is fairly widely distributed in plants and has been shown by 

 the last-named authors to be present in the chloroplasts. In order 

 to demonstrate the production of the first result of assimilation, i.e. 

 formaldehyde, the further stages in its conversion must be stopped 

 by killing the plant and the catalase it contains. They therefore 

 placed leaves, which had been boiled, in water saturated with carbon 

 dioxide and exposed them to bright sunlight. The leaves were 

 bleached by the oxidation of the chlorophyll, and some substance of an 

 aldehydic nature was produced, as shown by the red colour obtained on 

 placing them in rosaniline, previously decolorised with sulphurous acid. 



Two proofs were brought forward that this substance was formaldehyde : 

 (a) Some of the bleached leaves were soaked for twelve hoxirs in aniline 



water. The chloroplasts under the microscope were seen to contain crystals 



resembling methylene aniline. 



