112 PHYSIOLOGY 



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 formaldehyde. 

 Usher and Priestley, on treating a solution of carbon dioxide with 1-5 per 

 cent, uranium acetate or sulphate in bright sunlight, 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 2 + 3H 2 = 2H 2 2 + CH 2 



Such a reaction must be regarded as reversible since the hydrogen per- 

 oxide 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 cor- 

 puscles 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. formalde- 

 hyde, 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 hours in aniline water. The 



chloroplasts under the microscope were seen to contain crystals resembling methylene 



aniline. 



(&) The leaves were distilled in a current of steam. The distillate was shown to 



contain formaldehyde by the formation of methylene aniline crystals on treatment 



with aniline, and by the preparation from it of the characteristic tetrabrome derivative 



of hexamethylenetetramine. 



Usher and Priestley conclude that the first products of the photolysis 

 of carbonic acid are hydrogen peroxide and formaldehyde. Both these 

 substances are rapidly removed from the reaction. The hydrogen peroxide 

 is broken up by the catalase into water and oxygen which is turned out by the 

 plant. The formaldehyde is at once polymerised in the protoplasm of the 

 chloroplast with the formation first of a hexose and then of starch. The 

 formaldehyde, if not removed in this way, destroys the catalase. The 

 hydrogen peroxide, if not broken up by the catalase, destroys the 

 chlorophyll. 



