The Action of Light on Chlorophyll. 



393 



tends to disappear as soon as it is formed, whilst the aldehyde is a solid which 

 remains in the paper or in the film left on the glass. But it is not impossible 

 that the oxidising substance may be of service in connection with the chemical 

 reactions that take place in the chlorophyll, and may become used up in 

 this way. 



Instead of the extract of chlorophyll we may use dried leaves, or the 

 chlorophyll expressed from living leaves and spread upon paper. We may 

 also use layers of Euglena viriclis, algpe and other green organisms spread over 

 the surface of paper. In all these cases we can get by appropriate treat- 

 ment, after exposure to light, both the aldehyde and potassium iodide 

 reactions. 



We can also show that both these reactions take place actually inside a 

 leaf when the chlorophyll is sufficiently bleached. Thus if sunlight is 

 condensed by means of a lens upon a living Oxalis leaf which contains 

 abundance of starch, the chlorophyll in a small area of the leaf is bleached. 

 If the leaf is now treated with Schiff 's solution we get a strong aldehyde 

 reaction in the bleached part ; if treated with potassium iodide solution the 

 starch grains in and around the bleached area become coloured blue. The 

 last experiment is not an easy one to perform as it is very difficult to hit 

 just the right moment to stop the bleaching in order to get the potassium 

 iodide reaction. 



Is Formaldehyde produced by the Photo-decomposition of Chlorophyll ? 



The observations of Pollacci * Usher and Priestley,f Harvey Gibson,! an d 

 Schryver§ all show that formaldehyde is produced when chlorophyll is 

 exposed to sunlight in the presence of carbon dioxide but not in its absence, 

 or possibly in minute quantities only. It is therefore important to determine 

 whether the aldehyde produced in my experiments is composed of formaldehyde 

 or whether it contains formaldehyde. The test used by Harvey Gibson gives 

 a very pronounced reaction even when formaldehyde is present in quite small 

 quantities. I have obtained a reliable reaction with 1/1,000,000, and a very 

 pronounced reaction with 1/100,000. The test is carried out as follows: — 

 About 3 c.c. of pure concentrated sulphuric acid are placed at the bottom of a 

 small test-tube ; a few drops of a 5-per-cent. solution of gallic acid in absolute 

 alcohol are poured gently on to the surface of the sulphuric acid and the 

 liquid to be tested is then added ; if formaldehyde is present, a beautiful 



* ' Inst. Bot. d. R. Univ. di Pavia,' 1902 ; see ' L'Annee Biologique,' 1903. 



t ' Roy. Soc. Proc.,' 1906. 



J ' Ann. Bot.,' 1907. 



§ 1 Roy. Soc. Proc.,' 1909. 



