54 IRON COMPOUNDS IN THE CHLOROPLASTS 



colour is rapidly replaced by a green, and then photo- synthesis 

 is readily demonstrable by the evolution of oxygen and the 

 appearance of starch granules. 



The above reasoning constitutes the whole of the evidence that 

 chlorophyll is the primary cause in the first act of photo-synthesis. 

 It is to be observed that the entire chain of evidence is inferential, 

 and that in order to form a valid proof, chlorophyll would require 

 to be the only substance present in the chloroplast, which is very 

 far from being the case. No observer has ever obtained an appre- 

 ciable and satisfactory synthesis with pure chlorophyll in solution 

 or suspension when removed from the other constituents of the 

 chloroplast. Certain observers have observed minute traces of 

 formaldehyde formation with chlorophyll solutions or emulsions, 

 but even these traces of photo- synthesis have been stoutly denied 

 by other competent observers. In any case, the photo- synthetic 

 effect produced is infinitesimally small compared to that observed 

 in the intact green cell. 



The most recent and careful experiments upon this subject are 

 those performed by Usher and Priestley 1 and by Schryver. 2 Usher 

 and Priestley found that when a chlorophyll- containing extract 

 from green leaves was spread out as a film or emulsion on a gelatine 

 plate, small but distinctly demonstrable amounts of formaldehyde 

 were formed on exposure to sunlight. But in this case there is 

 gelatine containing inorganic colloids, since an ash is obtained on 

 combustion of the gelatine, and in the chlorophyll extract there 

 would undoubtedly be iron salts present, because about one-fourth 

 of the iron of green leaves comes away in the alcoholic extract. 



Schryver worked with an ethereal solution of chlorophyll allowed 

 to evaporate at room temperature on a strip of glass, and found that 

 although such films of chlorophyll on glass produced no formalde- 

 hyde in darkness even in presence of moist carbon dioxide, a minute 

 amount of formaldehyde was formed when the film was exposed to 

 sunlight even in absence of carbon dioxide, and a distinct reaction 

 when the film was exposed to sunlight in presence of moist carbon 

 dioxide. The amount of formaldehyde formed in all such experi- 

 ments is, however, very minute compared to the products of photo- 

 synthesis under natural conditions by the complete chloroplast. 



1 Roy. Soc. Proc., B, vol. Ixxvii., p. 369 (1906); B, vol. Ixxviii., p. 318 

 (1906); B, vol. Ixxxiv., p. 101 (1911). 



2 Eoy. Soc. Proc., B, vol. Ixxxii., p. 226 (1910). 



