558 Prof. B. Moore. Presence of Inorganic Iron 



containing parts of the plant, and only in these when they are exposed to 

 light. Also, when a plant is allowed to grow in darkness, the leaves are 

 found to be pale-yellow in colour, or chlorotic, instead of green. When a 

 plant grown in darkness, and, as a result, possessing chlorotic leaves, is then 

 exposed to light, the pale-yellow 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 appreciable 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* and by Schryver.f 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 and the inorganic colloids 

 it contains shown by its ash to be present, 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 formaldehyde 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 experiments is, however, very minute compared to the products 

 of photo-synthesis under natural conditions by the complete chloroplast. 



* 1 Eoy. Soc, Brae.,' B, vol. 77, p. 369 (1906) ; B, vol. 78, p. 318 (1906) ; B, vol. 84, p. 101, 

 (1911). 



t 'Roy. Soc. Proc.,' B, vol. 82, p. 226 (1910). 



