OF THE GREEN CELLS OF PLANTS 55 



Now the chloroplast contains a great deal more than chlorophyll, 

 and when all the chlorophyll has been removed by some such reagent 

 as hot alcohol there remains behind a colourless body, the so-called 

 stroma. The chloroplast after the extraction is still a solid-looking 

 body, and to all appearances the only thing that has happened is 

 that a thin layer of green colouring matter has been removed. There 

 is no shrinking or shrivelling up of the chloroplast. 



There is accordingly no experimental evidence that the primary 

 agent in the photo-synthesis may not be contained in the colourless 

 part of the chloroplast, and the chlorophyll may be evolved at a 

 later stage in synthetic operations induced by some constituent 

 of the colourless part. The function of the chlorophyll may be a 

 protective one to the chloroplast when exposed to light, it may be 

 a light screen as has been suggested by Pringsheim, or it may be 

 concerned in condensations and polymerisations subsequent to the 

 first act of synthesis with production of formaldehyde. 



All these views and others are possible, and the function of chloro- 

 phyll in the chloroplast remains for solution, but it has not been 

 proved that chlorophyll is the primary causative agent in the photo- 

 synthetic process where the chief energy uptake occurs with forma- 

 tion of formaldehyde. 



There are other pieces of experimental evidence apart from the 

 repeated failures to obtain satisfactory synthesis with isolated 

 chlorophyll which go to indicate that chlorophyll is not the trans- 

 former in the first link of the synthetic chain. 



In the first place chlorophyll itself is a product of photo- synthe- 

 sis, and therefore there must be some active photo- synthetic sub- 

 stance present in the chloroplast before the chlorophyll appears 

 which indeed first produces the chlorophyll by its activity. 



When a yellow etiolated leaf taken from the darkness is exposed 

 to the light it contains no chlorophyll, but photo-synthesis, in the 

 absence of chlorophyll, sets in, and chlorophyll itself is one of the 

 products, not the originator or agent, of this photo- synthesis. The 

 period from first exposure to light to the appearance of chlorophyll 

 is too short to determine whether oxygen production and starch 

 formation commence before chlorophyll is formed. 



In the next place Engelmann, 1 by the application of his ingenious 

 method of the oxyphile bacteria, has clearly demonstrated two 



1 Botanische Zeitung, 1881, p. 446, and 1887, pp. 394 ; 410, 418, 426, 442, 

 458. 



