140 PLANT PHYSIOLOGY. 



alcohol. This yellow pigment is supposed to beetiolin. There 

 are reasons for believing that etiolin is an antecedent of 

 chlorophyll. 



In the various authorities consulted there are two theories 

 of plant assimilation advanced, both worthy of consideration. 



The lirst theory is as follows : The chlorophyll of the 

 leaves, under the influence of light, acting upon water, 

 produces formaldehyde : 



CO, ' H,0 =- CH,0 - 0.,. 



The formaldehyde, combining with the nitrogen taken in by 

 the plant as ammonia or nitrates, forms nitrogenous bodies, 

 perhaps asparagin and leucin. From these is formed albumen 

 and lastly protoplasm. The starch of the plant results from 

 the decomposition of the protoplasm and is the immediate 

 product not of constructive, but of destructive metabolism. 

 That this is so is made evident from the formation of starch 

 in parts of the plant which are not green and are not exposed 

 to light. The sugar of the plant is formed from the starch by 

 hydrolysis. The formation of protoplasm goes on in ever>' 

 living cell of the plant and independently of the presence of 

 chlorophyll and the action of light. In green plants it goes 

 on more actively, however, in the cells that contain chloro- 

 phyll. In the green parts of the plant this synthetic process 

 begins with such simple substances as CO.^. H.,0 and min- 

 eral salts, whereas in the white portions it begins with com- 

 plex substances, asparagin, and so forth. The energy required 

 for the formation of jirotoplasm in the green parts of the plant 

 is obtained directly from the sun, while in the parts unexposed 

 to light this energy nuist be obtained in the main from the 

 oxidation of starch, sugar, etc., formed elsewhere. 



The second theory, and perhaps the one most widely 

 accepted, may be outlined thus : The chlorophyll, as before, 

 produces formaldehvde : 



CO, -I H.,0 -- O, H CH3O. 



