3 02 THE FOOD OF PLANTS 



may not exist which are able to liberate free oxygen by means of energy 

 derived from chemical decompositions, and which may thus enable aerobic 

 organisms to exist in the depths of the ocean. 



PART II 



THE ASSIMILATION OF CARBON DIOXIDE 



A. Photosynthetic Assimilation *. 

 SECTION 52. General. 



It is the chlorophyll-containing parts and organs which have this special 

 power of producing organic substance from carbon dioxide and water, by 

 means of the energy of radiant light, a process which is accompanied by an 

 excretion of free oxygen. For the correct interpretation of this process 

 Engelmann's 2 observation that purple bacteria have a similar power of 

 photosynthetically assimilating carbon dioxide is of great importance, 

 although the feeble assimilatory activity of these organisms cannot be 

 of much importance in the balance of nature. 



Since chlorophyllous plants obtain all their organic food from the 

 synthesis of carbon dioxide and water, they are able to grow in pure sand 

 if supplied with the other essential elements in the form of appropriate 

 inorganic compounds. Thus maize, barley, buckwheat, beans, and very 

 many other plants grow admirably under such cultural conditions, in which 

 the large amount of organic materials, ultimately far surpassing the weight 

 of the seed, are obtained by the photosynthetic assimilation of carbon 

 dioxide. The entire organic substance of the harvest taken from a field, 

 or of a tree removed from a forest, is derived from the same source, for 



1 [The term ' photosynthetic assimilation ' is a perfectly general one, and would include the 

 assimilation of other compounds by the aid of light, should any such processes be discovered in the 

 future. The photosynthetic assimilation of carbonic acid may be termed carbon dioxide assimilation. 

 ' Carbon-assimilation ' is obviously incorrect, for in analogy with the term ' nitrogen-assimilation ' it 

 would indicate that carbon could be directly assimilated. The uncouth term ' photosyntax ' is quite 

 unnecessary, and moreover has been erroneously used to indicate all cases of carbon dioxide 

 assimilation, although the occurrence of a power of chemosynthetic assimilation of carbonic acid 

 in certain Bacteria was already well-known.] 



2 Engelmann, Bot. Zeitung, 1888, p. 661. [The chromophyll of purple bacteria appears to be 

 a compound pigment, from which chlorophyll and a pinkish-red dye may be extracted.] 



