58 PLANT PHYSIOLOGY 



all probability not a constituent element of the chlorophyll 

 pigments, must be either that of a stimulant to the living 

 protoplasm to form chlorophyll, or else in some purely non- 

 vital way an assistant in the synthesis of these color-com- 

 pounds. The former supposition seems more probable, but 

 neither has any strong evidence in its favor. However, 

 neither light nor iron will bring about the production of 

 chlorophyll in cells which do not contain chromatophores as 

 living organs. The cells of animals, fungi, and certain of the 

 phanerogamic parasites and saprophytes, contain no chro- 

 matophores, no organs, therefore, capable of forming, con- 

 taining, and using the chlorophyll pigments. 



PHOTOSYNTHESIS 



The absorption of carbon-dioxide and the combination of 

 water with it by green plants have for years been mislead- 

 ingly termed assimilation or carbon-assimilation. The word 

 assimilation may much more correctlj^ and significantly be 

 applied to the working up, by the living organism, of sub- 

 stances already somewhat resembling in constitution the 

 substance of the organism itself. The word really means 

 making like. No organism can make carbon-dioxide and 

 water like itself till it has combined them. The food formed 

 by combining the two innutritions substances, carbon-diox- 

 ide and water, can be used directly only as fuel, for respira- 

 tion. It must be modified, added to, elaborated, changed 

 in various ways not yet understood, until it becomes chem- 

 ically and physically like the component matters of the 

 body of the organism. Combining carbon-dioxide and wa- 

 ter is food-manufacture — a synthetic process; the modifica- 

 tion of this food which makes it immediately available as 

 constructive material is assimilation — not necessarily a syn- 

 thetic process. Since, as we have seen, the energy by which 

 the synthetic process is accomplished comes from the sun, 

 the name photosynthesis has been proposed for it. 



Besides the substances absorbed — carbon-dioxide and 

 water — and the final products of the synthesis — starch or 

 some equivalent — nothing is known of the chemistry of pho- 

 tosynthesis. Only the relative numbers of the atoms form- 



