24 DISCOVERY OF PHOTOSYNTHESIS CHAP. 2 



liberated by the plants; and most important of all, he proved that the 

 increase in dry weight caused by the assimilation of a certain quantity of 

 carbon dioxide, is considerably larger than the weight of carbon contained 

 in it. Since the equivalent of all the oxygen contained in the decomposed 

 carbon dioxide is evolved into the air, the large weight increase cannot be 

 attributed to a coassimilation of oxygen from this source. The plants 

 grew in pure water and air containing air carbon dioxide; therefore the 

 only other possible source of weight increase was water (taken up in a 

 form not removable by drying). De Saussure thought at that time that 

 the assimilation of water is an independent process, merely coupled with 

 the decomposition of carbon dioxide. However, the experimental results 

 do not warrant such a separation of the two processes; what they prove 

 is the participation of water in photosynthesis. They require an amplifica- 

 tion of the over-all equation (2.3), which we may now write in the form: 



plant 



(2.4) Carbon dioxide + water + light > organic matter + oxygen 



Perhaps because water came into the picture several years later than 

 carbon dioxide, the concept of photosynthesis as decomposition of carbon 



dioxide 



light 



(2.5) CO. > C + O2 



followed by hydration of carbon 



(2.6) n C + m H2O > Cn(H20)„ 



has persisted in the literature for a long time. We shall see in chapter 3, 

 that photosynthesis is better interpreted as a reaction between carbon 

 dioxide and water — more exactly, as reduction of carbon dioxide by water 

 (or oxidation of water by carbon dioxide). 



The study of photosynthesis, after the above described rapid start in 

 the quarter century between 1779 and 1804, lapsed into an almost com- 

 plete quiescence for the next fifty years. Liebig (1803-1873) in his fa- 

 mous Chemistry in its Application to Agriculture and Physiology, severely 

 criticized the methods used by plant physiologists of that time in dealing 

 with the problems of material exchange between plants and the surround- 

 ing media. Boussingault (1802-1887) was the first to improve these 

 methods; to him is due the first redetermination of the gas exchange in 

 l)hotosynthesis, with a precision better than that achieved bv de Saussure 

 in 1804 (cf. C^hapter 3). 



8. The Conversion of Light Energy: Robert Mayer 



If the time between 1804 and 1864 (the year when Boussingault pub- 

 lished his first paper on photosynthesis) was sterile in the development of 

 tlie physiology and chemistry of ])hotosynthesis, it saw the foundation of 



