THE NATURE OF PHOTOSYNTHESIS 163 



it to a disturbance of respiration. Later Pringsheim ^^^ studied the effects 

 of lack of oxygen by subjecting the plants to a stream of hydrogen and 

 carbon dioxide. Under these conditions protoplasmic streaming soon 

 ceases, and if the cells are then illuminated no photosynthesis occurs. Cells 

 in this condition of "inanition" can be revived by the supplying of oxygen. 

 Even in continuous illumination in an atmosphere of hydrogen and carbon 

 dioxide, i.e., starting with active photosynthesis, cells of Cliara gradually 

 showed stoppage of photosynthesis and protoplasmic streaming. If this 

 condition of "inanition" is not continued too long photosynthesis and 

 streaming can be revived by addition of traces of oxygen. Pringsheim 

 interpreted these facts on the basis that photosynthesis and respiration 

 were intimately associated. He argued that with photosynthesis inde- 

 pendent of respiration, the former process in an atmosphere containing 

 carbon dioxide and with sufficient light intensity should continue to evolve 

 oxygen, and respiration and protoplasmic streaming could continue. This. 

 his experiments showed, is not the case. Ordinarily photosynthesis pro- 

 duces more oxygen than is used in respiration. 



Pringsheim's ^®- next deduction does not seem as well founded. He 

 concluded that in photosynthesis no oxygen is formed within the cell. 

 But rather that in the decomposition of carbonic acid in the cell a sub- 

 stance, possibly hydrogen peroxide, is formed which passes out by 

 diosmosis and on the surface of the cell decomposes with the liberation of 

 oxygen. He maintained that we have no experimental evidence of the 

 formation of oxygen within the cell. That all our conclusions are based 

 upon methods of gas analysis which can yield results only of the final end 

 products. Of the chemical nature of the hypothetical oxygen compound 

 Pringsheim did not venture a conjecture but emphasized that the carbon 

 dioxide decomposition and oxygen evolution are two separate processes. 

 This he sought to support by his observations of oxygen evolution from 

 dying cells. 



There is as yet not sufficient experimental evidence to permit the 

 formulation of a theory of either the kinetics of oxidation or of reduction 

 in living cells. In connection with the fact, which seems quite well 

 established, that a small amount of oxygen is essential for the reduction 

 of carbon dioxide in the photosynthetic process, similar conditions which 

 have been observed in the catalytic hydrogenation by means of platinum 

 are of interest. Adams and others ^^-^ have called attention to the fact 

 that oxygen is essential to the activity of platinum black as a hydrogena- 

 tion catalyst, though the role of oxygen in these reactions is not yet 

 definitely established. 



2*^ Pringsheim, Sit::b. prcuss. Akad. Wiss., 1887, 763. Friedel, Compt. rend., 131, 

 477 (1900). 



''== Pringsheim, Jahrb. wiss. Bot., 17, 178 (1886). 



''"■ Carothers and Adams, Jour. Am. Clicin. Soc, 47, 1047 (1925). Waldschmidt- 

 Leitz and Seitz, Ber. Chcm. Ges., 58 B, 563 (1925). 



