THE NATURE OF PHOTOSYNTHESIS 167 



The high photosynthetic rates observed by Willstatter and StoU in 

 leaves poor in chlorophyll make Pringsheim's protective theory very im- 

 probable. Pringsheim/'-*- starting from the erroneous hypothesis that di- 

 rect light accelerates respiration, developed the theory that chlorophyll acts 

 as a protection against light. He supposed that light might induce photo- 

 synthesis in colorless cells as well as those containing chlorophyll ; the 

 pigment acting as a protection against light and permitting the reaction 

 to become more prominent in the green cells than in the colorless ones. 

 The theory is quite untenable, for the cells containing chlorophyll are, if 

 anything, more sensitive to light than the colorless ones. 



From the fatigue effects of Pantanelli, the inhibition experiments of 

 Ewart and others and the disproportionality betvi^een chlorophyll-content 

 and photosynthesis as demonstrated by Willstatter and Stoll, it is apparent 

 that the protoplasm of the cell plays an important role in the process. Now, 

 of course, it must be realized that the nature of this protoplasmic factor 

 or enzyme has not been established. Willstatter and Stoll consider it 

 probable that it behaves as a dissociable oxygen compound (dissozierbare 

 Sauerstoft'verbindung) . 



In relation to the question of the dependence of photosynthesis on 

 protoplasmic activity mention should be made of the attempts which have 

 been made to detect photosynthetic activity in dead leaves. The first reports 

 regarding this subject were not clear and decidedly contradictory. Frie- 

 deP^^ was the first to claim that photosynthesis was possible in dried leaf 

 material to which water had again been added, but was himself unable to 

 substantiate his first findings. Recently Molisch has again made a very 

 careful study of this problem. For the detection of photosynthesis he 

 used a very sensitive luminescent bacteria method to the technique of which 

 he had previously made many valuable contributions. In the earlier ex- 

 periments on the photosynthetic activity of dried leaves, the latter were 

 ground in water and the mixture was then filtered. JMolisch altered the 

 procedure so that the entire paste was used ; this proved to be an impor- 

 tant point, for material prepared in this manner after being kept in the 

 dark for a time and subsequently illuminated, even for a very brief period, 

 showed the evolution of oxygen. The leaves were dried at 30 to 35° for 

 3 to 4 days and in some cases were kept in a desiccator over calcium 

 chloride or sulphuric acid for weeks. Some leaves which had been heated 

 to 84° for 5 hours still showed the capacity to form oxygen, but the 

 leaves could not endure a temperature of 96° for one hour. When heated 

 to higher temperatures in the moist condition the leaves lose the capacity to 

 form oxygen in the light. Similarly, if the leaves are treated with ether 

 and dried they no longer are able to form oxygen. Leaves which have 

 been killed by freezing are also able to emit oxygen when illuminated. 



"' Pringsheim, Jahrb. iviss. Bot., 12, 288 (1882). 



^'"Friedel Compt. rend.. 132. 1138 (1901). Harroy. ibid.. 133, 890 (1901). 

 Herzog, Zeit. phvsiol. Chcm., 35. 459 (1902). Molisch, Bot. Zeitg., 62, 1 (1904); 

 Zeit.f. Bot. 17, 577 (1925). 



