THE PRODUCTS OF PHOTOSYNTHESIS 183 



photosynthesis and vice versa; this jjhenomenon has already been dis- 

 cussed in Chapter 2. 



The quantitative relations of the amount of carbon dioxide absorbed to 

 that of oxygen emitted have been subjected to careful examination. These 

 values, termed the photosynthetic quotient, have been found to be very 

 close to unity and are also discussed in greater detail in Chapter 2. 

 Oxygen emission commences immediately on illumination of the green 

 parts and stops when the light exposure ceases. According to Kostyts- 

 chew ^ the amount of oxygen emitted is considerably less than that of 

 carbon dioxide absorbed during the first few minutes of illumination ; after 



CO2 

 a short period of illummation (15 to 30 minutes) the — =— ratio attams 



U2 



unity. 



. 2, The Carbohydrates of the Leaf 



The early work on photosynthesis was to a great extent confined to 

 investigations on the gaseous exchange of plants, and, owing largely to 

 the undeveloped state of organic chemistry, little advance was made re- 

 garding the substances which were formed from the decomposition of 

 the carbon dioxide. Ingen-Housz, Senebier and de Saussure realized that 

 the carbon dioxide absorbed during photosynthesis was converted into 

 material which was appropriated by the plant for the formation of new 

 tissue and was used to maintain respiration. But the chemistry of these 

 substances was treated only very meagerly. 



The first decisive step in identifying carbohydrates with the photo- 

 synthetic process was made by Sachs. Since the time of Ingen-Housz 

 and Senebier it was realized that photosynthesis was dependent upon 

 chlorophyll. Von Mohl, the discoverer of protoplasm in plants, in 1837, 

 had made an intensive study of chlorophyll, had described the chloroplasts 

 and recognized starch in them. A great deal of confusion existed re- 

 garding the nature and function of the chloroplasts, but von Mohl de- 

 veloped the conception that the starch which was associated with the 

 chloroplasts was of the nature of reserve food material. These investi- 

 gations together with those of other anatomists, notably Nageli, deter- 

 mined the structure, and to some degree also the function, of the chloro- 

 plasts. Gries " then demonstrated that the starch in the chloroplasts dis- 

 appears when the leaves were kept in the dark. It was, however, not 

 certain whether starch or chlorophyll was the first to appear in the 

 illuminated plant so that the genetic relationship of these two substances 

 was still uncertain. 



With these facts as a background Sachs^ undertook his classical re- 

 searches on the function of chlorophyll. These established the fact that 



' Kostytschew, Ber. hot. Ges., 39, 319 (1922). 

 'Gries, Ann. Sci. Nat. Bot., 8, 179 (1857). 



'Sachs, Jahrb. wiss. Bot., 3, 184 (1863); Bot. Zeifg., 20, 365 (1862); 22, 289 

 (1864) ; Arbeiten aus dem bot. Inst. Wiirzburg, 3, 1 (1884). 



