THE NATURE OF PHOTOSYNTHESIS 159 



of the phenomenon of the inhibition of photosynthesis by high light 

 intensity. 



It has been known for a long time that the photosynthetic rate de- 

 creases with the accumulation of the products of photosynthesis. This 

 is especially noticeable when the leaves which are being observed have 

 been removed from the rest of the plant. Boussingault ^" first noticed the 

 gradual decrease in the amount of carbon dioxide reduced by leaves which 

 had been removed from a plant. This is undoubtedly associated with the 

 fact that in excised leaves the capacity for translocation of synthesized 

 carbohydrates does not exist. Under normal conditions the material 

 formed in the leaves travels to other parts of the plant where it is con- 

 sumed or stored. When the leaves are cut from the plant this is not 

 possible, and accumulation readily occurs. 



SaposchnikofT ^'* has demonstrated the inhibitory power of an accumu- 

 lation of carbohydrates and that these cannot increase beyond a certain 

 point. When the leaves of Vitis vinifera contain 23 to 29 per cent carbo- 

 hydrates of the dry weight there was a cessation of photosynthesis. Leaves 

 which had accumulated a certain amount of carbohydrates showed a de- 

 creased photosynthetic rate, while a decrease in this accumulation of 

 carbohydrates resulted in an increased photosynthetic rate. It is evident 

 that the rate of the dififerent steps comprising the photosynthetic process 

 exercises a profound effect upon the total reaction and the movement of 

 the products away from centers of activity is of importance for con- 

 tinuous action. This applies not only to the accumulation of carbohydrates 

 but apparently also to oxygen, for pressures of oxygen above %o at- 

 mosphere tend to decrease the photosynthetic rate when high light inten- 

 sity is used. 



These facts are of particular importance in experimental work, when 

 excised leaves are used, though they must be taken into consideration 

 under all circumstances. When the rate of photosynthesis is high as under 

 conditions of high carbon dioxide concentration, strong illumination and 

 elevated temperatures, this effect is especially to be watched for. 



Ewart ^" made similar observations with a variety of plants, but found 

 that the plants which have thus been inactivated do not always regain their 

 photosynthetic capacity by being kept in darkness. The case of Allium cepa 

 is of interest because this plant does not form starch. When leaves of 

 this plant are exposed to bright light for a long time, 14 days, or for a 

 shorter period while being fed sugar, the evolution of oxygen finally 

 ceases. This inactivation apparently does not injure the cells or chloro- 

 plasts. After a few days in darkness the capacity for photosynthesis is 

 regained. It is possible that this phenomenon is associated with the 

 osmotic relations of the cell, for when the cell sap reaches a certain 

 concentration photosynthesis ceases. 



'''Boussingault, "Agronomic Chimie et Agriculture," Vol. 4, pp. 286, 312 (1868). 

 >'* Saposchnikoff, Ber. bot. Ges.. 11, 391 (1893) ; 9, 293 (1891) ; 8, 233 (1890). 

 "'Ewart, Jour. Linncan Soc, 31, 429 (1896). 



