180 PHOTOSYNTHESIS 



rise very high. Harder quotes Klehs who found that the leaves of the 

 beech tree which had been kept under constant and high illumination still 

 emitted carbon dioxide at 6250 Lux. The great importance of a considera- 

 tion of such circumstances to plants in nature needs no special elaboration. 

 The investigations of O. Warburg"* on the efficiency of the photo- 

 synthetic process are pertinent to the subject of the nature of the com- 

 pensation point. He found wide fluctuations in the quotient: absorbed 

 radiant energy to chemical work performed, depending upon the previous 

 treatment of the plants studied. It was established that when the plants 

 are cultivated under conditions of high light intensity they are capable 

 of utilizing only a small amount of the absorbed radiant energy. On 

 the other hand, when the plants are cultivated under conditions of low 

 light intensity they are capable of utilizing a relatively large proportion 

 of radiant energy absorbed. By cultivation under conditions of either 

 high or low light intensity, one type of plant can apparently be converted 

 into the other within a few days, and its photosynthetic efficiency altered 

 accordingly. It is doubtful on the basis of the work of Willstatter and 

 Stoll, that this alteration can be entirely at least ascribed to alterations 

 in chlorophyll-content, as they found no direct j)roportionality between 

 this and photosynthetic rates. 



It is apparent, therefore, that merely increasing the light intensity 

 or the period of illumination of a plant does not result in a higher photo- 

 synthetic efficiency. 



Some very interesting observations of the effect of hydrocyanic acid 

 on the compensation point have been made by Warburg"^ on the green 

 alga Chlorella. A 0.0001 normal solution of hydrocyanic acid exerts a 

 decidedly inhibiting effect on the photosynthetic activity; on removing 

 the plant from the HNC solution normal photosynthesis is again attained. 

 Respiration, on the other hand, is not affected in the same manner. A 

 0.01 N. solution of HNC slightly stimulates the oxygen consumption and 

 carbon dioxide emission and only after several hours exerts a poisonous 

 or inhibiting effect. By studying the influence of various concentrations 

 of HNC on photosynthesis Warburg found that oxygen evolution is 

 inhibited by very small amounts of HNC, but that there is a point be- 

 yond which even relatively high concentrations of the poison have no 

 effect. Warburg's results seem to indicate that in 0.05 normal HNC 

 solution the plants are incapable of taking up and reducing carbon dioxide 

 even in high light intensity. That is, plants with a compensation point 

 of 500 Lux in 0.05 normal HNC solution produced no more oxygen at a 

 light intensity of 19,000 Lux than with 500 Lux. Also the compensation 

 point is not affected by the HNC, for the carbon dioxide produced by respi- 

 ration is reduced by means of light of low intensity at the same rate in 

 plants exposed to cyanide as in plants under normal conditions. Thus 

 it appears that relatively high concentrations of HXC (0.05 normal) do 



"* Warburg, Z. physik. Chew.. 102, 246 (1922). 

 =" Warburg, Biochem. Zeit.. 103, 199 (1920). 



