THE NATURE OF PHOTOSYNTHESIS 173 



the water of salts extracted from roots of the plants. He concludes that 

 there are two elements necessary for photosynthesis, electrolytes and a 

 ferment, that in a mixture of these components from the plants (maize, 

 pea, bean, potato) and tlie electrolytes in water solution a reduction of 

 carbon dioxide in sunlight is attainable. In the absence of more exact 

 experimental data it is, however, difficult to judge of the value of these 

 results. 



Acids and Alkalies: \>ry dilute solutions of acids (0.0001 N.) exert 

 a stimulating effect on photosynthesis. Treboux -^* found that this was 

 the case with HCl, HXO3, H2SO4, CrOs, H3PO4, acetic, succinic, oxahc, 

 tartaric and citric acids, as well as with KHSO4 and KH2PO4. Higher 

 concentrations are inhibitory. Bose reports that one part of HNO3 in 

 two billion parts of water caused an increase of nearly 200 per cent in 

 the rate of photosynthesis. This author has also observed a stimulating 

 effect of traces of iodine and of the extract of thyroid gland. 



The effect of alkalies on the photosynthetic process is complicated 

 because this becomes involved in the carbon dioxide-concentration avail- 

 able to the plant. 



Potassium Cyanide in very dilute solutions produces inhibition of photo- 

 synthesis in aquatic plants. Lund and Holt ^^^ found complete but re- 

 versible inhibition in 0.00008 molal solution with fronds of the Pacific 

 Coast kelp Nereocystis. Higher concentrations (0.0006 molal) produce 

 permanent injury in the light, but when the plants are kept in these solu- 

 tions in the dark no permanent injury was observed. Warburg also ob- 

 served that the photosynthetic activity is affected by much lower concen- 

 trations of cyanide than is respiration. He found reversible inhibition 

 of photosynthesis by Chlorella in a 0.0001 normal solution of prussic 

 acid while respiration was not affected by a 0.01 normal solution. War- 

 burg further reports that while his algae were very sensitive to low con- 

 centrations of c}^nide in the absorption of carbon dioxide, the cyanide 

 did not prevent the utilization of the carbon dioxide produced in respira- 

 tion. This fact is brought out when different light intensities are used : 

 with high light intensity the rate of photosynthesis normally greatly ex- 

 ceeds that of respiration, and in a 0.05 normal HCN solution photosyn- 

 thesis is greatly inhibited. As the light intensity is decreased, and the 

 rate of photosynthesis consequently also decreases, an illumination intensity 

 is reached at which photosynthesis equals respiration (compensation point). 

 At this point all the carbon dioxide produced in respiration, but no more, 

 is utilized in photosynthesis. A 0.05 normal solution of HCN does not 

 affect this photosynthesis. In fact, according to Warburg, the degree 

 of inhibition of cyanide solutions depends greatly upon the intensity of 

 light. In a 0.0001 normal cyanide solution with high light intensity 



="^ Treboux, /. c, 65. Wider and Hartlieb, Ber. hot. Ges., 18, 348 (1900). 

 Bose, /. c, 64. 



"'Lund and Holt, Proc. Soc. Expt. Biol, and Med., 20, 232 (1923). Warburg, 

 Bxochem. Zeit., 100, 267 (1919) ; 103, 199 (1920). 



