172 PHOTOSYNTHESIS 



trates this difference ; the experiments were carried out with high carbon 

 dioxide-concentration, high hght intensity, and at 25 : 



Photosynthetic Respiration 



Inhibition of 50 Inhibition of 50 



Per Cent by milli- Per Cent by milli- 



Narcotic moles per liter moles per liter 



Methyl-urethane 400 1200 



Ethyl-urethane 220 . 780 



Propyl-urethane 50 100 



Butyl-urethane (iso) 17 43 



Amyl-urethane (iso) 12 32 



Phenyl-urethane 0.5 6 



Inorganic Compounds: Some inorganic salts, even in great dilution, 

 have a very decided effect on photosynthesis. The toxicity of solutions is 

 not only a matter of the concentration but also of the total amount of the 

 toxic salt available. For example, Treboux -" found that twigs of Elodea 

 in 100 cc. of 0.000015 per cent CUSO4, 0.00016 per cent ZnSO^ and 0.00015 

 per cent C0SO4 were but very slightly affected; in 500 cc. of solutions 

 one-tenth these concentrations, the plants were soon killed. The salts just 

 mentioned, and those of mercury, are very toxic. The inhibiting effect 

 on photosynthesis decreases gradually with concentration and appears to 

 be about the same as the toxicity for the plant cells. Ferrous sulfate 

 and ferric chloride, in not too dilute solutions, accelerate photosynthesis. 

 The action may be due to the fact that these solutions have an acid re- 

 action. Mercury vapor is very toxic. 



Briggs ^^^ studied the effect of a deficiency of various mineral nutrients 

 on photosynthesis. In cultures lacking one of the essential elements, 

 namely, potassium, magnesium, iron and phosphorous, photosynthetic 

 activity of Phaseolus vulgaris was decidedly less than in leaves grown in 

 solutions with all the essential nutrients present. The same depression was 

 observed whether external conditions such as light or temperature were 

 limiting factors. Briggs endeavored to explain this on the basis that a 

 lack of any of the essential elements resulted in a reduction of the re- 

 active surface of the chloroplasts ; the reactive surface affecting both 

 the temperature and light reactions. Briggs also found that the photo- 

 synthetic activity of a plant grown in what is usually considered a com- 

 plete mineral nutrient solution to be less than that of a plant grown in 

 soil. Very little is known regarding the role of mineral nutrients in 

 photosynthesis. ^^^ 



Galwialo ^^^ reports that in leaves which have been cut into pieces and 

 placed into water containing carbon dioxide, the rate of photosynthesis 

 soon decreases and that photosynthesis can be revived by the addition to 



'•"Treboux, /. c, 56. Rumm, Bcr. hot. Gcs., 11, 79 (1893). Frank and Kruger, 

 ibid., 12, 8 (1894). 



^Briggs, Proc. Roy. Soc, 94 B, 20 (1922). 



"^ Stoklasa and 'Matousek, "Beitrage zur Kenntniss der Zuckerriibe," Jena, 1916. 

 Andre, Cornpt. rend.. 162, 563 (1916). 



"' Galwialo, Biochnn. Zeit., 158, 65 (1925). 



n3 ; 



