THE NATURE OF PHOTOSYNTHESIS 129 



furnace exhausts, is piped in or on the ground. In general an increased 

 yield has been obtained. But the results are naturally dependent upon cli- 

 matic conditions and soil fertilization, and it is difficult to calculate any 

 regular or reliable ratio between increased yield and the carbon dioxide 

 factor. 



High concentrations of carbon dioxide exert a narcotic effect on photo- 

 synthesis. In pure carbon dioxide photosynthesis quickly ceases. Dif- 

 ferent species exhibit a wide variation in the amount of carbon dioxide 

 they can withstand.^-" Mosses and the lower plants are especially re- 

 sistant to the effects of asphyxiating gases. It is highly probable that 

 the effect of high concentrations of carbon dioxide is due to a toxic 

 influence on the protoplasm. Higher concentrations of this gas, 15-25 per 

 cent, retard and finally completely inhibit growth.^-^ Darwin ^" has also 

 found that in an atmosphere rich in carbon dioxide the stomata tend 

 to close. The efifect of high concentrations of carbon dioxide on photo- 

 synthesis is undoubtedly of a complex nature in which other functions 

 of the plant play an important role. 



d. The Influence of Temperature. 



The wide range of temperature under which plants are found to 

 thrive in nature indicates that the photosynthetic process can be carried 

 on at these temperatures, or at least that the photosynthetic apparatus 

 is not permanently injured at the extreme temperatures noted. Thus 

 McGee ^-^ found that joints of a prickly pear (Opuntia) reach a tempera- 

 ture of 55° in the open without injury. On the other hand the marine 

 algae of the polar regions must be exposed to temperatures of about 

 zero degrees for long periods ; while Jumelle ^-* reports that Picea excelsa 

 still showed photosynthesis at — 35° and Junipcriis at — 30 to —40°, 

 while Physcira cilaris and Cladonia rangifernia ceased at - — 25° and 

 Primustri at — 37°. Miss Henrici ^^^ found the threshold temperature of 

 certain alpine phanerogams at — 16° and for some lichens at — 20°. 

 Ewart ^^^ has shown that recovery of the capacity to do photosynthetic 

 work after exposure to extreme temperatures depends very much upon 

 the length of time of exposure. Similar results were obtained by 

 Wurmser ^^^ and Jacquot who found that marine algae, subjected to tem- 

 peratures of 36 to 45° for from 1 to 15 minutes had a lower rate of 

 photosynthesis when returned to the normal temperature of 16°, the de- 



"" Ewart, Journ. Linncan Soc. 31, 404 (1896). 



""Chapin, Flora., 91, 348 (1902). Brown and Escombe, Proc. Roy. Soc, 70 B, 

 397 (1902). 



'=" Darwin, Phil. Trans. Roy. Soc, 190, 531 (1898). 



^ McGee, Carnegie Inst, of Washington, Year Book, 20, 47 (1921). 



""Tumelle, Compt. rend., 112, 1462 (1891). 



•^''Henrici, Verli. Natur. f. Ges. Basel, 32, 107 (1921). 



"'Ewart, Joxirn. Linncan Soc, 31, 368 (1896). 



"'Wurmser, Bull. Soc. chim. Biol, 5, 305 (1923). 



