THE NATURE OF PHOTOSYNTHESIS 133 



becomes highly important. Under natural conditions the problem becomes 

 much more complex with the introduction of such factors as the angle 

 of incidence of the light, etc. 



It may here also be mentioned, that since the photosynthetic process 

 is endothermic, it has been suggested that photosynthetic activity would 

 reduce the transpiration from a leaf to a certain degree. A plant in 

 an atmosphere freed of carbon dioxide and so incapable of photo- 

 synthesis, would thus show a higher rate of transpiration than when 

 the plant was photosynthetically active. Claims of having observed 

 such a phenomenon have been affirmed and denied from several sources. ^^' 

 In view of the fact that in land plants but a small proportion of the 

 absorbed energy is converted into chemical energy, the "cooling effect" 

 of photosynthesis would be relatively slight and it is possible that the 

 phenomenon, when observed, was actually due to some other cause. In 

 this connection the observation of Detlefsen "^ and of Puriewitsch '^^ 

 are of interest. Both found that the amount of radiant energy absorbed 

 was greater when leaves were in an atmosphere containing carbon dioxide 

 than in one free of this gas. The photosynthetically active leaf absorbs 

 1.7 to 11.7 per cent more energy than the leaf in a carbon dioxide free 

 atmosphere. 



Blackman and Matthaei ^^* have made careful determinations of the 

 internal temperature of leaves exposed to bright illumination. They used 

 small thermocouples of copper and constantan connected with a galvanome- 

 ter. One junction was inserted in the mid-rib of the leaf, and the other 

 junction was placed in a water bath. The internal temperatures of the 

 leaves were measured by bringing the temperature of the water bath to 

 a point where the galvanometer deflection was zero. The temperature 

 of the two couples was then the same and the temperature of the water 

 bath was taken as that of the leaf. The leaves used for the determina- 

 tion of the photosynthetic rates were in a special frame submerged in 

 a water bath. The temperature of the bath varied only slightly, while 

 "the leaf temperature oscillated up and down with the varying fine shades 

 of natural illumination through a range of 9°." 



Miss ]\Iatthaei has made a study of the influence of temperature on 

 photosynthesis by determining the amount of carbon dioxide absorbed. 

 The smoothed curve which she constructed from a number of separate 

 determinations is shown in Figure 12. This gives the rates of photo- 

 svnthesis from — 6° to 43° under optimal conditions of light and carlx)n 



dioxide supply. 



Miss ]\Iatthaei's work was carried out with the viewpoint of Black- 



"^^Deherain, Ann. sci. nat. (VI), 4, 177 (1876). Jumelle, Rev. gen. hot., 1, 2,7 

 (1889); 2, 417 (1890); 3, 241 (1891). Burgerstein, "Transpiration der Pflan- 



zen " 46. 



'"'Detlefsen, Arh. hot. Inst. Wiirzhurg., 3, 534 (1888). 



"'Puriewitsch, Jahr. iciss. Bot., 53, 210 (1914). 



"^ Blackman and Matthaei, Proc. Roy. Soc. London, B 76, 402 (1905); Phil. 

 Trans. Roy. Soc. London, B 197, 75 (1904). 



