318 The Physiology of Plants BOOK in 



allowed to fall upon the plant. He estimated the photo- 

 synthetic value of the rays he blocked out by measuring 

 the decrease in the amount of oxygen exhaled when only part 

 of the spectrum fell upon the leaf. Reinke counteracted 

 the unequal dispersion of the rays by his prism by con- 

 centrating different regions of the spectrum to strips of 

 equal size by means of cylindrical lenses. It is very 

 strange to find that Reinke published none of the results 

 he obtained with the apparatus. 



Opinion remained divided up to the end of the century 

 on the question of the second maximum in the blue. As 

 we have seen, Engelmann emphasized it strongly in 1884. 

 Timiriazeff held it established in 1885. In 1886 Bonnier 

 and Mangin said that the ultra-violet rays can in some 

 cases cause a small photosynthesis. Kohl supported it in 

 1897. On the other hand, Pfeffer could not convince him- 

 self of its existence in 1880, and Reinke claimed that only 

 one maximum exists. 



The most accurately plotted curves do not show an 

 exact correspondence between the rays of light actually 

 absorbed and those effecting the decomposition of carbon 

 dioxide. 



The position of the maximum in the red was differently 

 determined by the various writers whom we have quoted. 

 Timiriazeff said it corresponds to rays of wave-length 

 685-655 /A/A, and was confirmed by Engelmann. Pfeffer 

 said 655-590 /A/A, and Reinke 720-685 /A/A. 



But few investigations were made up to the end of the 

 century to determine accurately the relation of temperature 

 to the activity of the photosynthetic processes. The most 

 accurate ones were those of Kreussler in 1890 ; he showed 

 that a gradual increase of construction goes on as the 

 temperature rises up to about 25 C. In some plants it 

 rises to a maximum at 40 C., above which there is invariably 

 a rapid decrease, photosynthesis ceasing generally at 45 C. 



