THE NATURE OF PHOTOSYNTHESIS 111 



By throwing a si>ectrum on a leaf Timiriazeff "^ endeavored to demon- 

 strate the capacity of different wave-lengths for photosynthetic work. 

 The method has been used by several investigators and consists essen- 

 tially in allowing the light to act for a certain length of time, extracting 

 the chlorophyll, and treating the leaf with an iodine starch reagent. The 

 starch formed in photosynthesis thus becomes blue to black and the depth 

 of color serves as a rough measure of the rate of photosynthesis. By 

 means of the iodine reagent the starch is thus "developed" producing an 

 effect not entirely dissimilar to the development of a photographic plate. 



Ursprung ^° has used the method with prism and grating spectra and 

 a variety of light sources. Timiriazeff had obtained hardly any photo- 

 synthetic effect with blue and violet rays. Ursprung found the lower limit 

 of starch formation at about 759 ^i ^. There was no starch formation in 

 the infra-red. In the violet he found starch formation up to 330 [i [i. 

 With the use of quartz apparatus no photosynthetic action could be de- 

 tected in the ultra-violet. Considering the region of maximum activity 

 as that in which starch formation is first detected, the region 687-656 |.i j.i 

 would receive this value. This applies for light from the sun. a carbon 

 arc or filament lamp with prism and grating spectra. There appears to 

 be little doubt that all light between wave-lengths 760-330 ^i |i is capable 

 of producing starch if sufficient time of illumination is allowed ; in the 

 normal solar spectrum staixh formation in the blue requires longer time 

 than in the red. The exact spectral limits of photosynthesis have not been 

 determined and it is, in fact, probable that considerable variation exists in 

 different plants in this regard. In general, the method of measuring the 

 rate of photosynthesis by the appearance of starch in the leaf cannot be 

 considered as being very accurate. The formation of starch is itself not 

 dependent upon the presence of light, as is indicated by the fact that 

 starch can be formed by the leaf from glucose, sucrose and other sub- 

 stances in the dark. Between the actual reduction of carbon-dioxide by 

 'light and the appearance of starch in the leaf there are a number of steps 

 all of which may be influenced by light and other factors. 



A somewhat different line of approach was followed by Wurmser.®^ 

 This author has made a careful study of the effect of light on chlorophyll 

 solutions and the decoloration of such solutions in different wave-lengths. 

 The rate of decomposition of an acetone solution of chlorophyll by light 

 of different wave-lengths was utilized by Wurmser as a measure of the 

 incident light. He used the green alga Ulva lacfuca and the method of 

 Osterhout and Haas for determining the rate of photosynthesis. The 

 latter method, which is described in another chapter of this book, is based 

 upon the fact that certain marine algae, when exposed to light convert 

 some of the dissolved bicarbonates into carbonates and thus increase the 

 hydroxyl ion concentration of the solution, the rate of increase of the 



"Timiriazeff, Proc. Rox. Soc. London, B. 72, 424 (1903). 



'"Ursprung, Ber. bot. Ges.. 35, 44 (1917). 



^Wurmser, "Recherches sur rassimilation chlorophjilienne," Paris, 1921. 



