176 



PLANT PHYSIOLOGY 



absorbs light selectively; part of the wave lengths are trans- 

 mitted. If sunlight is passed through a chlorophyll solution 

 and then spread by means of a prism into its constituent wave 

 lengths, portions of the spectrum are absorbed by chlorophyll. 

 Absence of the wave lengths absorbed will cause dark bands; 

 while others that are transmitted appear as bright bands of the 

 hues of the spectrum, some of which will be transmitted in differ- 

 ent degrees. In this manner, the absorption spectrum of 

 chlorophyll is obtained (Fig. 52). 



The most intense absorx:)tion takes place in the region of the 

 red and orange rays, between the Fraunhofer lines B and C 



700 B 



5ooF 

 J-L. 



400 



7ooB C 



600D 



E 5ooF 



•hOO 



Fig. 52. — Absorption spectrum of chlorophyll. Above, chlorophyll a; below, 



chlorophyll h {after Willstatter and Stoll). 



(wave length, 630 to 680 mm). An almost equally intensive 

 absorption will be found in the blue-violet portion of the spec- 

 trum, to the right of line F (470 mm) (1mm = -OOIm). In addition 

 to these, several other less distinct absorption bands may be 

 detected. The absorption spectra of chlorophyll a and b differ 

 somewhat. In b, the absorption in the blue-violet region covers 

 a somewhat larger region, while in the red-orange portion the 

 area is smaller. 



As the concentration of the chlorophyll increases, the absorp- 

 tion bands become broader and merge (Fig. 53). Finally, there 

 are two absorption bands: one on the right side of the spectrum, 

 covering the red and yellow rays; the other on the left part of 

 the spectrum, embracing the blue-violet rays. The green and 

 part of the red rays that lie at the lower limit of visibility can 

 pass without being absorbed. It is the combination of these 

 two spectral portions that imparts to the chlorophyll the green 



