SPECTRAL PROPERTIES OF CELLULAR PIGMENTS 



29 



absorption peak for one form of protochlorophyll is located. It is 

 clear from the comparison of action and absorption in this figure that 

 direct absorption of light by protochlorophyll itself is necessary for 

 its transformation to chlorophyll. In another experiment a leaf already 

 containing both chlorophyll and protochlorophyll was illuminated with 

 light of 670 m/j-, which was absorbed by chlorophyll but not by proto- 

 chlorophyll. This wavelength did not cause any further transforma- 

 tion. This action spectrum was measured with the usual point-by-point 

 procedure, each point requiring a 2-hr chemical analysis. 



When Dr. Halldal was working at this laboratory, he wanted to 

 measure the action spectrum for the phototaxis of dinoflagellates. A 

 method was devised for measuring the complete action spectrum in 

 a single experiment, much like the microprojection procedure of Engle- 

 mann. The procedure (Halldal, 1958b), however, was modified so that 

 quantitative results could be obtained and the data presented as 

 curves. The motile algae were put in a small Incite box. This box was 

 illuminated uniformly by a reference beam from the left, as shown in 

 Fig. 8. On the right side of the box he projected a spectrum with a 



REfERENCE 

 BEAM 



PROJECTED 

 SPECTRUM 



RED 



GREEN 



BLUE 



Fig. 8. A simple method for making motile algae plot their own action 

 spectra for phototaxis (Halldal, 1956). 



vertical intensity gradient. The spectrum was bright at the bottom 

 and weak at the top. The spectrum was produced by a grating, and 

 the gradient of the light intensity came from a wedge-shaped slit. The 

 gradient was further sharpened by a photographic density wedge 

 attached to the slit. These motile algae will swim in the direction of 

 the light which is most effective in phototaxis. Without the pro- 

 jected spectrum they all swim to the side of the vessel facing the 

 reference beam and stick on the wall. However, in the parts of the 



