SPECTRAL PROPERTIES OF CELLULAR PIGMENTS 37 



length on the other axis. The same principle of crossed gradients may 

 be applied to such processes as growth and pigment formation. Since 

 the temperature and light intensity which influence a biological process 

 must often be investigated in some detail before it is practical to make 

 action spectrum measurements under the right conditions, it may be 

 worth while to make a preliminary study of the interrelation of such 

 variables as temperature and light intensity. 



A large petri dish made of a thick aluminum plate is used for these 

 crossed gradient experiments. One edge of this aluminum plate is kept 

 cold by flowing water through a long hole just inside its left edge. The 

 opposite edge is similarly kept warm. There is therefore a flow of heat 

 across the plate giving a stable temperature gradient from left to right. 

 A thin layer of agar on the surface of the plate is inoculated with 

 algae. Now at right angles to the temperature gradient there is a light- 

 intensity gradient estabhshed by diffuse illumination from above. Thus, 

 at each temperature we have a complete series of intensities. There- 

 fore, the growth of the algae on a plate of this type will automatically 

 plot out their survival limits and will also indicate their intensity and 

 temperature requirements for optimum grov/th. Halldal (1958b) has 

 taken samples of algae from different parts of the plate to see the 

 effects of temperature and light intensity on the formation of different 

 pigments as shown in Fig. 12. It is possible that experiments of this 

 type may be useful in establishing relations between intensity of light 

 and temperature for other photobiological processes. The device has 

 been described by Halldal and French (1958). 



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Barer, R. 1955. Spectrophotometry of clarified cell suspensions. Science, 121, 

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Bateman, J. B., and G. W. Monk. 1955. Spectral absorption of turbid systems 

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Duysens, L. N. M. 1956. The flattening of the absorption spectrum of suspen- 

 sions, as compared to that of solutions. Biochim. et Biophys. Acta, 19, 1-12. 



Emerson, R., and C. M. Lewis. 1943. The dependence of the quantum yield of 

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French, C. S. 1955. Fluorescence spectrophotometry of photosyntlietic pigments. 

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 American Association for the Advancement of Science, Washington, D. C. 



French, C. S. 1957. Derivative spectrophotometry, Proc. I.S.A., pp. 83-94. 



