286 



PHYSIOLOGY, SYMBIOSIS, ETC. 



The results so far described would seem to be against a weak light 

 and cold medium as being the best conditions for the Fucaceae, 

 but the great development of this group in northern and arctic 

 seas cannot be overlooked, and the explanation must be the 

 working together of factors other than merely light and temper- 

 ature. Hyde (1938), however, explains this development in arctic 

 waters as due to the indirect effect of lowering the temperature 

 because this results in an excess of assimilation over respiration. 



c 



O 



■M 



2x 



100 

 2x40 



5°C 



30 25 20 15 10 

 Temperature 



Fig. 184. Diagram of paper model to show the combined effects of Hght and 

 temperature on the rate of apparent assimilation of Fucus serratus. (After Hyde.) 



She found that between 15 and 20° C. the assimilation rate could 

 be increased by raising the light intensity, and that there was a 

 certain light value (2 x 500) which yielded an optimum in the rate 

 of assimilation. This effect, however, is not observed at low light 

 intensities and low temperatures, whilst above 25° C. an increase 

 of the light intensity causes a marked decrease in the assimilation 

 rate (cf. fig. 184). 



Ehrke (193 1), after carrying out experiments both in the field 

 and laboratory, found that the respiration of most algae increased 

 with rise of temperature, and often has not reached its maximum 

 even when the high temperatures have reduced the assimilation 



