400 THE ALGAE 



decrease in the absorption of the red wave lengths as depth in- 

 creases. Fig. 216 illustrates absorption and action spectra for algae 

 at different wave lengths 



Fig. 217 illustrates the assimilation rate of algae from all three 

 groups under different hght intensities. All exhibit an optimum 

 light intensity for assimilation, which in the case of the Uttoral algae 

 is in the same region as that of cormophytic land plants. In sub- 

 littoral algae, such as Laminaria saccharina^ the optimum is at a 

 much lower Ught intensity and with some plants, Macrocystis, 

 Egregia, it has been shown to vary with different parts of the plant. 

 Because certain algae are more efficient metabolically at low Ught 

 intensities or when the spectral composition of the Ught has been 

 modified, some workers have suggested that one can recognize sun 

 and shade algae. Different bases have been used by workers in 

 making this distinction, and whilst there may be some ecological 

 value in the distinction, nevertheless further experimental work is 

 desirable before any final conclusions are reached. The compensa- 

 tion depth or Ught intensity at which photosynthesis just balances 

 respiration may be very important. This differs for various species 



Fucus platycarpus '' 



y Fucus serratus 



0-5 12 5 10 50 100 150 



LIGHT INTENSITY (thousands of metre candles) 



Fig. 217 Assimilation of different algae in relation to light intensity. 



(After Stocker and Holdheide.) 



