PHYSIOLOGY 



283 



the same way as aerial shade plants. The colour of the plant would 

 therefore seem to act in a purely physical fashion and not in any 

 physiological manner, an interpretation that has also been sup- 

 ported by Seybold (1934) (cf. p. 293). It must, however, be 

 remembered that depth studies cannot yield valid conclusions on 

 the role of pigments unless the spectral composition of the light has 

 been determined in order that an adequate comparison can be made 

 with the behaviour of green algae under similar conditions. 



Phaeophyceae 



A study by Stocker and Holdheide (1937) of the assimilation of 

 the principal brown fucoids which zone our shores (Fiicus platy- 

 carpuSy F. vesiculosus, F. serratus) when compared with that of a 



rucui bUtycA.r,bu5 



14 

 12 







-1 



100 150 



12 5 10 50 



Light intensity — in thousands of metre-candles 



Fig. 181. Assimilation of different algae in relation to hght intensity. (After 

 Stocker and Holdheide.) 



Laminaria, a green and a red alga produced some interesting results. 

 Assimilation by all these algae decreases under very bright light, 

 and this agrees with Tshudy's results for clear days when he found 

 that the maximum assimilation did not take place at the surface 

 (cf. fig. 181). The optimum Hght intensity for assimilation was 

 found to be in the same region as that of the cormophytic land 

 plants, even though these do not usually show a drop beyond 

 50,000 lux. The optimum temperature for photosynthesis in 



