398 THE ALGAE 



assimilation of Hormosira under submerged and exposed condi- 

 tions at Auckland led to results that agreed with these earlier 

 workers (Fig. 214). 



Some work from Japan on sporeling growth in relation to Hght 

 intensity is of relevance here. Except for Monostroma, sporeling 

 growth in green algae is greatiy retarded under decreased light, 

 whilst that of red and brown algae tends to be increased. Spore- 

 lings of green algae also grew more rapidly under yellow, red or 

 blue light than under green light. Length of the Hght period may 

 also be involved. Thus it has been found that Enter omorpha spore- 

 lings grow better under long-day conditions, whereas those of 

 Monostroma grow best at first imder mediimi day length but that 

 later short days are optimal. 



There has been considerable discussion over many years in con- 

 nexion with the colours of the algae in relation to assimilation and 

 depth. Quite early Englemann put forward the hypothesis that the 

 colour of the alga is complementary to that of the incident hght. 

 Thus in the Chlorophyceae maximum assimilation takes place in 

 the red region of the spectrum, and as these wave lengths are 

 rapidly eHminated in sea water, green algae tend to grow in the 

 upper part of the Uttoral. In the Phaeophyceae, maximum absorp- 

 tion takes place in the green region and they can therefore live at 

 greater depths, and in the Rhodophyceae maximum absorption 

 takes place in the blue region. There has also been considerable 

 argument as to whether the coloured pigment merely acted as 

 a passive colour screen or whether there was active absorption 

 by the pigment. Recent work on fluorescence in chlorophyll 

 and in other pigments has led to the view that the coloured 

 pigments in Rhodophyceae and Phaeophyceae actively absorb the 

 available incident hght, expecially in the regions complementary 

 to the colour, but that the energy so absorbed is largely trans- 

 mitted to chlorophyll molecules where the actual photosynthesis 

 occurs. 



In all studies on assimilation in relation to depth with algae it is 

 important to consider not only the absorption curves of the algae, 

 but also the action spectra. The latter is the more important be- 

 cause it indicates the extent to which the different portions of the 

 absorbed spectrum are utiHzed in photosynthesis. Fig. 215 illus- 

 trates the absorption spectra of a green and red alga at different 

 depths, and it will be seen that in the Monostroma there is a rapid 



