410 THE ALGAE 



normally under diffuse light maximum photosynthesis takes place 

 (see p. 394). 



So far as the lower submerged limit is concerned, one may postu- 

 late that an alga will go deeper in the water the nearer its absorption 

 and action spectra approach that of the shade type (i.e. algae most 

 efficient metabohcally imder low light conditions), and the lower is 

 its compensation point. Under these conditions, for example, a 

 green shade alga would be able to go to a lower limit than a red sun 

 alga. 



Despite all that has been said in the previous pages, our know- 

 ledge of the factors controlling zonation is scanty. No real progress 

 will be made until key species are studied exhaustively. Work of 

 this nature has been commenced with Macrocystis pyrifera^ Egregia 

 laevigata and Hormosira hanksii. The last-named species occurs in 

 Australasia and an analysis of sea temperature relationships over 

 its area shows that it is found within a range of 15° C. The region 

 that it occupies on the shore also rises with decreasing latitude, but 

 the exact significance of this has yet to be worked out. Detailed 

 studies of the alga have shown that it exists in a number of eco- 

 logical forms and eventually each one of these forms will need to be 

 studied so that the vertical limits reached imder different condi- 

 tions can be understood (Fig. 121). There are several interesting 

 free-living forms found in mangrove swamps and these occupy 

 higher levels on the shore than normally do the attached counter- 

 parts on rocky shores. Shade, higher humidities during exposure 

 and lower water loss may well be responsible for this phenomenon. 

 A study of water loss that can occur during periods of exposure 

 follow the trends that have been found for other fucoids. There is, 

 however, in the form investigated, a relationship between degree of 

 hydration and rate of respiration, and it is evident that young plants 

 differ from adult ones in this respect (Fig. 210). The difference is 

 sufficient to suggest that certain metaboUc paths in the young plants 

 may be different to those in the adult. This is an aspect the impHca- 

 tions of which will require to be explored more fully. The relation- 

 ship of photosynthesis to depth and sea temperature has also been 

 studied (Fig. 220) and the effect of varying periods of desiccation 

 upon the photosynthetic rate. Once the basic facts have been est- 

 abhshed, various refinements can be added. Thus the effects may 

 vary depending upon whether high tide occurs at sunrise or whether 

 it occurs in the middle of the day. The effects will also vary in 



