124 THE METABOLISM OF ALGAE 



Laminaria saccharina, this alga has been selected for de- 

 tailed consideration here. 



Lamiimria saccJiarina, which grows at and just below low 

 tide level, consists of a broad ribbon-like frond arising from 

 a stipe terminating in the holdfast by means of which it is 

 attached to the substratum. This plant is the sporophyte; 

 the microscopic gametophyte generation with which it 

 alternates need not concern us here. Sporophytes are pro- 

 duced at all times of the year but generally it is only spring 

 sporophytes which persist until maturity. On the British 

 coast their life span does not exceed three years. Growth of 

 the plant is continuous throughout life but shows seasonal 

 variation, being most rapid from January to June. Gro\\th 

 also changes with the age of the sporophyte, the maximum 

 both in rate and total amount being reached during the 

 second period of rapid growth. Tissue is worn away con- 

 tinuously from the distal end of the frond so that normally 

 the age of the oldest frond tissue is not more than 5 to 

 7 months. Reproductive tissue develops when the plants 

 are from 8 to 12 months old.^'^^ 



Analyses of L. saccharina have been made using samples 

 taken from naturally occurring populations and including 

 plants of all ages. The changes which can be followed are 

 thus seasonal ones in average chemical composition rather 

 than those which occur in the life cycle of the individual 

 plant. During the period of rapid growth both the mean 

 fresh weights and mean total protein content of the plants 

 increase in spite of the continuous wearing away of the 

 fronds. Protein content expressed as a percentage of dry 

 weight is highest at the beginning of this phase (Fig. 19) 

 but falls progressively as the nitrate in the seawater becomes 

 exhausted. The contents of mannitol and laminarin show 

 reciprocal changes to those of protein and, in general, 

 laminarin is almost completely absent when growth is most 

 rapid. There is a decrease in growth rate in June to July, 

 coinciding with the minimum of phosphate and nitrate 

 concentrations in the seawater, and loss of material from 

 the fronds then equals or exceeds the gains. Photosynthesis 

 continues at a high rate for about two months after this 



