I 



V VII IX xi| i 



1947 ' 



1 1 — r 



V VII IX 



1948 



Fig. 223 Annual variation of alginic acid (top) and mannitol 

 (bottom) in Laminaria digitata and L. saccharina. Top: A — L. 

 digitata (loch plants), B — the same (open sea), C-L. saccharina. 

 Bottom: A — L. digitata (open sea), B — the same (loch plants). 



(After Black.) 



the littoral rockweeds (Fucales) and sub-littoral oarweeds (Lamin- 

 ariales) of Europe. Regular analyses of plant populations have been 

 made so that the results represent changes in the average composi- 

 tion rather than those in a single individual. During the period of 

 rapid grov^h (spring) fresh weight, ash, protein content and alginic 

 acid increase v^hilst laminarin and manitol behave in a reverse 

 fashion (Fig. 222). The summer decrease in growth is correlated 

 with the decrease of PO4 and NO3 in the sea at that time. During 

 this period, however, the high photosynthetic rate permits the ac- 

 cumulation of mannitol and laminarin (Fig. 223). The degree of the 

 variations depends upon habitat, especially for Laminaria digitata 

 and L. saccharina, being least in the open sea and greatest in lochs 



423 



