PELVETIA 



203 



swollen fertile ends are a particularly conspicuous feature (Fig. 109). 

 The structure of the thallus shows no important differences, but 

 the branches of the frond are much narrower and channelled on 

 their upper surface, features which aid in the retention of mois- 

 ture ; moreover, the thallus is thus mainly illuminated by oblique 

 light, so that the heating effect of the sun, and therefore transpira- 

 tion, is presumably diminished during the hot hours of the day. 

 The principal differences exhibited by Pelvetia can therefore be 

 related to the special conditions of its habitat. 



FIG. 109. Plant of Pelvetia canaliculata, about natural size, showing the 

 basal attaching disc and the prominent fertile conceptacles. 



On rocky shores the Brown Algae Pelvetia, Fucus, and Lami- 

 naria often form three zones at successively lower levels, occurring 

 in the order named. Other members of the group are free-floating, 

 as, for example, Sargassum bacciferum (Fig. no), huge stretches 

 of which characterise the Sargasso Sea in the Mid- Atlantic. Many 

 of the larger Brown Algse are edible, and indeed extensively 

 cultivated in Japan, whilst in the West of Scotland Fucus and 

 similar forms are abundantly used as manure. 



Most of the Red Algse (Rhodophycea) are much smaller plants 

 preferring weakly illuminated habitats, so that they either grow at 

 considerable depths below low-tide level or in shady rock-pools. 



