ZONING OF SEAWEEDS 75 



in the branches, which occur mostly in pairs in various 

 parts of the fronds. The length varies from 2 or 3 inches 

 to several feet. It is obvious that the air-vessels serve 

 as floats. We cross the zone of Bladder- Wrack, and at 



Fig. 24. — Knobbed-Wrack Fig. 25. — Notched-Wrack (Fucus 

 (ascophyllum nodosum). serratus). 



once encounter Knobbed-Wrack {Ascophyllum nodosum. 

 Fig. 24), a tough, leathery, olive-green species, varying 

 from 2 to 6 feet in length, the thallus consisting of 

 narrow, branching straps, several of which arise from 

 the one base. It bears large, oblong air-vessels, due to 



