FUCUS (THALLUS) 



201 



Tlie slight notch at the tip of each branch of the frond harbours 

 an apical cell by whose divisions growth is ei^ected. The paired 

 air-bladders (Fig. io8, a.b.), which appear as occasional large 

 swellings on either side of the midrib, and to which the specific 



Fig. io8. — Fiicus. On the left, part of a thallus of the Bladder Wrack 

 (F. vesictilosiis) ; on the right, of the Serrated Wrack {F. serratus). 

 a.b., air-bladder ; /., fertile conceptacles ; m., midrib of thallus ; 

 5., sterile conceptacles. 



name is due, increase the buoyancy of the plant when submerged, 

 and may also, like the air-canals of aquatic Phanerogams, serve 

 for purposes of respiration. 



Scattered irregularly over the whole surface of the frond are 

 slightly protruding dots (Fig. io8, s.) marking the positions of 



