280 BOTANY: PRINCIPLES AND PROBLEMS 



Fucus, the best known genus, is exceedingly abundant on 

 rocks between tide marks in the temperate regions. The vegeta- 

 tive body of this alga is a flat, repeatedly forking thallus well 

 provided with air bladders (Fig. 158). The swollen tips of certain 



Fig. 158. — Fucus vesiculosus. Portion of thallus showing air-bladders, 

 (a) and receptacles (r). On the surface of the latter are the many small openings 

 of the conceptaeles. 



branches are known as receptacles and bear the sexual organs. 

 Scattered over these tips and just below the surface are many 

 small chambers or conceptaeles each opening to the outside by 

 a pore. Lining the wall of these chambers are masses of branch- 



