GROUP I. THALLOPHYTA : ALG.E. 



269 



In some of these plants (e.g. Fucus vesiculosus, Ascophyllum, Halidrys, 

 Cystoseira, Sargassum) there are large intercellular spaces, filled with air, 

 which project on the surface, and are known as air-bladders ; they serve as 

 floats. In Halidrys and Sargassum the air-bladders -are borne on special 

 branches. 



The sexual organs are in all cases borne in depressions of the surface 

 known as concepfades (Fig. 157). The conceptacles are commonly confined 

 to special portions of the thallus ; either to the tips of the branches (e.g. 



FIG. 166. Pucus vetieulosMS, about half nat. size : b air-bladders j / fertile branch. 



Fucus, Cystoseira) or to special branches, the gametophores (e.g. Himau- 

 thalia, Ascophyllum). From the inner surface of the conceptacle there 

 arise a number of hairs (pqraphyses) among which the sexual organs 

 are borne. The oogonia (Fig. IBS) are nearly spherical, and are borne on a 

 short stalk consisting of a single cell ; the antheridia (Fig. 158) are the 

 lateral branches of some of the hairs. The plants may be monoecious 

 (e.g. Fucus piatgcarptu, Halidrys, Pelvetia, Cystoseira), or dioecious (e.g. 

 Himanthalia, Ascophyllum, Fucus vesiculosus and serratus); in the former 

 case each conceptacle contains both antheridia and oogonia. 



