194 STRUCTURAL BOTANY 



separated from the old plant, affords a simple but effective 

 means of vegetative propagation. The regular repro- 

 duction, however, of Pelvetia, in common with all other 

 Fucacese, is exclusively by the sexual method. 



2. EEPRODUCTION 



As we have seen, each receptacle or enlarged end of 

 a branch contains numerous conceptacles or cavities in 

 which the reproductive organs are placed. When ripe, 

 the conceptacle is an approximately spherical hollow, 

 communicating with the exterior by a narrow pore (see 

 Figs. 81, B, and 82). It arises as a depression on the 

 surface of the thallus, the part which is to form the 

 bottom of the hollow becoming arched over by the 

 greater growth of the surrounding tissue. The formation 

 of the cavity is, however, in some cases at any rate, 

 helped by the breaking down of a central cell, so as to 

 leave a gap in the tissue. 



In Pelvetia the conceptacle contains organs of three 

 kinds : (1) sterile hairs or paraphyses arising all over 

 the wall of the conceptacle, with their free ends con- 

 verging towards the pore (see Fig. 82); (2) branched 

 filaments, on the lower parts of which the antheridia 

 are borne ; (3) the sessile oogonia, which are placed 

 chiefly in the lower half of the conceptacle (see Fig. 

 82). This species is therefore hermaphrodite, for the 

 organs of both the sexes occur in the same conceptacle. 

 In most other Fucacese the plants are dioecious, all the 

 conceptacles of each individual containing organs of 

 the same sex. 



The antheridia arise as single cells, borne laterally in 

 small numbers near the base of the branched filaments 



