The sexual reproduction takes place by oogamous fertilisation. The oogonia and antheridia are formed inside special organs (conceptacles), and are surrounded by paraphyses. The conceptacles (Fig. 70 _B_, Fig. 71 _b_) are small, pear-shaped or spherical depressions, produced by a special ingrowth of the surface cells of the thallus, and their mouths (_ostioles_) project like small warts; they are either situated near the end of the ordinary branches of the thallus (_Fucus serratus_, Fig. 71 _a_) which may be swollen on this account (_Fucus vesiculosus_, Fig. 70 _A_, _b_), or on special short branches (_Ascophyllum_, _Sargassum_). The vertical section of a conceptacle is seen in Fig. 70 _B_ (see also Fig. 71 _b_) where, in addition to the paraphyses, oogonia only are seen (_F. vesiculosus_ is diœcious--male plant, yellow-brown; female plant, olive-brown); but in some species antheridia, together with oogonia, are produced in the same conceptacle. The oogonia are large, almost spherical cells, situated on a short stalk, in each of which are formed from 1–8 (in _Fucus_, 8; in _Ascophyllum_, 4; in _Halidrys_, 1; in _Pelvetia_, 2) rounded, immotile oospheres. The wall of the oogonium ruptures, and the oospheres, still enclosed in the inner membrane, are ejected through the mouth of the conceptacle, and float about in the water, being finally set free by the bursting of the inner membrane. The antheridia are oblong cells (Fig. 70 _C_, _a_), many of which are produced on the same branched antheridiophore (Fig. 70 _C_); the numerous spermatozoids are provided with 2 cilia and are very small (Fig. 70 _D_, two antheridia surrounded by spermatozoids, one being open). The spermatozoids, still enclosed by the inner membrane of the antheridium, are similarly set free, and fertilisation takes place in the water, numerous spermatozoids collecting round the oosphere (Fig. 70 _E_), which is many times larger, and by their own motion causing it to rotate. After fertilisation, the oospore surrounds itself with a cell-wall and germinates immediately, attaching itself (Fig. 70 _F_) to some object, and by cell-division grows into a new plant. [Illustration: FIG. 70.--_Fucus vesiculosus. A_ Portion of thallus with swimming bladders (_a_) and conceptacles (_b_). _B_ Section of a female conceptacle; _h_ the mouth; _p_ the inner cavity; _s_ oogonia. _C_ Antheridiophore; _a_ antheridium; _p_ sterile cells. _D_ Antheridia out of which the spermatozoids are escaping. _E_ Fertilisation. _F_ Germinating oospore.] [Illustration: FIG. 71.--_Fucus serratus_. _a_ Portion of a male plant which has been exposed to the action of the open air for some time; small orange-yellow masses, formed by the antheridia, are seen outside the mouths of the male conceptacles (nat. size). _b_ Cross section through the end of a branch of a female plant, showing the female conceptacles (× 4).]