SEXUAL REPRODUCTION (FUCUS) 225 



septa. The antheridia are oval cells, which likewise possess 

 relatively thick walls and occupy the ends of most of the short 

 lower segments of richly branched hairs (Fig. 120, I).i In each 

 are formed numerous minute bicihate spermatozoids (Fig. 120, L), 

 containing a well-marked nucleus, but only traces of a chloroplast. 

 The mature antheridia have a yellowish colour, which they impart 

 to the entire conceptacle, and by this means, in the dicecious 

 species, the male plants can be distinguished. 





Fig. 121. — The Bladder Wrack [Fiiciis vesictilosus). A, Transverse section 

 through a fertile conceptacle containing oogonia, in different stages 

 of development. B, Small part of a transverse section through the 

 thallus, more highly magnified, a., assimilating laj'er ; m., medulla ; 

 s., storage cells ; w., wall of conceptacle. 



WTien the se.xual cells are ripe, the outermost layer of the 

 antheridium or oogonium, as the case may be, breaks open and 

 sets free the contents which remain enclosed in the inner part 

 of the wall. The gradual extrusion of the packets of ova, or 

 spermatozoids, from the opening of the conceptacle often occurs 

 between the tides, and is probably largely brought about by 

 expansion of the mucilage, secreted by the hairs, combined with 



' These antheridial hairs are best examined by teasing out the contents 

 of a male conceptacle in a drop of water. 



15 



