THE ALG.E 



197 



CO 



In Pelvetia, then, the oogonium forms two ova which 

 are surrounded by a thick and very gelatinous cell-wall 

 showing three distinct layers (see Fig. 86). When 

 ripe, the outer layer of the oogonial wall gives way, and 

 the two ova, surrounded by the thick mucilaginous inner 

 layer of the cell-wall, are set free. 



The expulsion of the spermatozoids and ova from the 

 conceptacles generally takes place in Pelvetia when the 

 tide has gone down and left 

 the plants exposed to the air, 

 though it may also go on 

 under water. The cavity of 

 the conceptacle is full of 

 mucilage secreted by the hairs 

 which line it. The surround- 

 ing tissue presses on the full 

 conceptacle and forces out the 

 mucilaginous contents through 

 the pore ; mixed with this 

 extruded mucilage are the 

 spermatozoids and ova. If we 

 hang up Pelvetia, or some other 

 Fucaceous seaweed, freshly 

 taken from the water, we can 

 see the little slimy drops 

 appearing at the pores of the 

 conceptacles ; and these drops, 

 if examined under the micro- 

 scope, are found to contain spermatozoids or ova or both, 

 according to the dicecious or hermaphrodite character of 

 the species. In Pelvetia we should find both organs in 

 the same drop. The spermatozoids are expelled while 

 still enclosed in the inner antheridial membrane; the 



FIG. 



_ Oogonium (oo) of 

 Pelvetia, already divided to 

 form the two ova. p. para- 

 physes. To the left of the 

 oogonium some antheridia 

 are also shown. Magnified 

 about 110. (After Thuret 

 and Bornet. ) 



