THE BEOWN SEAWEEDS. 



43 



the plants are completely exposed as the tide recedes. The 

 commonest species, F. vesiculosus (Fig. 26, A), is distinguished 

 by the air sacs with which the stems are provided. The plant 

 is attached to the rock by means of a sort of disc or root from 

 which springs a stem of tough, leathery texture, and forking 

 regularly at intervals, so that the ultimate branches are very 



/an. 



FIG. 26. A, a branch of common rock weed (Fucus} , one-half natural size, x, 

 end of a branch bearing conceptacles. B, section through a conceptacle con- 

 taining oogonia (og.}, x 25. C, E, successive stages in the development of 

 the oogonium, x 150. F, G, antheridia. In G, one of the antheridia has dis- 

 charged the mass of spermatozoids (an.), x 150. 



numerous, and the plant may reach a length of a metre or 

 more. The branches are flattened and leaf-like, the centre 

 traversed by a thickened midrib. The end of the growing 

 branches is occupied by a transversely elongated pit or de- 

 pression. The growing point is at the bottom of this pit, 

 and by a regular forking of the growing point the symmetrical 



