LECTURE XIV. 



Fucus or sea-wrack is one of the best-known sea-weeds. It con- 

 tributes more than perhaps any other sea-weed to cover the half- 

 tide rocks of our coasts. It has an olive-brown colour. When 

 examined it is seen to be composed of a small disc-shaped sucker, 

 by means of which it is attached to the rock, and a cord-like stem 

 which supports a flat branching portion. The bifurcations of this 

 portion are in one plane and the branches are flattened in the 

 same plane. The stem appears to be continued up into the 

 flattened portion or frond as a cord-like mid-rib which may be 

 traced upwards almost to the extreme tips. The mid-rib bifurcates 

 with the branching of the fronds. The surface of the plant is 

 slippery and mucilaginous to the touch and the edge of the frond 

 is more or less notched. Some kinds are possessed of bladders 

 filled with gas formed in the tissue of the fronds at irregular inter- 

 vals along the mid-rib; and often the ends of the fronds are 

 swollen into cushion-like enlargements. The general surface of 

 the plant is smooth but it is studded over with what appear to be 

 small warts when the plant is out of water. These structures are 

 most noticeable and most numerous on the swollen ends of the 

 branches. When the plant is submerged it is easily seen that the 

 wart-like bodies are tufts of fine hairs which issue from the top 

 of a slight prominence. 



If the frond of Fucus is cut across it will be seen that it is 

 composed of two distinct layers. The outer or cortical layer, 

 which is comparatively tough and is coloured brown, and an 

 inner medullary substance which is transparent and almost colour- 

 less and has a mucilaginous consistency. 



If a thin transverse section of the frond be prepared and ex- 

 amined microscopically we shall see that the cortex is composed 

 of several layers of cells coloured a golden olive-brown. The 

 outer ones, which form a continuous coating covering over the 

 whole frond, appear square in outline in the transverse section. 

 Their walls are thin except those which form the outer surface of 

 the plant, and these outer walls together form a sheet which may 



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