THE ALGAE 



137 



Fucus 



Fiicus is a very common Brown Alga which occurs all round the British 

 coast attached to rocks (Fig. 124), and lives between high and low tide marks, 

 the various species showing distinct zonation. It is attached to the rocks by 

 means of a specialized basal disc called the holdfast. The thallus is dark 

 brown in colour or almost black when dried, and is dichotomously branched, 

 the lower part being narrow and almost round, while the upper parts are 



Fig. 124. — Pliotograph showing the growth of Ftuiis on rocks 

 of the foreshore, Ventnor, I.O.W. 



flatter and broader but with a thick midrib (Figs. 125 and 126). The plants 

 vary considerably in size according to the conditions under which they are 

 growing. If the plants are living on exposed and rather dry situations near 

 high-tide mark they are rarely more than 6 in. in length, but if living under 

 conditions of more complete immersion they may be anything up to 3 ft. in 

 length, according to their age. 



When the plants are in a reproductive condition the ends of the branches 

 become enlarged and covered with tiny wart-like projections, each of which 

 has a minute pore at the centre. These bodies mark the position of cavities 

 in the tissue of the thallus, which are called conceptacles, and in which the 

 reproductive organs are borne (Fig. 127). At the tip of each branch there 

 is a small groove, at the bottom of which lies the single four-sided apical 

 cell by means of which the thallus grows. The apical cell cuts off broad 

 segments on two sides and thinner segments on the sides at right angles 

 to the first, thus producing a flattened thallus. From time to time it also 

 divides equally into two halves, thus starting a dichotomy of the shoot. 

 Cells are also cut ofi" from the base of the apical cell and these become the 



