CH. n] THE BEACH AND ROCKS 13 



extending below the low-tide mark far into the 

 deeper w r ater. Such fixed surfaces, except where 

 they may be exposed to the grinding action of the 

 sand washed past by the movement of the waves, 

 are found covered by seaweeds of various forms 

 and colours. These constitute a vegetation of 

 Algae, widely different from that of the land. Their 

 characters of outline, of function, and of propagation 

 are peculiarly their own. 



The spread of this characteristic Flora of the sea 

 is limited upwards by the high-tide mark, and 

 though some Algae appear content with a short 

 immersion at high water, they are all dependent 

 upon access to sea-water at intervals, while some 

 require to be constantly submerged. Some straggle 

 up estuaries and river-mouths till the brackish water 

 is reached, but do not thrive in fresh water. On the 

 other hand certain genera, such as Batrachospermum 

 and Lemanea, allied to the Red Seaweeds by their 

 structural and propagative characters, are common 

 in freshwater streams but do not occur in the 

 sea. These plants are green in colour, and might 

 paradoxically be described as green freshwater 

 red seaweeds. Passing through the zone of sea- 

 weeds downwards to the low-tide mark and beyond 

 it, it becomes a point of interest to enquire what 

 are the limits of depth to which this Algal Flora 

 may extend. It appears that at about 150 feet 



