130 THE PLANTS 



The Bottom is another factor of great influence. A 

 rocky and protected shore is a favourite haunt of 

 algae, though many species prefer the high surf of the 

 exposed shore. A rocky coast with many tide-pools 

 filled with fresh sea water at every flood, and lying 

 exposed at ebb tide, is a wonderful place for algae. 

 Collectors should bear this in mind when studying sea 

 charts in search for a fit place where they may expect 

 algae. 



A sandy bottom is poor in algae, on account of the 

 shifting of the sand by the movement of the water, 

 and only species with long hair-like rootlets can con- 

 trive to live in it. For the same reason, algae avoid 

 places covered with fine mud. If, however, a bit of 

 stone or wood rises out of the mud, it will soon be 

 covered by algae. 



Salinity. Sea water contains, on the average, in the 

 open sea from 3-5 to 4 per cent, of salts ; but on ap- 

 proaching the coast this percentage decreases in 

 different degrees, according to the quantity of fresh 

 water brought, by land drainage, into the sea. Marine 

 algae flourish luxuriantly in this medium, but if the 

 percentage of salt falls considerably for instance, 

 when big streams pour their fresh water into inland 

 seas (Black Sea, Baltic Sea, White Sea), and the water 

 becomes brackish true marine algae grow scarce. 

 The transition from a rich algal vegetation to a poor one 

 is, however, gradual, and such inland seas or big estu- 

 aries are interesting spots for investigation. Marine 

 algae have an optimum, maximum, and minimum limit, 

 with regard to salinity as well as with regard to light, 

 and those limits lie for some species far asunder. 



The Movement of the water influences also the growth 

 of algae. Some like places where the tidal waves rush 

 along the coast ; others prefer exposure to the high 



