The Lower Plants 61 



times these little floating islands are veritable zo- 

 ological gardens, as a great variety of small marine 

 animals, sometimes including fish, seek shelter 

 among the fronds of the kelp. 



The brown algse are preeminently marine plants. 

 Very rarely do they invade fresh water, and then 

 only in the immediate vicinity of the sea. A large 

 number of them are plants living between tide 

 marks, and they show very perfect adaptation to 

 this condition. Their leathery, gelatinous fronds 

 retain the water very tenaciously, and they may be 

 exposed to the air for a long time without injury. 

 Indeed in the far northern regions like Alaska, 

 where as a rule the air is cool and moist, they often 

 grow so near the high tide mark as to be out of the 

 water for the greater part of the time. It is sup- 

 posed that the brown pigment with which they are 

 provided is a protection against too great illumina- 

 tion when they are uncovered by the receding tide. 



These large brown algse sometimes show a 

 marked degree of specialization. The massive, 

 tough, and leathery plants often develop structures 

 resembling the stem, root, and leaves of the higher 

 plants. As they very often grow where they are 

 exposed to the full force of the ocean surf, they are 

 provided with powerful root-like organs or hold- 

 fasts, by which they cling tenaciously to the rocks. 

 Thin flat leaf-like organs are of common occurrence, 

 and in connection with these there may be floats, 

 or air vesicles, which buoy up the leaves and keep 



