THE SARGASSO WEED. 285 



shallow water, it sunk, but where the depth of 

 water was great, the bulk of water was in itself 

 sufficient to keep the weed afloat ; the use of the 

 air vesicles is most probably intended for the pur 

 pose of bringing some portion of the plant in con- 

 tact with atmospheric air, or by keeping the plant 

 to the surface of the water, to receive a greater 

 benefit from air and light, or to prevent the young 

 shoots or other parts of the plant being injured by 

 the violent action of the waves. We find in the 

 fucus buccinalis, or trumpet weed of the Cape, that 

 stem is hollow, accommodating itself in length 

 to the depth of water in which it grows ; is 

 attached to the ground by ramified roots ; the 

 stem terminating in a croM^n of broad leaves ex- 

 panded on the surface of the water, and kept in 

 that situation by a broad air bladder, in which the 

 stem terminates under the crow^n ; it may aid the 

 plant also in maintaining itself against the force 

 of the waves in the exposed situations in wdiich it 

 is usually found. 



Air-bladders are not confined to sea weeds ; 

 several plants growing in fresh water are similarly 

 provided. Of these, the Jussioea tenella, found in 

 the rivers of Amboyna, is a remarkable example. 

 Along its stalk are many large oval tubercles, 

 full of air, and each of these is compounded of 



