ECOLOGY 



77 



which grow entirely submersed either in the sea, in fresh water 

 streams or in quiet ponds. The most striking pecuHarities 

 common to plants Hving under such conditions is the almost 

 complete absence of mechanical supporting tissue. Almost 

 without exception, submersed aquatics are not rigid enough 

 to support their own weight when taken from the water. 

 Obviously the buoyancy of the water makes such a highly 

 developed supporting system superfluous. 



185. Some aquatics utilize the buoyancy of the water for 

 support by specialized bladder-like floats which represent 

 modified leaf blades, petioles, or other organs. Very generally, 

 also, the tissues of such plants contain extensive systems of 

 passages filled with air. These serve not only to aerate the 

 tissues, but at the same time act as floats. 



186. Other characters common to plants of this class are the 

 undeveloped condition of the root system, which usually serves 

 only as a hold-fast, and the absence of root hairs. The absorp- 

 tion of water is carried on chiefly by the epidermis of the stem 

 and leaves. For the epidermis^ not being exposed to the dry 

 air, is not cutinized and, consequently, is in condition to serve 

 the function of water absorption. 



187. The leaves of submersed aquatics are commonly very 

 narrow or finely divided. This oft'ers several advantages under 

 the conditions; the ratio of absorbing surface is increased, 

 mutual shading lessened and there is less resistance to currents 

 of water which would tend to dismember the plant. Besides, 

 in a submersed plant, there would be no apparent advantage 

 offered by a broad leaf over an equal expanse of narrow leaves. 



188. Plant surfaces continually in contact with water have 

 no stomata, hence the gases absorbed in the case of submersed 

 aquatics are taken from the water by osmose. 



189. In marked contrast with the finely divided leaves of 

 submersed plants are the broad leaves of the floating aquatics. 

 The under surface of leaves of this type is destitute of stomata, 



