CHAPTER LVI. 



AQUATIC PLANT SOCIETIES. 



I. General Considerations. 



IN this chapter are treated the vegetation forms which are for 

 the most part submerged. Some float on the water with their 

 upper surface exposed. In others the leaves float on the water, 

 while in still others the leaves may be lifted considerably above 

 the surface. They differ from the plants of swamp societies in 

 that the plants of the latter have all or a large part of their body 

 out of the water, and are usually supplied with mechanical tissue 

 for self-support. (These semi-immersed plants are sometimes 

 called semi-aquatics.) 



1107. General characters of aquatic plants (see also Chapter 

 XLVII, Hydrophytic structures). A typical aquatic plant has 

 at best only a slight development of mechanical tissue; it is sup- 

 ported by the water; it is provided with air by large air-spaces 

 throughout the tissue; all or nearly all parts being surrounded 

 by water, absorption takes place directly and there is little or no 

 development of root-hairs, and only a feeble development of 

 fibrovascular tissue; where roots are developed they serve chiefly 

 as holdfasts. Water-plant societies grade into swamp societies 

 and these into the vegetation of soil shores. In fact all three 

 types may be present in the same pond or lake or shore, shallow 

 water, or deep-water vegetation. 



1108. Two general divisions of aquatic plants. In studying 

 plant societies growing in water it is convenient to divide them 



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