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INTRODUCTION TO PLANT GEOGRAPHY 



[chap. 



shallow enough for colonization by other plants which, through long 

 and flexible upgrowing stems and/or leaf-stalks, enable their leaves 

 to reach the surface of the water, so constituting the ' floating-leaf ' 

 stage and crowding out most of the submerged aquatics. Examples 

 of such plants with floating leaves are Potamogeton natans and some 

 other Pondweeds, some Bur-reeds [Sparganium spp.), and most 

 members of the Water-lily family (Nymphaeaceae). In a sense also 

 belonging here is the detached pleuston of Duckweeds {Lemna 



Fig. 169. — Leaves of Sacred Lotus {Nelumbo nucifera) projecting out of the water, 

 and Pistia stratiotes floating on the water, in the Phihppine Islands. 



spp.), Pistia stratiotes, Water-hyacinth {Eichhornia crassipes), Frog- 

 bit {Hydrocharis morsus-ranae), Water Buttercups (Ranunculus spp.), 

 etc., which frequently help to consolidate the surface vegetation of 

 this stage. Fig. 169 shows a dense community really belonging to 

 this stage although the leaves of the dominant Sacred Lotus (Nelumbo 

 nucifera) project out of the water. Floating freely on the surface 

 are abundant plants oi Pistia (cf. Fig. 164, B). Such prolific aquatic 

 vegetation markedly reduces light-penetration and the turbulence of 

 the water {cf. also Fig. 164, C). Various Podostemaceae are par- 

 ticularly characteristic of running warm waters, the vegetation of 

 which may be separated on this and other grounds. 



