CHAPTER VII 

 THE LARGER AQUATIC VEGETATION 



BY RAYMOND H. POND 



Late Professor of Botany, Texas Agricultural College 



NEARLY all of the larger plants which have distinct roots, stems, 

 and leaves grow attached to the muddy substratum. This habit 

 of the larger plants to grow as attached organisms is so universal 

 that it can hardly be regarded as an accident and it is reasonable 

 to suppose that such attachment offers some advantage to the 

 organism. Even the simple filamentous algae are often attached. 



When a plant is floating free any portion of it may be exposed 

 to the surface light, or to the air, because the water movements 

 may turn its body in any direction and such a plant is better off 

 without specialized organs which would be destroyed by exposure. 

 It is common to see drifting plants which are dying rapidly be- 

 cause, among other reasons, the roots are exposed to the intense 

 light at the surface of the water. The small, free-floating forms 

 are simple in structure because no portion of the organism has a 

 distinct environment of its own and changes in position are so 

 frequent that all parts of the body are equally exposed. The 

 common duckweed, Lemna, moves with the changing currents and 

 shows a marked differentiation into an upper and a lower side. 

 Notable, however, is the fact that its movement is always in a 

 horizontal direction so that the upper side is uniformly up while 

 the lower side is down, 'with its roots in the water, and shaded by 

 the cap-like upper side. Thus it is that Ceratophyllum, which is 

 usually regarded as a dicotyledon and which certainly occupies a 

 much higher station in the natural system than Lemna, shows less 

 differentiation in outer structure than the latter. In the case of 

 Ceratophyllum attachment is purely accidental so far as special 

 organs for the purpose are concerned. Well-developed roots have 

 never been found on this plant although the rudiment of a root is 



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