THE LARGER AQUATIC VEGETATION 203 



order given. It must be remembered, however, that this reten- 

 tion of dissolved substance by the soil is neither absolute nor per- 

 manent. We may suppose that in the case of a lake there are two 

 opposing processes operating in which the soil on the one hand 

 tends to withdraw salts from the water and retain them, while the 

 water on the other hand tends to bring salts from the soil into 

 solution. As a consequence of those two processes the lake water 

 contains certain salts in much greater proportion than others 

 which seem to be firmly held by the soil. Just how such condi- 

 tions influence the plant is only partially known. 



It seems as though the substances needed by the plant are the 

 ones most firmly retained by the soil, — especially the nitrogen, 

 phosphorous, and potassium compounds, — and yet it is hardly 

 possible to say that the water does not contain enough of those 

 substances in solution for the larger plants. While plants cannot 

 select certain substances and reject others they can to a certain 

 extent regulate the amount of a substance which they absorb. 

 It is evident that all of the substances absolutely necessary for 

 the growth of plants must be present in the water in solution 

 because there are so many forms which Hve as freely swimming 

 objects and depend wholly upon the water in which they live for 

 food. Lemna, the so-called duckweed, Hves in the water and de- 

 pends upon the water only for food (mineral), but Lemna has 

 been analyzed and found to contain substances in much greater 

 proportion than does the water in which it grows. One investi- 

 gator found the ash of this plant to contain 13.16 per cent of potas- 

 sium, and 8.73 per cent of phosphoric acid, while the mineral resi- 

 due obtained by evaporating some of the water in which the Lemna 

 was growing contained those substances in the respective propor- 

 tions of 3.97 per cent and 2.619 P^^ cent. However, the fact that 

 Lemna can obtain all the food necessary from the water alone and 

 that some other plants must be rooted in the soil to thrive is no 

 more remarkable than the fact that some animals are herbivorous 

 while others are carnivorous. 



Water plants may be found growing in clayey, sandy, gravelly, 

 or loamy soil. From field observation one would say that the 

 loamy soil supports the greatest variety of species. Wherever the 



