236 



With less nitrogen we may expect to find less plankton. The 

 contrast of Illinois and Spoon rivers shows the same tendency, 

 though the difference in the chemical conditions in the streams 

 is less than that in the lakes, while the contrast in plankton 

 production is much greater. Likewise in seasonal changes, the 

 greatest developments of the plankton — the spring maxima — 

 appear at the close of a period of high nitrogen content. 



On the other hand, precise comparisons and correlations 

 cannot be maintained, in part because of the operation of other 

 factors, — such as temperature and vegetation, — and in part be- 

 cause of the fundamental difference between chemical and 

 biological phenomena. The fertility of a body of water must 

 be judged, not by chemical analyses only, but in conjunction 

 with other phenomena which condition growth and reproduc- 

 tion. It is also evident that isolated chemical analyses throw 

 as little light upon the fertility of a body of water as isolated 

 plankton examinations do upon its productiveness. The routine 

 of seasonal changes must be discovered in both before trust- 

 worthy data for estimation of fertility can be obtained. 



Vegetation. 



The aquatic environment at Havana impresses the visiting 

 biologist who for the first time traverses its river, lakes, and 

 marshes, as one of exceedingly abundant vegetation, indeed al- 

 most tropic in its luxuriance. The aquatic flora of the ponds, 

 lakes, and streams of New England, of the Middle States, and of 

 the north central region is, as a rule, but sparse in comparison 

 with that which here constantly meets his eyes. He will note 

 the entire absence of beds of Chara and patches of Nitella, and 

 will find the Potamogetons fewer both in species and numbers- 

 The shore-loving Juncacece, Cyperacea?, and Equisetums are also 

 less in evidence, for here the shore itself is a shifting region, 

 lacking the permanence which these plants demand. On the 

 other hand he will find acres upon acres of "moss," as the fish- 

 ermen call it — a dense mat of mingled Ceratophyllum and Elodea 

 choking many of the lakes from shore to shore, and rendering 



