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 Nitell/a, and 
will find the Potamogetons fewer both in species and numbers. 
The shore-loving Juncacew, Cyperacee, 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 
