214 
limits of the growing period of vegetation into November or 
even December. This seeming inertia in the seasonal move- 
ment of the nitrates seems to be due on the one hand to the 
gradual utilization of the accumulations of the winter and con- 
tributions of the spring floods by the spring plankton ; and on 
the other, to the slow accumulations of the autumn and to 
utilization by the autumn plankton, which, as in Thompson’s 
Lake in 1897, often attains a considerable development. 
A somewhat intimate connection between the nitrates and 
the plankton maxima can be detected in many instances in the 
diagrams. When the plankton increases, the nitrates often 
exhibit a depression, the extent of which, however, is not 
always proportionate to the change in the plankton. This 
absence of any constant ratio between the apparent changes in 
these two factors indicates the operation of other factors, one 
doubtless due to defects in the quantitative plankton method, 
and another due to changes in the component organisms of the 
plankton. 
In the Illinois River in 1895-96 (Pl. XLIII.) the plankton 
maxima of April and October are accompanied by a marked fall 
in nitrates; on the other hand, those of November, June, and 
August appear with rising nitrates, the last two accompanying 
floods. The depressions in nitrates in October, December, Feb- 
ruary, June, July, and September are not in any case associ- 
ated with arise in the plankton, though often with the initial 
stages of the flood. In 1897 (Pl. XLIV.) the April-May, July, 
September, and October maxima are all associated with de- 
pressions of the nitrates. The February and March depressions 
of nitrates occur with floods, while in November and Decem- 
ber no correlation is apparent. In 1898-99 (Pl. XLV.) the effect 
of the May, June, and July maxima can scarcely be detected 
in the nitrate curve, while those of December and March pro- 
duce corresponding depressions. In this diagram neither plank- 
ton nor nitrates show marked changes after July. 
In Spoon River the development of plankton is apparently 
so slight and the nitrates are relatively so abundant that no 
