298 
of the initial stages of the rise in vernal production indicated 
in later years, especially in 1896 and 1898. 
The two April collections average 3.18 cm.’—about 37 per 
cent. below the mean for this month in the three subsequent 
years. An inspection of the vernal plankton curves for this 
and subsequent years at this and other stations (Pl. [X.-XIL., 
XXVI.-XXXIV., XXXVI.-XLIT.) reveals the presence of a ver- 
nal volumetric pulse* of plankton, which, as a rule, marks the 
maximum period of production in the year, and follows imme- 
diately upon the vernal rise in temperature. 
In 1895 our collections were too infrequent to detect the 
location and extent of this vernal pulse. In other years, as 
seen in the plates to which reference is made above, the best- 
defined vernal pulses appear in the closing days of April and 
the first week of May. From the character of the best delineat- 
ed vernal pulses—e. g. those of 1896 and 1898 (Pl. X. and XII.) 
—it is probable that the apex or crest of the pulse is narrow, 
that is, the maximum development lasts but a few days. If 
this be the case, our two collections in April may miss entirely 
the period of culmination. The second collection, upon the 
29th, would appear to be located at the probable season (tem- 
perature?) to detect the maximum development. If this be the 
case the vernal development of 1895 is much reduced, and might 
be correlated with the suppression of overflow stages and con- 
sequent reduction of contributions from the impounded back- 
waters. ‘T'wo facts lead me to think that two well-developed 
vernal maxima may have been present in 1895. First, a com- 
parison of the vernal pulses of 1896 (Pl. X.) and 1898 (PI. XII.) 
indicates that the pulse of the former year culminates about 
*I use the term plankton “pulse” to designate the phenomenon of a periodic in- 
crease of the plankton volumetrically, as a-whole, from a minimum to a maximum, 
followed by a decline to another minimum, the rise and fall being more or less 
gradual, and the data forming when plotted a more or less symmetrical curve, re- 
sembling that known as the “probability of error” curve. A typical example of this 
phenomenon and resulting curve is seen in the case of the April-May plankton of 
1898 (Pl. XII.). * 
The term may also be applied to a periodic increase in individual members of the 
plankton similar in its graphic delineation to that of volumetric changes. The 
pulses will be designated by the months in which the major part of their course is run. 
