76 
there are three rises to overflow stages (Pt. I., Pl. XII. and XIII.) 
with intervening declines of a month’s duration. There is a pulse 
of Dinobryon in each of these periods of declining flood. The pulse 
of 275,200 on December 20 follows the November flood, and it is 
followed by a minimum of 1,500 on the rising flood of January 10. 
The numbers slowly increase until a meteoric rise on February 7 
to 6,486,700 and on February 14 to 22,621,440 is followed again by 
another decline, to 25,920 on February 28, with the sudden flood of 
that week. During the maximum flood stage in March (Pt. I., PL. 
XIII.) no Dinobryon was recorded, but it reappeared again on 
March 21. The suspension of our plankton operations interrupted 
the further tracing of the fluctuations. 
From the facts above detailed it is very evident that the pulses 
of Dinobryon occur in channel waters at times when the run-off of 
impounded backwaters is making its greatest contribution to the 
river plankton. These are times of greatest stability of the en- 
vironment in all respects save river level and its sequences. The 
impounded waters have come from regions of slight current and 
decaying vegetation, and there has been time in those localities for 
the decay of sewage and debris, and for the growth of planktonts 
such as Dinobryon. These conditions of the environment are 
therefore favorable for the growth pulses of Dzinobryon. The 
phenomenon of pulses of growth is not, however, to be considered 
as merely the result of declining floods. These afford a favorable 
environment and doubtless determine within certain limits the 
time and the extent of the ptilse. The phenomenon is one common 
to most plankton organisms, and occurs in Dinobryon of lakes where 
floods are of little significance. 
Any evidence of recurrent minor pulses in Dinobryon at brief 
intervals is lacking. 
Dinobryon has been found in our plankton through practically 
the whole range of temperatures, but it disappears when maximum 
summer heat is reached and does not return until the water cools 
to 45° or lower. Large pulses, such as that of February 21, 1899 
(22,621,440), have developed at temperatures approximating 32°, 
and largely under the ice. The vernal pulse of April-May has been 
recorded at temperatures ranging from 60° to 79°, but generally 
nearer the former. No well-defined optimum temperature appears, 
and the seasonal distribution suggests that the high temperatures 
