304 
follows: ‘The method used is that of flushing with a hose, a crew 
of men being kept constantly at work, taking them about a period 
of three weeks to cover the entire system. The water is allowed to 
run through a fire-hose at each point for a period of about ten 
minutes.”’ This system was in use during the years of our opera- 
tions, and it offers no occasion for the periodic pulses in growth of 
the organisms in question. Investigation of the discharges of dis- 
tillery and cattle-yard wastes into the stream has not revealed any 
periodic fertilization of the river waters from these sources. The 
available data thus fail to exhibit any periodic rhythm in food 
matters in solution and suspension in the river water with which 
these pulses of chlorophyll-bearing organisms might be correlated. 
Frequent reference has been made in previous pages to the 
appearance of pulses upon the decline of floods. Flood waters bring 
into the river, as shown by the chemical analyses, large quantities of 
silt and organic wastes in suspension and solution. They inundate 
great tracts of fertile territory rich in vegetation, and thus add to 
the available sources of food for the phytoplankton. Decline of the 
flood affords time for decay and solution of some of the food matters, 
and time also for breeding, and its run-off adds to the volume of the 
plankton in channel waters. A comparison of the hydrographs of 
the years in question (Part I., Pl. X.—XIII.) with these recurrent 
pulses (Pl. I.) will show that many if not most of the pulses appear 
on declining flood waters, and that many of the larger ones follow 
the major floods. Closer analysis, however, shows that there are 
sometimes two pulses of chlorophyll-bearing organisms on the 
decline of a single flood, and that they may also occur upon rising 
flood or even in its entire absence. Floods unquestionably affect 
the amplitude of the pulses, and to some extent modify their location. 
They seem inadequate, however, to explain their recurrence and 
their tendency toward a uniform interval. Minima between pulses 
also recur on declining floods. 
Energy as well as matter is necessary for the growth of the 
phytoplankton, and its source is primarily the radiant energy of 
the sun. A plot of the tri-daily air temperatures at Havana -for 
1894-1896 (Part I., p. 478, Fig. C) inclusive, exhibits many irregu- 
larities, a few of which partake of the nature of recurrent pulses at 
approximately monthly intervals, but they are too fewand tooirregu- 
lar to be the basis of the recurrent growth of the phytoplankton. 
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