329 
conditions prevalent during this pulse (Pl. XLV.) does not re- 
veal anything peculiar to this season. The nitrogenous sub- 
stances are neither greater nor less than at other times when 
production was at a minimum. It is only evident that there 
is an overplus of these substances in which all correlation be- 
tween chemical and plankton contents of the stream is lost. 
The slight quantitative rise in nitrates (.75 to 1.1 parts per mil- 
lion) a week prior to the culmination of the plankton pulse 
(Pl. XLV.) may contribute to the increase, but similar rises in 
nitrates elsewhere are not followed by like results. We may 
therefore dismiss chemical factors—in so far as our data reveal 
them—as affording only the basis of nutrition, but neither re- 
vealing nor explaining their meteoric utilization in this remark- 
able pulse of growth and reproduction of the plankton 
organisms. 
The thermal factor peculiar to this season of the vernal 
pulse is the vernal rise in temperature. This pulse in produc- 
tion follows immediately upon it, rising with it and culminat- 
ing shortly before summer heat is reached. The vigor and ra- 
pidity with which growth and reproduction ensue in the aquatic 
world is comparable with that which we see in field and wood 
at this same season of the year. On the very days in which this 
plankton pulse culminates, the bursting buds are releasing leaf 
and flower in growth unsurpassed for rapidity during the whole 
year. The animal world, notably the insects, also begin their 
rapid multiplication at this period. The same fundamental 
causes, whatever these may be, underlie these responses of or- 
ganisms to the vernal rise in temperature both on land and 
inthe water. The prolongation and gradual ascent of the vernal 
pulse in 1898 may have made its cumulative effect much greater, 
and thus increased the amplitude of the vernal pulse beyond 
that of years of more sudden approach of spring. 
The hydrographic conditions in 1898 thus conduce to the 
presence in channel waters of an unusually abundant plankton at 
this season. Reference to Plates, XXIX., XXXIX., and XLII. 
will show that the backwaters have a greater plankton con- 
