46 
declining floods about the end of April at about 60°. It can not be 
temperature which limits the occurrence of the species, for this 
apparent optimum recurs again in October. This is the period of 
declining nitrates (Pt. I., Pl. XLITI.—XLV.), but they rise again in 
the autumn, and in our sewage-fed waters they contain even in the 
midsummer minimum a quantity adequate to support an abundant 
growth of Asterionella. Whipple and Jackson (99) have found on 
analysis that Astertonella to the number of 10,000,000,000 per cubic 
meter yield but .079 parts per millionof organic nitrogen. The nitrates 
inour watersrarely fall below .25 parts per million, which, with the other 
forms of nitrogen that may be available, would seem to afford nurture 
not only for Astertonella but also for competing organisms. These 
authors have also found that silica to the amount of 1.78 and manganic 
oxide to .03 per million are contained in Astertonella to the number 
per cubic meter above quoted. As was shown in Ft. I., p. 234, the 
silica is present in great excess (26 to 81 parts),and the manganic 
oxide, though not reported in the analyses of November waters, is 
present on June 15 to the amount of .07 parts per million—more than 
double the amount required to support Asterionella to a maximum 
twelve times as great as any recorded in our plankton collections. 
This also occurs at a season when Astertonella is usually declining 
rapidly in numbers. Such chemical data as are available thus afford 
us no explanation of the limitation of Asterionella in our waters to 
the vernal pulse alone. 
Some evidence bearing on a factor which may be operative in 
producing this phenomenon is to be found in the hydrographic con- 
ditions attending the vernal pulse. As previously noted, this 
appears each year with the decline of the spring flood. A repetition 
of the overflow in 1898 at the end of May brought with it a repetition 
of the vernal pulse of Astertonella in early June. With the decline 
of the flood the backwaters make their major contribution to the 
channel plankton, and it is during this period that Astertonella 
reaches its maximum and also declines. If the spring flood is sup- 
pressed, as in 1895 and 1896, the spring pulse of Asterionella is cor- 
respondingly feeble. The environmental conditions are thus more 
favorable in the impounded backwaters than in the main stream. 
Whipple and Jackson (99) have noted in frustules of this diatom 
the appearance of structures which they interpret as spores. If these 
are spores, and if the sedimentation of spore-bearing frustules occurs 
