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 XLIII.-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 Asterionella to the number of 10,000,000,000 per cubic 

 meter yield but .079 parts per million of organic nitrogen. The nitrates 

 in our waters rarely fall below. 25 parts permillion, which, with the other 

 forms of nitrogen that may be available, would seem to afford nurture 

 not only for Asterionella 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 Asterionella to the number 

 per cubic meter above quoted. As was shown in Pt. 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 Astenonella to a maximum 

 twelve times as great as any recorded in our plankton collections. 

 This also occurs at a season when Asterionella 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 Asterionella 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 Asterionella 

 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 



