56 



in September in 1898; in November in 1896, 1897, and 1898; and in 

 December in 1894. 



The optimum temperatures, omitting the pulse of August 30, 

 1897, at 80, all lie below 72, averaging 65 for the vernal pulse 

 and 62 for the autumnal. But three pulses in all, exceeding 

 100,000, lie at temperatures above 70, and but three below 50. 

 In the case of this species likewise temperatures seem to be potent 

 factors in limiting its seasonal occurrence. The fluctuations in 

 nitrates do not seem to bear any constant relation to its develop- 

 ment. The midsummer minimum of the diatom may appear, as 

 in 1896, during an abundance of nitrates (0.5 to 3.0 parts per mil- 

 lion Pt. I., PI. XLIII.) unusual for the season. On the other hand, 

 a minimum of nitrates (.1 to .35) in August and December, 1898, 

 coincides with a suppression of this species in the plankton. Thus 

 in the presence of food, temperature seems to be a determining 

 factor in the seasonal distribution of this organism. Whipple ('94) 

 expresses the opinion that the growth of diatoms occurs at those 

 seasons of the year when the water is in vertical circulation ; that is, 

 when it passes 39.2. In our waters this generally occurs early in 

 March and late in November. In this species the only pulses 

 which it seems might exhibit the effect of this phenomenon are 

 those of December and March, and neither of them are in any way 

 constant or prominent. Neither of the major pulses, vernal nor 

 autumnal, can be attributed to it. The latter pulse occurs prior 

 to the autumnal overturning of the water. 



The vernal pulse usually follows the spring volumetric maxi- 

 mum, and the autumnal one generally appears during a volumetric 

 minimum. No immediate quantitative effect of this species upon 

 the plankton is apparent. 



In European waters this is a common planktont, and Apstein 

 ('96) reports vernal maxima in March, April, and May, and an 

 autumnal one of minor value in November. 



The number of cells in the filaments varies from one to sixty, 

 and in filter-paper collections averages four, while in the silk 

 catches it varies from seven to fifteen from year to year. The fila- 

 ments average somewhat longer during the periods of maximum 

 growth, reaching twelve to twenty-five. This species also occasion- 

 ally bears the flagellates found upon M. granulata var. spinosa, but 

 not in such abundance. It is quantitatively much less important 



