54 



XLIII.-L.). Other diatoms, as in the hiemal pulse of Fragilaria, 

 develop in numbers at temperatures approaching 32, but not M. 

 granulata var. spinosa. Whipple ('94) concludes from the records of 

 examinations of potable waters in Massachusetts that temperature 

 has possibly a slight influence on the growth of diatoms, but that it is 

 of so little importance that it does not affect their seasonal distribu- 

 tion ; and, on the other hand, that a sufficient supply of nitrates is one 

 of the most important conditions for their growth. The seasonal dis- 

 tribution of Melosira was not separately discussed in his paper 

 though included in his general statements. In our waters the data 

 at hand seem to show conclusively that abundance of nitrates is of no 

 avail in the case of Melosira when the temperature falls below 60. 

 There are times, therefore, in the case of this, our most important 

 diatom, when temperature is more potent than food as a factor con- 

 trolling its growth. 



Melosira does not appear in its maximum pulses at the time of the 

 major volumetric pulse of the total plankton of April-May, nor do 

 its fluctuations seem to bring about directly any considerable 

 changes in the volume of the plankton. For example, the extreme 

 pulse of 111 ,456,000 on July 21, 1897, occurs at the time of a sudden 

 drop in the amount of plankton (Pt. L, PL XL). The amount of 

 plankton on July 14, 21, and 30 is 8.16, 0.92, and 1.05 cm. 3 per m. 3 , 

 and the corresponding numbers of Melosira are 66,528,000, 111,456,- 

 000, and 13,176,000. 



The diatoms here discussed are predominantly of the type 

 designated as var. spinosa, marked by the spinous prolongations 

 from the valves at the ends of the filaments. The cells of the forms 

 in our plankton are proportionately much longer, as a rule, than 

 those figured by Schroder ('97), usually attaining one and a half to 

 two times the length without proportional increase in diameter. 

 Not infrequently in the height of the growing season much elongated 

 and curved cells and filaments are to be found. In one instance an 

 unusual number of filaments approaching M. varians in form though 

 still, of the spinous type were found. It is not improbable that 

 several so-called species of Melosira have been included with this 

 variable species in the enumeration. 



Melosira is the bearer of numerous passive planktonts, the most 

 abundant of which is Bicosceca lacustris Clk. Associated with this, 

 and often on the same filament, is the elegant little craspemonad 



