D2 
to cause a pulse of development of Fragilaria, for nitrates are 
abundant when the diatom declines and is at its minimum. It 
does not seem possible to find in the unstable environment of this 
organism any external factor which shows a causal connection with 
its periods of growth. 
Apstein (’96) found that this diatom reached its major pulse 
in March and April in Dobersdorfer See, and a minor one in 
November. 
The cells of this diatom form long twisted bands, visible to the 
unaided eye. They reach a much greater length in this species 
than in the preceding one, and are longest during the height of the 
growing period, decreasing rapidly in length as it declines. The 
average number of cells in a ribbon at the time of the maximum lies 
between 150 and 200, and at other times is usually below 100 and 
often below 25. 
The vernal pulse of this species coincides with that of F’. croto- 
nensis, and appears either with or just after the volumetric pulse. 
The December pulses may in part serve as primal food sources for 
the fairly constant minor volumetric pulse of December. 
Gomphonema constrictum Ehrbg.*—Average number, 501,923. 
This species appears irregularly, with a predominance of occur- 
rences in May and November, and is apparently adventitious. 
Melostra granulata (Ehrbg.) Ralfs var. spinosa Schroder. 
—Average number of cells, 1,181,125 (filter-paper, 34,762,365). 
In 1897 it was more than five times as abundant. “Wigage 
filter-paper collections as a whole it is about fifty times as abun- 
dant as in those of the silk net. A much greater proportion 
of single cells and short filaments occurs in the latter collections, 
since the longer filaments are the more readily retained by the silk. 
In the discussion which follows, the data from the silk collections 
will be used, since they cover the whole period. The data from 
the filter-paper collections indicate very nearly the same seasonal 
routine, and the differences between the results by the two methods 
lie in the proportions of the numbers rather than in the direction of 
movement in the fluctuations. The pictures of the seasonal 
changes in occurrence of the diatom given by the two methods are 
essentially alike aside from greater irregularity during minimum 
periods, resulting from the larger margin of error in the filter-paper 
method as I used it. 
