35 



is at a lower level than the summer one. In. 1894 the interval of 

 collection is too great to follow the seasonal distribution, but there 

 are hints of summer and autumnal pulses. In 1896 there were no 

 May collections, and the largest number, 6,060,665, appears June 

 19, five minor pulses on July 18, August 21, September F2, October 

 11, and November 5 intervening before the hiemal pulse of 3,574,- 

 028 appears on November 27. Other pulses follow on December 18, 

 January 6, February 4, March 4, and March 17, before the vernal 

 pulse of 1896 culminates at 105,440,858 on April 24. This is fol- 

 lowed by minor pulses on May 18, June ] 1, July 18, August 8, and 

 September 16, and by the hiemal pulse of December 3 of 346,982,- 

 928*. The vernal pulse of 1897 appears April 27 at 6,207,473,520, 

 but is surpassed by a pulse on July 14 principally of Melosira 

 spinosa of 11,459,289,600, and minor pulses then follow on August 

 17, September 29, October 26, and December 7 and 21. The hie- 

 mal pulse of this year is insignificant. In 1898 three minor pulses 

 appear, January 21, February 15, and March 22, and the vernal 

 pulse culminates May 10 at 3,865,257,360. Minor pulses follow on 

 June 14, July 19, August 9, August 30, September 27, October 25, 

 and November 22, and the hiemal pulse culminates December 15 at 

 436,535,790, followed in 1899 by minor ones on January 10, Febru- 

 ary 14, and March 14. 



Some of the pulses here indicated are due to the development of 

 single species, as that of Melosira on July 14, 1897. Most of them, 

 however, are composite, including a number of species. This is 

 especially true of the vernal pulse, w T hich in 1898 is due to the com- 

 bined increase in Fragilaria virescens and F. crotonensis, Cydotella, 

 Asterionella, Navicula spp., and Synedra acus. Asterionella culmi- 

 nates early in the vernal pulse and the majority of the others 

 towards its close. Melosira varians is among these, but M. 

 spinosa contributes less' to this pulse than it does to later ones. 

 Minor pulses are also composite, as, for example, that of August 9, 

 1898, which is due to Melosira spinosa, Cydotella, and Navicula. 



FACTORS CONTROLLING DIATOM PRODUCTION. 



The fact that many of these pulses represent the combined 

 fluctuations of a number of species leads us to look for some factor 



* Filter-paper collections included in this and in following years. 

 (4) 



