30 



by the experimental method. The data here cited are suggestive 

 only; not conclusive. 



The relation of Pediastrum to the volumetric pulses of the plank- 

 ton is not a constant one, though there is some correspondence in 

 their fluctuations. The extreme maximum (3,264,800) of June 19, 



1895, is coincident with a plankton pulse of 30.42 cm. 3 , but the num- 

 ber of collections is insufficient to show the relative fluctuations of 

 the plankton and Pediastrum at that season. In May and June, 



1897, and in October, 1898, the Pediastrum pulses culminate shortly 

 after the volumetric pulses. In July and September, 1897, arid 

 in August, 1898, they coincide. 



Polyedrium trigonum Nag.* Average number, 432,692. Ap- 

 pears from June through September, disappearing when falling 

 temperatures reach 60. In 1897 it continues through October 

 with the higher temperatures (averaging 65) of that year. There 

 are slight indications of a September pulse. 



Polyedrium trigonum forma minus Reinsch and var. tetragonum 

 (Nag.) Rabh., P. bifurcatum Wille, and P. gracile Reinsch, were 

 also recorded in a few collections during the period of occurrence 

 of P. trigonum. They are all evidently summer planktonts. 



Raphidium polymorphum Fresen.* Average number, 2 1 ,450,000. 

 Occurs in every month of the year and in a majority of the collec- 

 tions. In 1897 a vernal maximum of 201,600,000 occurs on April 

 27 and an autumnal one of 28,800,000 on September 21. In 1898 

 a vernal pulse culminates May 3 at 24,000,000, and thereafter 

 throughout the summer at intervals of three to six weeks there 

 occur five other pulses, the greatest of which culminates July 19 at 

 75 ,600,000. A pulse of 90,000,000 on a declining flood in February, 

 1899, indicates an adaptation on the part of this organism to the 

 whole range of temperatures. A pulse of 25,200,000 December 3, 



1896, further illustrates this adaptability. Records in 1897 and 



1898, however, suggest that the optimum lies above 60. It is thus 

 a perennial planktont. 



Raphidium longissimum B. Schroder. Appeared sparingly in 

 February, August, October, and December, suggesting that it has 

 also a perennial distribution. 



Richteriella botryoides (Schmidle) Lemm.* Average num- 

 ber, 6,399,705 (in 1897). From May to November, with a vernal 

 pulse of 25,200,000 on May 25, and an autumnal one of 100,800,000 



