ARTICLE I. Plankton Studies. V. 1 The Plankton of the Illi- 

 nois River, 1894-1899. Part II. Constituent Organisms and their 

 Seasonal Distribution. BY C. A. KOFOID. 



INTRODUCTION. 



This paper gives the results of a statistical study of a series of 

 quantitative plankton collections made in the channel of the Illinois 

 River near Havana, 111., at the Illinois Biological Station, in 1894- 

 1899. The environmental conditions and the volumetric results of 

 this investigation have been given in Part I. (Kofoid, '03), published 

 in Volume VI. of this Bulletin. 



Of the 235 collections made in channel waters and used in the 

 quantitative study, only 182 were subjected to numerical and 

 qualitative analysis. The omitted collections were intercalated at 

 brief intervals of one to several days between those enumerated, 

 principally in the summer of 1895 and during the winter flood of 

 1896 and the summer of the same year. The collections chosen for 

 this study, whenever possible, represent a weekly interval, and a 

 full list of all collections, with environmental data, may be found in 

 Table III. of Part I. The chronological distribution of the collec- 

 tions studied by the statistical method is given in the table on 

 the following page. 



The work of enumeration and the primary tabulation was com- 

 pleted at Urbana December 31, 1900, when my formal connection 

 with the State Laboratory ceased. The manuscript has been 



1 The four preceding numbers of this series, all by the present writer, have 

 been published as articles of the Bulletin of the Illinois State Laboratory of 

 Natural History, as follows: 



Article I., Vol. V. Plankton Studies. I. Methods and Apparatus in Use in 

 Plankton Investigations at the Biological Experiment Station of the University of 

 Illinois. 



Article V., Vol. V. Plankton Studies. II. On Pleodomna illinoisensis , a New 

 Species from the Plankton of the Illinois River. 



Article IX., Vol. V. Plankton Studies. III. On Platydorina, a New Genus 

 of the Family Volvocidce, from the Plankton of the Illinois River. 



Article II., Vol. VI. Plankton Studies. IV. The Plankton of the Illinois 

 River, 1894-1899, with Introductory Notes upon the Hydrography of the Illinois 

 River and its Basin. Part I. Quantitative Investigations and General Results. 



