299 



April 24 and that of the latter about May 3. The temperature 

 curves of 1896 pass 60° about sixteen days before they reach 

 that point in 1898. This may be the cause of the earlier cul- 

 mination of the vernal pulse in 1896. Now in 1895 there is a 

 suggestion in the temperature curve of an early spring, and the 

 suggestion is borne out by the records of the U. S. Weather 

 Bureau for central Illinois. The normal mean temperature 

 for Illinois as a whole in April is 51.8°. In 1896 it was 54.8°. 

 In this case we might expect to find an earlier vernal pulse 

 culminating, as in 1896, before the end of April, so that our col- 

 lection of the 29th would fall upon its decline rather than upon 

 its apex. I use the term vernal maxima advisedly, for I am 

 inclined to the view that the period from April 29 to June 19 

 witnessed a remarkable development of the plankton. The 

 reasons for this view are found, first, in the fact that the catch 

 of June 19 contains many Moina micrura, numerous males and 

 epphippial females being among them, whose presence suggests 

 the close of a period of rapid multiplication by parthenogenesis. 

 The catch of June 19, though large, may thus represent the 

 decline of a still larger population. In the second place, the 

 qualitative collections made with a tow-net in the river in the in- 

 terim between the quantitative collections of April 29 and June 

 19 indicate an exceedingly abundant plankton rich in Moina. 



From the available data in 1895 and the course of the ver- 

 nal production in other years it may be inferred with some de- 

 gree of possibility, if not indeed of probability, that the vernal 

 production in this year was accelerated by the early spring, 

 and that a pulse appeared prior to April 29, and that this was 

 followed in May-June by another pulse of much larger propor- 

 tions and longer duration, a part of which (probably the decline) 

 is detected in catches of June 19 and July 6. Of the occurrence 

 of this latter and larger pulse there is little doubt, though the 

 data are not available for its location and delineation. 



The unusual and prolonged low water of these spring 

 mouths thus seems to result in a marked increase in the plank- 

 ton content. The causes which lead to this are not far to seek. 



