446 
and the plankton accordingly foliows similar lines of develop- 
ment. This is noticeably prominent in the planktographs of 
the river and Quiver and Thompson’s lakes, as will be seen by 
a comparison of Plates XII., XXIX., and XXXIX. The princi- 
pal features of the common course of production are the coinci- 
dence of the May and June pulses, the subsequent low level of 
development throughout the summer and early autumn, and 
the December rise. These three bodies of water were submerged 
in the common flood of overflow in February—June, and the 
succeeding minor flushes of summer and autumn caused re- 
current ingress and egress of water from and to the channel. 
The similarity in the course of production in these three local- 
ities and the lessened differences in the amplitude of produc- 
tion in this year are in no small measure the consequence of 
this equalizing action of this interchange due to floods. 
Phelps Lake (Pl. XLII.) is the only one of our backwaters 
which diverges from this marked agreement, and its divergen- 
ces are increased by its intimate connection with Spoon River 
during high levels and its isolation during the remainder of 
the year. 
The vernal pulse of this year is noticeable for its amplitude, 
its meteroric appearance and disappearance, and its coinci- 
dence in different localities. It follows a prolonged period of 
extreme overflow, and a very gradual and somewhat tardy rise 
in vernal temperatures. It appears, moreover, at levels of 10— 
11 ft., just when great stretches of bottom-lands are contribut- 
ing their last run-off to the channel. The submergence of the 
bottom-lands did not occur until late in February in this year, 
so that the period of vernal increase in the plankton was not 
preceded by a long interval of flood, as in 1896 and 1897, which 
might carry away in suspension or solution those organic sub- 
stances in the vast amount of vegetable detritus which covered 
the bottom-land as a result of the low water of the preceding 
autumn, and which may have been utilized by the plankton in 
this extraordinary vernal development as a result of the juxta- 
position of flood and vernal growing season. 
