475 
MONTHLY MEANS OF VERNAL TEMPERATURES FOR ILLINOIS AND OF PLANKTON 
PRODUCTION—CM.? PER M®*. 
1896 1897 18098 
Temper- Temper- Temper- 
ature |Plankton| ature |Plankton| ature | Plankton 
(Fahr.) (Fahr.) (Fahr.) 
North section... 31.9 34.4 39.0 
Central section. 36.4 41.1 44.4 
March .|South’rnsection| 40.6 .07 46.8 38 48.7 33 
State, average.. 35.6 39.5 43.4 
Normal). sce... 37-6 | 37.6 37.6 
North section. . 55.3 47.2 46.9 
Central section 590.9 51.3 50.5 
April ..|South’rn section} 63.6 5.67 BELO |) eSaht 53-0 4.40 
State, average. 59.0 50.4 49.6 
Normalliee si 51.8 51.8 51.8 
North section. . 67.2 57-5 59.1 
Central section. 70.6 59-7 63.0 
May...|South’rn section) 71.7 1.30 62.1 5.62 66.8 II.30 
State, average.. 69.5 59-3 62.2 
Normalizeeccs|) O0s8 61.8 61.8 
The most notable instances of correlation between deflec- 
tions of temperature and plankton production are to be seen in 
the early spring of 1896 and the late autumn of 1897. These 
correlations have already been noted in connection with the 
discussion of production in the river and the several back- 
waters. The mean temperatures of the air here given corrob- 
orate our conclusions based on the relatively scanty data of 
water temperatures delineated in the thermographs of the plates 
accompanying this paper. Thus, the spring of 1896 was 2° be- 
low normal in the state as a whole in March, but was 7.2° 
above normal in April, and 9.4° above the mean for 1898. Mean 
plankton production is also higher in April in 1896 than in any 
other year. Indeed, in this month the descending scale of mean 
temperatures in 1896-1898 is accompanied by a similar scale 
of decreasing mean production of plankton, and, as has been 
noted in the discussions of the course of production in the 
river and backwaters, the vernal pulse of 1596 is from 10-14 
days earlier than in 1898, when, as this table of mean tempera- 
tures shows, the April mean of air temperatures was 9°-10° 
below that of 1896. | 
