r9tt] DACHNOWSKI—CRANBERRY ISLAND 129 
normal sum total of effective daily temperatures above 43° F. 
(6° C.), the estimate for which is derived by multiplying the mean 
average monthly minimum temperature of that period by the num- 
ber of days, is 10414° F. (2520° C.), as against 6466° F. (1422° C.) 
for Marquette. The normal mean temperature of the six consecu- 
tive hottest weeks of the year, effective also in determining the 
austral limit of species, is 75°. The warmest month is July with 
an average monthly maximum of 86° F. (30° C.), as contrasted 
with 77° F. (25°C.) at Marquette. The coldest months are Jan- 
uary and February with an average monthly minimum of 22° F. 
and 23° F. (5° and 5°5 C.) respectively, as contrasted with 10° F. 
and g° F. (12°5 and 12°o C.) respectively for Marquette. The 
dates of the latest killing frost in spring and the earliest in autumn, 
although not the exact limits of physiological activity in plants, 
or the limits of the growing period of most plants, are neverthe- 
less an unquestionably important factor. Six months of the year 
are normally free from frost about Columbus. 
Though the relation between rainfall and the amount of water 
needed by plants is of great importance in regard to differences 
in vegetation, the rainfall and its distribution during the seasons, 
and the number of rainy days, are of greater significance than is 
the amount of rain. At Columbus precipitation is quite evenly 
distributed, reaching an optimum of 10.5 inches (26 cm.) during 
spring and summer, when the vegetative functions of bog plants 
are more active, with a minimum of 8.5 inches (21 cm.) during 
the season of low temperature and in the quiescent period of 
plants. Columbus exceeds the annual precipitation at Marquette 
by 4.7 inches (11.8 cm.); the average number of days with rain- 
fall during the year, however, is considerably less than in northern 
Michigan, 144 days as against 161 days. In the north the greater 
precipitation is in the form of snow. Marquette has over five 
times more snow than Columbus, 125.7 inches (315 cm.), as con- 
trasted with 23.5 inches (59 cm.) here. In this vicinity the 
longer growing season of the plants has therefore correspondingly 
more of the precipitation available. On account of the higher 
temperature more moisture is needed, and hence the evaporation 
also is much greater here than at Marquette. Cold air does 
