235 
In Europe, however, the connection between the temperature of the 
air and vegetation is so intimate that some investigators maintain 
that on the ¢ occurrence of a given temperature the plant enters at once 
upon a corresponding definite stage of development, while others 
maintain that in order to enter into this stage a definite sum total of 
heat must be received. Therefore the former determined the stages 
of development by the ordinates of the annual curve of temperature, 
while the latter determine them by the area of the space that is 
bounded by such ordinates. It is evident that if under a given lati- 
tude the temperature of the atmosphere is the principal factor, while 
under another latitude the moisture of the atmosphere ts the princi- 
pal factor, then neither of these should be entirely overlooked, but the 
part played by each must be examined. To this end the study of the 
geographical distribution of plants gives very little information. 
Again, the study of the influence of per riodic variations of the atmos- 
phere on plants is useless in the attempt to distinguish between the 
effects of temperature and moisture, because as a general rule the 
atmospheric conditions all attain their maxima and minima at about 
the same time. The study of the nonperiodic variations gives prom- 
ise of greater success. But in studying the relation of temperature to 
vegetation the data given by thermometers hung in the shade, as to 
the temperature of the air, can have little to do with the life of the 
plant as compared with the temperature given by a thermometer 
exposed to the full sunshine by day and the radiation from the sky 
by might. 
Dove then discusses the observations of maximum sunshine and 
minimum radiation thermometers made in the botanic garden at 
Chiswick, near London, from 1816 to 1840, and shows among other 
things that when the mean temperature of the air is low the freely 
exposed radiation thermometer is especially low, and when the aver- 
age temperature is high the freely exposed solar thermometer is es- 
pecially high. He then investigates the observations of earth tem- 
perature made by Quetelet, of Brussels, from 1834 to 1843, and shows 
that the upper layers of soil, whether dry or wet, have temperature 
variations parallel to those of the temperature of the air. He then 
studies the phenological observations of Ejisenlohr at Carlsruhe 
from 1779 to 1830. These show that a plant enters into a defipite 
stage of development when the air attains a definite degree of tem- 
perature rather than when the plant has received a definite sum total 
of heat, this conclusion being, of course, based upon the internal 
agreement of the computed figures for these fifty-one years of 
observations. 
Analogous results were obtained by him by studying similar ob- 
servations made in the State of New York and at Wurttemberg, 
Germany. 
With regard to the influence of rainfall, Dove finds that it is not 
so plain as that of temperature, and that it is not so much the quan- 
tity of rainfall that is important as the frequency; too great fre- 
