235 



In Europe, however, the connection between the temperature of the 

 air and vecvtation is so intimate that some investigators maintain 

 that on the occurrence of a given tem[)erature the phmt enters at once 

 upon a corresponding definite stage of develojDment, "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 'develoi3nient 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. Tt is evident that if under a given lati- 

 tude the temperature of the atmosphere is the principal factor, Avhile 

 under another latitude the moisture of the atmosphere is the princi- 

 pal factor, tlien neither of these should be entirely overlooked, but the 

 part played by each must be examined. To this end the study of the 

 geogra23hical distribution of plants gives very little information. 

 Again, the study of the influence of periodic 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 

 ex]:)Osed to the full sunshii\e by day and the radiation from the sky 

 by night. 



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 Eisenlohr at Carlsruhe 

 from 1779 to 1830. These show that a plant enters into a definite 

 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- 



