-1859] WRITINGS OF JOSEPH HENRY. 187 



to employ the temperature obtained from the mean of the 

 observed thermometer in the sun and in the shade during 

 the day. To render this principle of use in practice, a series 

 of observations in different seasons of the year, on the tem- 

 peratures of thermometers in the sun and in the shade would 

 be necessary. Besides this, since vegetation is comparatively 

 but little advanced at night, the length of the day should be 

 taken into account, which in the neighborhood of the equa- 

 tor is 12 hours, and in the vicinity of the polar circle, nearly 

 24 hours. Another correction is necessary in order to obtain 

 strictly comparative results, namely, that which is due to the 

 fact that different plants begin to show signs of vitality in the 

 spring at different temperatures. 



Allowing the truth of the proposition of the definite 

 amount of heat required for the full development of each 

 plant, we have a ready explanation of the fact that some 

 grain will come to maturity in climates of very different tem- 

 peratures, the less intensity of heat being compensated for 

 by the longer duration of the day. Though each species of 

 plant may require a definite amount of heat for its perfect 

 maturity, yet this is by no means the measure of the power 

 expended in the organization, though it may bear a definite 

 ratio to it. The chemical ray of the sun decomposes carbonic 

 acid, and thus furnishes the greater part of the material of 

 which the plant is composed, and in the process of germ- 

 ination and assimilation, probably furnishes a portion of the 

 power necessary to carry on these processes. 



The following table is selected from the memoirs of M. 

 Quetelet, of Belgium, and contains the times of leafing, blos- 

 soming, and fructification of plants found in this country as 

 well as in Europe. The selection has been made at my re- 

 quest by Dr. L. D. Gale, of Washington, and it is hoped that 

 the times will be compared with those pertaining to the same 

 periods of the developments of the same plants in different 

 parts of this country. 



The observations from which the original table was con- 

 structed were made in the garden of the Royal Observatory, 

 at Brussels, and according to the author, they may be ap- 



