1909. J NATURAL SCIENCES OP PHILADELPHIA. 105 



Weak Chromic Acid. — 



Lansing, Mich. Auburn, Ala." 



Leaf buds eased February 4. 



Leaf buds burst February 8. February 11. 



First leaf unfolded February 1 1 . February 14. 



The fact that there is such a difference between the time when the 

 leaf and flower buds of the same variety and species of fruit and forest 

 trees open is an index of the extent of their acclimatization. The 

 native trees have, since their migration from the original forest centre, 

 been acclimated to their new localities of growth, while the introduced 

 fruit trees since their introduction to America have become similarly 

 ameliorated to the American climate, so that they react in nature and 

 in the experimental culture solutions exactly as do the native plants. 



Bearing of Experimental Data on the Advent of Spring. 



The climatic differences between the north and the south, representing 

 the amount of acclimatization which each species has had to adopt, 

 may be demonstrated by a study of the advent of spring. The tem- 

 perature of 42.8° F. (6° C.) is that at which the protoplasmic contents 

 of the ordinary plant find the limits of their activity. The advent of 

 spring may be considered as taking place at the approach of an isotherm 

 one degree higher, or 43.8° F. This isotherm, progressing steadily in 



Map illustrating the advance of the isotherm of spring (43.8° F.), as well as the 



regions ( ) and stations (#) from which the experimental 



twigs were obtained. 



e Started two days later. 



