40 



THE PLANT LIFE OF MARYLAND 



at Sunnyside. In the autumn, however, the average first occur- 

 rence of 32° at Sunnyside follows more rapidly on that of 40° (16 

 days) than it does at Pocomoke City (24 days). 



In the first volume of the Publications of the Maryland Weather 

 Service (p. 487) is given a map showing the date of the advent of 

 spring throughout the state, which is reproduced herewith. The 

 isochronals on this map indicate the average date of the permanent 

 occurrence of a daily mean temperature of 44°. This temperature 

 was taken by Mr. Walz, following Professor Harrington, on the as- 



Fig. 2. — Map of Maryland Showing the Advent of Spring. 



sumption of Herve Magnon* based mi a study of cereals, that plant 

 protoplasm is inactive below a temperature of 42.8° Fahr. While 

 there are many plants in the Maryland flora which would not be 



wakened to activity by a temperature legree above 43° and very 



many others which would be wakened by temperatures lower than 

 this, yet the isochronals for 44 may serve very well to indicate the 

 character and rate of advance of other daily mean temperatures as 



*Herve Magnon, Des Conditions climatologiques des annees 1869-1879 en 

 Normandie, et leur influence sur la maturation des racoltes. Comptes 

 Rendus, Vol. 89, 1879. 



