38 THE PLANT LIFE OF MARYLAND 



entiation of winter than of summer climate in the state. The data 

 showing the mean daily range of temperature indicate a maximum of 

 2G.2" for Boettcherville, ranges which are nearly the same for Sunny- 

 side and Green Spring Furnace, and appreciahly lower ranges for the 

 other four stations, with a minimum of 16.1° for Baltimore. 



The length of the growing season is a datum which must be de- 

 termined for each plant species in order to have physiological value, 

 owing to the specific differences of plants as respects the temperature 

 which serves to awaken them to activity in the spring and to cause 

 them to die or to resume winter dormancy in the autumn. For the 

 purposes of Floristic Plant Geography we may, however, very well 

 contrast the differences in length of growing season in the different 

 parts of Maryland by ascertaining the number of days falling be- 

 tweeen the last vernal and first autumnal occurrence of certain low 

 temperatures. The freezing point is the temperature which is of the 

 most uniform influence in the demarcation of the growing season, 

 particularly upon its termination, for it. is only by the occurrence of 

 heavy frosts that the foliage of deciduous trees ceases to be functional 

 and annual plants die. As respects the commencement of the grow- 

 ing season the last occurrence of a minimum temperature of 32° is 

 not equally significant, for many plants begin activity before the date 

 of its occurrence and many others not until after. For the purpose 

 of estimating the length of the growing season in different parts 

 of [Maryland two sets of data have been secured, — the first show- 

 ing the number of days between the last occurrence of 32° in the 

 spring and its first occurrence in the autumn, the second showing the 

 number of days between the last and first occurrence of 40°, a tem- 

 perature arbitrarily chosen as being about that at which light frost 

 may occur, and above which most plants of the native flora may be at 

 least somewhat active. 



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