268 LIVINGSTON— CLIMATIC AREAS [April i8, 



but this appears to be far more broadly applicable than any other. 

 The actual data of mean length of the frostless season in the United 

 States have never been published, but Day's chart (already referred 

 to) presents a general view of the range in length of this period 

 which this country affords. Data corresponding to those from which 

 Day's chart of the frostless season was compiled have been deduced 

 from the average dates of last and first killing frosts as given in the 

 io6 Sumaries by Sections" published by the Weather Bureau. 

 These deduced data have been used in deriving the other climatic 

 indices considered below. 



Tciiipcratiire integration. — The mean length of the frostless sea- 

 son is of course primarily a temperature condition, but it tells us 

 nothing of the normal temperatures which may prevail within the 

 period designated, only that killing frosts do not normally occur. 

 In order to be able to relate the temperatures of the frostless season 

 to plant activities it is thus obvious that we shall need to sum or 

 integrate the temperatures over the period of active growth. As 

 has been said, the mature plant itself is to be regarded as a summa- 

 tion of all of the accelerations and retardations which have occurred 

 during its life, so that our integration of temperatures should at- 

 tempt to consider these, not merely as they affect our thermometers, 

 but rather as tlicy affect plants. This is, however, practically im- 

 possible until we have at our disposal a much larger fund of infor- 

 mation concerning the general relation of plant activities to tempera- 

 ture, and such information is not apt to be forthcoming until such 

 time as the laboratory for controlled conditions, mentioned above, 

 may become a fact instead of a mere dream. Various procedures of 

 temperature integration have been devised by different writers and 

 appear to be more or less valuable in this connection, but the physio- 

 logical basis for such procedures remains still to be established. 

 Under the circumstances, it seems best here to give attention to but 

 a single one. This is the method of direct summation of the daily 

 normal means throughout the period in question. 



""Summary of the Climatological Data of the United States, by Sec- 

 tions," U. S. Department of Agriculture Weather Bureau. No date. The 

 io6 pamphlets appear to have been prepared about 1909-10. The data ex- 

 tend for the most part through igo8 or 1909. 



