38 



BULLETIN 700, U. S. DEPARTMENT OF AGRICULTURE. 



place. On the basis of many plants studied this minimum may be 

 placed at approximately 40° F. Hence in the periodic and seasonal 

 temperature summations, the daily mean temperature less 40, the un- 

 effectiye growing temperature, were added. These summations of 

 effective temperatures were made, in the case of each of the type sta- f 

 tions, for the period in toto during which the plants were grown, as 

 well as for shorter periods. This method involves a slight error, since 

 in a few instances during the growing season the mean dropped be- 

 low 40° ; but the error thus introduced is so small as to be quite 

 negligible. 



The sum of the daily mean temperatures was obtained from the 

 hourly corrected thermograph readings and added according to defi- 

 nite periods. These summations are presented chiefly for purposes 

 of comparison with the other two methods of summation described. 



The temperature summations by the three methods are given in 



Table 11, in Section A, of the table for the plants that were started 



June 13 and grown until killing frosts arrested their activities, and 



in Section B, for those started several weeks later and grown until 



inclement weather set in. 



» 

 Table 11. — Temperature summations, in degrees Fahrenheit, for period of 

 growth of potometered plants in type stations. 



SECTION A.i 



Type. 







Sum of 



Duration 



Sum of 



positive 



of period. 



daily mean. 



tempera- 

 ture. 



Days. 



°F. 



°F. 



81 



5,034 



1,789 



95 



5,445 



1,404 



91 



4,631 



991 



Sum of 

 physio- 

 logical tem- 

 perature 

 efficiency. 



Oak-brash. 

 Aspen-fir. . 

 Spruce-fir . 



Index. 

 2, 473. 7 

 1, 560. 6 

 730.5 



SECTION B. 



Oak-brush. 

 Aspen-fir. . . 

 Spruce-fir . . 



52 



4,330 



1,528 



70 



3,932 



1,132 



65 



3,285 



685 



1.938.2 



1, 025. 8 



486.1 



1 Section A of the table has reference to fig. 19, and Section B to fig. 20. 



It should be pointed out that the temperature summations in the 

 case of the oak-brush type are for a period of 81 days, which marks 

 approximately the time required for the maturity of the plants. The 

 summations in the aspen-fir and spruce-fir associations are for 95 

 and 91 days, respectively. Owing to the relatively low temperatures 

 the plants in the two latter types did not reach maturity, killing 

 frosts having occurred early in September. From the temperature 

 summations in figure 19, therefore, it should be understood that the 



