CLIMATE AND PLANT GROWTH. 49 



of the water requirements of wheat and brome grass without heads 

 to those with heads. The difference is rather pronounced. In the 

 case of wheat in the oak-brush type (fig. 24) it is 37 per cent, in the 

 aspen-fir association 24 per cent, and in the spruce-fir type 18 per 

 cent. In the case of less mature wheat specimens (fig. 25) the dif- 

 ferences between water requirement of the plants without heads and 

 those plants with heads is 19 per cent, 15 per cent, and zero ; for brome 

 grass (fig. 26) it is 62 per cent, 43 per cent, and 14 per cent. 



The differences in the water requirement figures serve to show one 

 of the responses of the plants to the different amounts of heat avail- 

 able in the three associations. In the oak-brush type, where the num- 

 ber of heat units is greatest, the plants are matured or nearly so, and 

 a large proportion of the total dry matter of the plants is deposited 

 in the seed heads. At the middle and upper stations, where the 

 summed seasonal temperature efficiency is lower, the plants are less 

 mature, and a correspondingly lower proportion of the total dry 

 matter of the plants is deposited in the heads. This difference in the 

 stage of maturity would seem, then, to account for the difference in 

 the water requirements of the plant with heads and without heads, 

 and the difference itself affords an approximate measure of the rela- 

 tive development and maturity of the plants in the different types. 



It is noteworthy that in the figures representing the ratio of the 

 water requirements of the plants based on (1) the tops, including 

 heads and (2) the tops without heads (figs. 24, 25, and 26) the curves 

 in each case fall, from the lowest to the highest type, in a manner 

 roughly proportional to the fall in the temperature summations. 

 This agreement in slope shows that the plants mature more slowly 

 as the number of effective temperature units decreases. 



EFFECT OF TEMPERATURE AS INDICATED BY PERIOD REQUIRED FOR PRODUCTION OF 



FLOWERS. 



Additional data showing the relation of the development of the 

 plant to temperature were obtained by noting the number of days 

 required for the first appearance of flowers in the species grown in 

 the type stations. In each instance temperature summations and aver- 

 age mean temperatures were recorded for each period, the results of 

 which are summarized in Table 14. 



56866 18 Bull. 700 4 



