50 



BULLETIN 700, U. S. DEPARTMENT OF AGRICULTURE. 



TABLE 14. Periods required for the production of flowers in the vegetative types 

 and temperature summations and average mean temperatures for the respec- 

 tive periods. 



Figures 27, 28, and 29 platted from Table 14 show a rather pro- 

 nounced parallelism in the different figures in the trend of the curves 

 from the lowest to the highest station representing the number of 

 days required for the flowering of the species, on the one hand, and 

 in the curves representing the number of heat units up to time of the 

 production of flowers, on the other. Provided no other factor 

 was operative in holding back growth in the case of the plants in 

 question it would appear that temperature was the controlling factor 

 in this instance. In the case of peas, flowers appeared 21 days earlier 

 in the oak-brush type than in the aspen-fir type and 45 days earlier 

 than in the spruce-fir type, the period between planting and flower- 

 ing in the spruce-fir type being more than twice as long as the corre- 

 sponding period in the oak-brush type. Wheat spikes appeared in 

 the lowest type in 40 days; while in the central and highest types 

 they began to show 12 and 31 days later, respectively. In the case 

 of mountain brome grass, panicles showed in 39 days in the lowest 

 type; but in the central station they did not begin to show until 13 

 days later and in the spruce-fir type 24 days later. One of the most 

 interesting facts brought out in these observations is that in spite of 

 the fewer days required for flowering in the oak-brush type a great 

 many more flowers were produced than in the other types. In the 

 case of wheat, for example, 40, per cent more heads appeared in the 

 oak-brush than in the aspen-fir type, and over 100 per cent more than 

 in the highest type. 



The fact that there is very little slope in the effective temperature 

 summation curves and in the average daily mean temperature curves 

 in figures 27, 28, and 29 shows that practically the same number of 

 heat units were required in each type for the production of flowers. 

 On the physiological basis of temperature summation for the entire 

 season, as has previously been pointed out, there were notably more 

 heat units in the lowest type. The lowest temperature efficiency was 



