142 Journal of Agricultural Research voi. xiii, no. 2 



The total degrees of positive temperature^ received by the sorghums 

 at Puyallup was 1,615° F-. at Chula Vista, 1,895°, at Bard, 4,236°, and 

 at Chillicothe 3,028°. The difference between the total seasonal heat 

 units at Chula Vista and Puyallup is rather small, and it is difficult to 

 believe that this difference in heat units is alone responsible for the 

 failure of the sorghums to reach the heading stage at Puyallup, while 

 they matured perfectly at Chula Vista. 



Evidently there was an excess of heat at Bard above what the plants 

 could utilize, since they ripened at Chula Vista with 2,341° F. and at 

 Chillicothe with 1,208° less positive heat units than were available at 

 Bard during the period covered by their growth. The old theory of 

 botanists that a given total of heat units will produce the same phase 

 of vegetation regardless of latitude, longitude, or local climatic condi- 

 tions is thus disproved, but the results do substantiate Linsser's law of 

 growth as described by Abbe (i, p. 214). 



This law can be stated as follows : In two different localities the sums 

 of positive daily temperatures for the same phase of vegetation is pro- 

 portional to the annual sum total of all positive temperatures for the 

 respective localities — that is, the heat required in any locality to pro- 

 duce a given phase of development in vegetation bears a constant ratio 

 to the total positive heat units available in that place. This ratio has 

 been styled the "physiological constant." If 50° F. is considered as 

 the minimum temperature for growth in the sorghums, the yearly total 

 of positive heat units at Chillicothe in 1915 was 5,618° F.; at Bard, 

 1915, 7,989°; and at Chula Vista, 1916, 3,600°. The positive heat 

 units required to bring the sorghums to maturity at Chillicothe were 

 3,028° F., at Bard, 4,236°, and at Chula Vista 1,895°. It appears, 

 therefore, that the physiological constant of sorghum for the period 

 from planting to maturity is about 0.53. The conformance of the sor- 

 ghums in these three cases to Linsser's law is rather remarkable, the 

 exact ratio in each case being as follows : 



Chillicothe 3,028:5,618, or o. 539 



Bard 4,236:7,989, or . 530 



Chula Vista 1,895:3,600, or . 526 



Although Linsser's law seems to furnish a rule for the behavior of 

 sorghums in respect to temperature, it does not take into account the 

 effect of sunlight and other factors, which are also important. 



The data for a more exact comparison of the growth attained by 

 certain varieties of sorghum at Bard and Chula Vista have been assem- 



* These totals were calculated according to the second method of Angot described by Abbe (r, p. yg), 

 except that 50° F. was taken as the zero point of growth instead of 43" F. Sachs {2) determined the mini- 

 mum temperature for corn as 49.1° F., and as sorghum is notably more sensitive to low temjjeratures than 

 corn, 30° F. was chosen arbitrarily as the minimum temperature for growth, and only those temperatures 

 above 50° F. were used in compiling the totals. For Chula Vista, Bard, and Chillicothe the totals are 

 based on the average season required by the sorghums for maturity. At Puyallup it includes the period 

 from the date when the sorghums were planted until the first killing frost in the fall. 



