Oct. 1, 1924 
Inheritance of Earliness in Wheat 
341 
In calculating the time-temperature 
efficiency units it is important to con¬ 
sider the effectiveness of varying de¬ 
grees of temperature in promoting 
growth. As plants approach the op¬ 
timum temperature the growth rate is 
approximately doubled for each rise of 
18° in temperature above the basic 
temperature in conformity with van’t 
Hoff’s law of chemical reaction velocity, 
as set forth by Livingston and Living¬ 
ston (13). The daily number of time- 
temperature efficiency units was deter¬ 
mined from thermograph records during 
the entire heading period of 1923. From 
these data the total number of such 
units required to bring each row from 
date of first heading to date of last 
heading were calculated. 
Table III.— Time-temperature coeffi¬ 
cients (by exponential system) adapted 
to hourly record on thermograph at 
Davis, Calif. (Doubled growth rate 
assumed for each 18° F. (10° C.), and 
using only temperatures above 40° F.) 
Degrees 
above 
40° F. 
Time- 
tempera¬ 
ture 
units 
Degrees 
above 
40° F. 
Time- 
tempera¬ 
ture 
units 
1. 
1. 00 
28... 
2. 82 
2_ 
1.04 
29_ 
2.93 
3. 
1.08 
30_ 
3. 05 
4. _ 
1.12 
31_ 
3.16 
5_ 
1.17 
32.. 
3. 29 
6. 
1. 21 
33_ 
3. 42 
7.. 
1.26 
34_ 
3. 55 
8. 
1.31 
35. 
3. 68 
9.. 
1.36 
36_ 
3. 84 
10. 
1.41 
37. 
4.00 
11_ 
1. 47 
38_ 
4.16 
12. 
1. 53 
39. 
4. 32 
13.. 
1. 59 
40_ 
4. 49 
14.__ 
1. 65 
41. 
4. 67 
15 _ 
1. 71 
42...... 
4. 85 
16_ 
1. 78 
43.. 
5. 03 
17. 
1. 85 
44_ 
4. 85 
18_ 
1. 92 
45__ 
4. 67 
19. 
2. 00 
46. 
4. 49 
20_ 
2. 08 
47_ 
4. 32 
21 . 
2.16 
48_ 
4.16 
22. 
2, 25 
2, 33 
2.42 
49.__ 
4.00 
23. 
50. 
3. 84 
24__ 
51_ 
3. 68 
25. 
2. 52 
52_ 
3. 55 
26.. 
2. 62 
53_ 
3. 42 
27. 
2. 72 
According to the method of calcula¬ 
tion used, the value of a unit at 41° 
for one hour is 1, at 59° it is 2 for each 
hour, at 77° it is 4 for each hour, and 
so on to the optimum temperature. 
The added increments of efficiency, 
calculated for each degree, are shown 
in Table III. At temperatures above 
the optimum the growth rate was 
assumed to decrease at the same rate 
at which it previously had increased. 
Table IV gives the daily time-tem¬ 
perature efficiency units for the entire 
heading period from March 26 to 
May 10. 
Table V is presented for purposes of 
comparison to show the number of 
time-temperature units, as arbitrarily 
defined previously, which are required 
to bring head rows of parent varieties 
from first heading to complete heading. 
Sunset apparently requires from 850 to 
1,050 such units, while Marquis, coming 
later in the season at higher tempera¬ 
tures, requires a smaller number. It 
will be observed in Table VII that the 
class of greatest frequency of the so- 
called homozygous rows is found at 
901 to 950 time-temperature units in 
both the early and the late groups. 
Table IV.— Daily time-temperature 
efficiency units from March 26 to May 
10, 1923, at University Farm, Davis , 
Calif. 
Date 
Daily 
time- 
temper¬ 
ature 
units 
Date 
Daily 
time- 
temper¬ 
ature 
units 
Mar. 25_ 
62. 5 
Apr. 19_ 
51.5 
26.. 
61. 5 
20_ 
46.2 
27_ 
66. 0 
21 
37.1 
28_ 
62.4 
22.. 
45.1 
29_ 
52.9 
23_ 
47.1 
30 . ... 
51. 8 
24 
58. 5 
31_ 
44. 5 
25 _ 
70. 8 
Apr. 1_ 
37.3 
26_ 
50.0 
2_ 
35. 6 
27. 
42.0 
3_ 
35.0 
28...... 
39.2 
4. 
35. 5 
29. 
41. 7 
5.. 
42.3 
30_ 
50.1 
6_ 
38.5 
May 1. 
49.4 
7_ 
39.3 
2_ 
48.9 
8_ 
42.9 
3_ 
65. 5 
9_ 
38; 9 
4_ 
53. 9 
10_ 
49.8 
5_ 
58. 5 
11 . ... 
46. 0 
6_ 
64.1 
12_ 
53. 2 
7_ 
63. 3 
13_ 
! 60.3 
8. 
70.4 
14_ 
58. 6 
9_ 
74.1 
15_ 
1 64.9 
10 . ... 
60. 8 
16_ 
39.0 
11. 
66. 7 
17. 
34.8 
12.. 
72.6 
18.. 
j 44.8 
Table V.— Total time-temperature efficiency units required to bring the recorded 
numbers of individual head rows of parent varieties from first heading to complete 
heading 
Numbers of efficiency units 
Parent 
varieties 
501- 
550 
551- 
600 
601- 
650 
651- 
700 
701- 
750 
Sunset__ 
Marquis.... 
1 
6 
6 
6 
2 
751- 
800 
801- 
850 
851- 
900 
901- 
950 
951- 
1,000 
1,001- 
1,050 
5 
7 
6 
1 
4 
