Oct. 1, 1924 
Inheritance of Earliness in Wheat 
345 
Table VIII.— Frequency distribution of the F 3 head-row population with reference 
to the number of days required for each row to pass from first heading to last 
heading 
Number of days 
To- 
13 
14 
1516 
17 
18|l9 
20 21 
22 
23 
24 
25 
26:27 
28 
29 
3031 
1 
32I33 
34 
35 
36(3738 
39 
40 
41 
42 
tal 
Number of rows. 
1 
8 
I 0 J 22 
30 
30J57 
82 69 
77 
37 
22 
25 
1421 
26 
34 
47]34 
6738 
30 
8 
10 5 6 
4 
3 
1 
1 
819 
Those classed as early began heading 
before April 10. Of these, 310 were 
homozygous and 317 were heterozygous. 
The minimum frequency class (ordi¬ 
nate 1,251-1,300) was included with 
heterozygotes. This makes a ratio of 
1.00 early to 1.02 late, while the ex¬ 
pected ratio is 1 : 2 . The reasons 
assigned above for the discrepancies ob¬ 
served may also apply here. 
per row. While the curves from the 
two sets of data are similar in general 
outline, a more definite segregation is 
shown by the latter. The class of great¬ 
est frequency in the homozygous group, 
which developed in the period of low 
time-temperature units, contains 95 
compared with 82 in the same group 
when the distribution is shown by the 
days required to develop from first 
heading to complete heading. 
Fig. 5. —Curve showing frequency distribution of the F 3 head-row population based on the number 
of days required for each row from first heading to last heading 
In Table VIII and Figure 5 are 
shown the distribution of the F 3 head- 
row population based on the number of 
days required by each row to develop 
from the appearance of the first head to 
the stage when all heads were out. 
There is shown a large group requiring 
a comparatively short period and a 
smaller group requiring a compara¬ 
tively long period for this development. 
The division evidently is at 26 days. 
When half of the individuals of this 
class were assigned to each group, 58.25 
per cent of the rows appeared homo¬ 
zygous and 41.75 per cent appeared 
heterozygous. This is a ratio of 1.19 
to 0.83, which is approximately the same 
as the segregation on the basis of the 
total time-temperature units required 
99184—25f-3 
Inspection of the data shows that 
rows requiring the same number of 
time-temperature units in a number of 
cases show quite a wide divergence in 
the total number of days required. 
This indicates that there was a distinct 
slowing down of plant growth due tp 
unfavorable temperature conditions, 
which should be taken into account in a 
study of this kind. Although a con¬ 
sideration of the time-temperature rela¬ 
tion apparently permits a somewhat 
more accurate analysis of the genetic 
factors involved in the inheritance of 
earliness than time alone, it is evident 
that more information is needed con¬ 
cerning the response of the wheat plant 
to temperature and other factors which 
influence growth. 
