Oct. 1, 1924 
Inheritance of Earliness in Wheat 
339 
in each row was recorded, as well as 
the date when the last head made its 
appearance. This gave the date of 
first heading of progeny from each F 2 
plant, as well as the interval of time 
required from first heading until all 
plants were fully headed. 
As. a wheat plant is considered a 
unit in heredity, any spike from that 
plant will exhibit in its homozygous 
progeny the characters of its parent. 
A homozygous head row from a reces¬ 
sive late plant will begin heading late; 
a homozygous early head row will 
begin heading early; and a heterozy¬ 
gous head row will begin heading early, 
as it contains both early and late 
plants. In the F 3 generation, there¬ 
fore, a similar distribution for first 
heading should be obtained as in the 
Fp. Table II gives the frequency 
distribution of date of first heading 
of the F 3 head rows. 4 There is a sharp 
segregation into an early group and 
a late group almost identical with that 
in the F 2 generation. 
If the division is made between the 
April 2 and April 3 dates of first head¬ 
ing in the F 3 , the totals in the early 
and late classes are 607 and 184, re¬ 
spectively, or practically the same as 
were placed in the early and late classes 
in the F 2 . These totals are not made 
up entirely of the same individuals as 
made up the early and late classes in 
the F 2 , however, as about 4 per cent 
of the segregates have shifted back or 
forth with respect to date of first 
heading. 
TJSE OF HEADING DATA AS A METHOD 
OF ANALYSIS OF Fg ROWS 
A further analysis of the F 3 popula¬ 
tion was made to determine, if possible, 
the segregation for rows homozygous 
and those heterozygous for duration of 
heading. 
In taking agronomic notes on pure- 
line varieties it has been observed that 
within a group, such as common wheat, 
elub wheat, etc., the period from first 
heading to fully headed is fairly uni¬ 
form and constant (figs. 1 and 2). On 
account of a number of factors, such 
as lower temperatures, shorter period 
of daylight, etc., varieties heading 
early in the season require a somewhat 
longer period to progress from first 
heading to fully headed than do those 
eoming later, but those heading at 
about the same time require approxi¬ 
mately an equal time for this period of 
development. It should be possible 
in segregating hybrid material, there¬ 
fore, to separate the early and late 
homozygotes from the heterozygotes 
by the length of time required to come 
from first heading to the fully headed 
stage. The homozygotes would re¬ 
quire a comparatively short time to 
complete the heading process, while 
the heterozygotes would require a 
longer time, in some cases extending 
over the entire season, depending upon 
the degree of heterozygosity. 
The season of 1923 at Davis was 
marked by high temperatures in Feb¬ 
ruary and March, so that the prog¬ 
eny rows from the early F 2 segregates 
began heading in late March. A heavy 
rain followed by cooler weather oc¬ 
curred from March 30 to April 2. 
This retarded the heading of the late 
segregates and slowed down the prog¬ 
ress of those already heading. How¬ 
ever, most of the progeny rows from 
the early segregates began heading be¬ 
fore the storm occurred, so that earli¬ 
ness of heading probably was not af¬ 
fected much in this group, and the late 
segregates probably were affected about 
equally. 
USE OF TIME-TEMPERATURE EFFICIENCY 
UNIT AS A METHOD OF ANALYSIS 
To get the most accurate measure of 
the time required for heading in the 
various rows, it was decided that tem¬ 
perature also must be considered, as 
it materially affects the rate of plant 
growth. High temperatures with rapid 
development occurred both early and 
late during the heading period, while 
in the intermediate period the rate of 
heading was slower. The time re¬ 
quired for heading therefore was con¬ 
verted into time-temperature efficiency 
units. A time-temperature efficiency 
unit was taken arbitrarily to mean 1° 
F. for the duration of one hour above 
a certain basic temperature, below 
which the wheat plant probably makes 
no appreciable growth. 
The minimum temperature of growth 
for the seedling wheat plant was found 
by Sachs {14, P . 365) to be 41° F., 
the optimum temperature, 83.7° F., 
and the maximum temperature, 108°. 
Other investigators have found that 
the growth of many plants ceases with 
falling temperatures at 40° to 43°. 
In growth-temperature studies with 
plants, 40° has been the basic tempera¬ 
ture commonly used. It will be con¬ 
sidered in this study also as the tem¬ 
perature below which little or no 
growth occurs in wheat. The opti¬ 
mum temperature was taken as 83.7°, 
as determined by Sachs, although it is 
probable that plants approaching ma¬ 
turity are not so sensitive to higher 
temperatures as seedling plants. 
4 There were 792 head rows in the F 2 (Table I), but the seed from one failed to germinate, hence 
791 in F3. 
