Oct. 23,1916 Daily Transpiration during Normdl Growth Period 
195 
MEASUREMENTS IN’ 1915 
The daily weighings in 1915 were begun as soon as the plants had 
started to grow, and the total transpiration period of wheat, oats, barley, 
rye, and flax is included in the transpiration-evaporation graphs pre¬ 
sented in figure 9. These graphs show the march in the transpiration 
independently of the fluctuations due to daily changes in the weather, 
assuming the latter to be represented by the fluctuations in evaporation. 
On the same assumption, the march of the weekly transpiration under 
uniform weather conditions* is expressed numerically in Table XIX in 
per cent of the total transpiration. 
Table XIX. — Weekly transpiration under uniform conditions of evaporation expressed 
in terms of percentage of the total, IQ15 
Week of growth. 
Crop. 
Wheat, Kubanka 1440. 
Wheat, Galgalos. 
WTieat, Washington Bluestem 
Oat, Swedish Select. 
Oat, Burt. 
Barley, Hannchen. 
Rye, spring. 
Flax, North Dakota (C. 1 .13). 
Flax, North Dakota (C. 1 .19). 
Flax, Smyrna. 
1st. 
2d. 
3d. 
4th. 
Sth. 
6th. 
7th. 
8th. 
9th. 
10th. 
nth. 
12 th. 
13th. 
z 
2 
4 
8 
9 
12 
12 
13 
13 
7 
8 
4 
2 
2 
3 
S 
8 
9 
II 
13 
IS 
12 
10 
8 
4 
I 
X 
3 
6 
7 
9 
11 
14 
14 
12 
10 
9 
4 
I 
2 
6 
8 
11 
14 
18 
13 
11 
11 
5 
O 
3 
7 
10 
12 
16 
19 
14 
12 
7 
I 
2 
8 
xo 
14 
16 
18 
1$ 
10 
6 
I 
4 
8 
11 
12 
13 
14 
12 
10 
8 
7 
2 
4 
8 
10 
17 
17 
16 
13 
8 
5 
2 
3 
6 
10 
18 
18 
14 
13 
10 
4 
2 
2 
3 
6 
9 
15 
19 
IS 
12 
12 
5 
2 
The wheats reached their maximum transpiration in the eighth week 
following emergence and were harvested in the twelfth or thirteenth 
week. Oats, barley, and rye, which were planted a little later, reached 
their maximum in the seventh week of growth and were harvested in 
the tenth or eleventh week. The flax varieties reached their maximum 
in the sixth week, although they were not harvested until five weeks 
later. 
THE TRANSPIRATION COEFFICIENT DURING THE EARLY PERIODS OF GROWTH 
The transpiration-evaporation graphs during the early development of 
the crops either approximate a straight line, or curve upward as in the 
case of Sudan grass. The latter form suggests an exponential relation¬ 
ship, which would mean that the rate of increase in the transpiration co¬ 
efficient is proportional to the transpiration coefficient itself. 
T 
Tet T represent the transpiration, E the evaporation, and — = & the 
transpiration coefficient (referred to evaporation) at the time t in the 
development of the crop. If the rate of increase of the transpira- 
dk • 
tion coefficient — is proportional to the transpiration coefficient, then 
lit 
in which o' is a constant of proportionality. 
55858°—16 - i 
