174 Journal of A gricultural Research voi. vii, No. 4 
COMPARISON OF THE TRANSPIRATION OF THE DIFFERENT CROPS 
The graphs in figures 2 and 3 illustrate in a striking manner the great 
fluctuations in the water required daily by plants to maintain normal 
growth. During the period from July 8 to 10, 1914, for example, the 
wheats under observation required about three times as much water 
each day as during the period from July 17 to 19. In 1914 the maximum 
rate of transpiration of the series as a whole occurred about July 9, at 
which period the grains were headed, the legumes were in bloom or coming 
into bloom, and corn was beginning to tassel. 
Alfalfa, Sudan grass, and amaranthus, of which several cuttings were 
made from the same root system, showed a continuous increase in the 
transpiration rate up to the time of cutting. The transpiration of Minne¬ 
sota Amber sorghum, Algeria com, and lupine was relatively uniform 
from about July 9 to near the end of the growth season. Among the 
small grains barley and rye showed the least change in the transpiration 
rate. The grain crops were harvested at the stage when similar crops 
in the field are cut with the binder, and it is interesting to note at this 
time transpiration was approximately one-fourth to one-half the maxi¬ 
mum rate. 
The season of 1915, like that of 1914, shows a nearly uniform evapora¬ 
tion rate throughout the more active growth period if the daily fluctua¬ 
tions are ignored. The season was very rainy. This is reflected in the 
radiation graph which is far more irregular than in 1914, but shows the 
same gradual decline as the season advances. 
The fluctuations of the transpiration graphs give evidence again of the 
great variation in the daily quantity of water required to maintain normal 
growth, as is shown by comparing the transpiration during the period 
from July 21 to 23 with the adjacent three-day periods. 
While the fluctuations of the different crops from day to day are simi¬ 
lar, marked differences in the pitch of the graphs are noticeable as the 
season advances. The short season crops in general show a gradual 
increase in transpiration from seedtime to a little past the middle of the 
growth period and then an equally gradual decline to harvest. 
The idea is often advanced that wheat and similar crops increase their 
water demand suddenly at or just before the time of heading. The 
measurements of the writers, however, lend no support to this conclu¬ 
sion. (See transpiration-evaporation ratios.) Aside from fluctuations 
due to weather the transpiration increases uniformly during this period. 
WATER LOSS DURING PERIODS OF MAXIMUM TRANSPIRATION 
It has already been shown in the case of annual crop plants that the 
transpiration rises to a maximum near the middle of the growth period 
and then decreases until the plants are harvested. This is especially 
true of grain crops. On the other hand, the transpiration of perennial 
forage crops, such as alfalfa, increases steadily to a maximum at the 
