Transpiration as a Factor in Crop Production. 
175 
Under comparable greenhouse conditions an increase in 
relative atmospheric humidity which lowered the evaporation 
rate from a free water surface 47 per cent reduced the amount of 
water transpired per gram ash content 38 per cent, and per gram 
dry matter 40.5 per cent. 
In comparable potometers situated in the cornfield during 
two years (1913 and 1914), a difference in climatic factors, which 
lowered the evaporation rate from a free water surface 40 per 
cent during July and August, reduced the transpiration per gram 
ash content 22 per cent and per gram dry matter 27 per cent. 
(Table 72.) 
EFFECT OF DIFFERENCES IN SOIL FERTILITY UPON RELATION OF 
TRANSPIRATION TO ASH CONTENT. 
An ash determination was made for all the plants grown dur- 
ing the season of 1914 in the new fertility series and also for the 
plants grown during both 1913 and 1914 in the old fertility series 
which was commenced in 1911. A full description of the experi- 
ment with other tables and conclusions is given in pages 143 to 158. 
It will be recalled (p. 145) that the plants reported in Table 73 
were entirely normal according to the fertility of the soil, while 
those in Table 74 were not entirely normal due to soil deterio- 
ration from prolonged confinement in the potometers. As a 
result, the data in Table 73 are normal. The differences secured 
from the old fertility series according to the various soil types 
are so marked, however, that the data appear of value in corrobo- 
rating the results in Table 73 and to help establish a general 
principle. 
The results for the separate experiments have been averaged 
together in Table 75. In all three tests the amount of water 
transpired per gram ash content was reduced somewhat as the 
fertility of the soil was increased. It would probably be more 
accurate to say that the amount of solute per unit water taken 
in by the plant was increased as the fertility of the soil was in- 
creased. The denser soil solution did not, however, reduce the 
actual transpiration rate, for in fact the total amount of water 
transpired per plant increased consistently as the strength of 
the soil solution increased. This correlates rather closely with 
the relative amounts of water used per gram dry matter. The 
greater the availability of the soil solutes within the limits of 
these tests (1) the greater was the amount of solute taken in 
per unit water, (2) the greater was the dry matter produced, (3) 
the greater was the total amount of water transpired, (4) the 
greater was the total amount of solute taken into the plant, and 
(5) the smaller was the amount of water used per unit dry matter. 
