SUMMARY. 89 
As a whole, the investigations relative to the effect of fertilizers on 
the water requirement show clearly that transpiration is not a 
measure of growth, even under the same atmospheric conditions, 
since the water requirement is profoundly affected by the plant food 
available in the soil. 
The type of soil used may affect the water requirement in so far 
as it determines the amount of plant food available to the crop. 
There is no indication that the texture of the soil, independently of 
the plant food it contains, affects the water requirement of plants. 
The water requirement is influenced by limiting the amount of soil 
available to the roots of the plants, the water requirement decreasing 
within limits as the amount of soil is increased. This would follow 
as a result of an inadequate supply of piant food. In some cases the 
water requirement has been increased as a result of previous cropping. 
This would follow from the reduction of the available supply of plant 
food by the previous crop. 
No direct measurements have been made of the effect of soil tem- 
perature on the water requirement. Leather found, however, that an 
increase in the soil temperature due to the sun shining on the pots 
did not materially affect the water requirement. 
The water requirement has been shown to be profoundly affected 
by the atmospheric conditions. The water requirement of the same 
crop varies greatly according to the period of the year in which it is 
grown. Similarly, the same crop will give a widely differing water 
requirement when grown in different regions during the same period. 
All of the investigations which have so far been made indicate 
that shading increases the water requirement. This would follow 
if the shade were so dense as to reduce photosynthesis. 
The water requirement is greater hi dry than in moist air. Mont- 
gomery and Kiesselbach have found the water requirement of corn 
to be proportional to the evaporation from a small free water surface. 
From their data the writers have shown that the water requirement 
was approximately proportional to the saturation deficit of the air. 
Sorauer found the water requirement to increase when the carbon- 
dioxid content of the air was diminished, which is analogous to a 
deficiency in mineral plant food. 
Wimmer has shown that the water requirement of sugar beets and 
celery is increased by the attacks of parasitic nematodes. The 
infested plants grow more slowly, which would account for the 
increase in the water requirement. 
Montgomery has compared the water requirement of narrow-leaf 
and broad-leaf types of corn, and found that the narrow-leaf strain 
had a lower water requirement. It is, however, possible that this 
difference may have been due to a superior varietal efficiency of the 
narrow-leaf type independent of its leaf area. 
285 
