54 
WATER REQUIREMENT OF PLANTS. 
small percentage. In poor soils the water requirement may be 
reduced one-half or even two-thirds by the addition of fertilizers. 
Often the high water requirement is due to the deficiency of a 
single plant-food element. As the supply of such an element ap- 
proaches exhaustion the rate of growth as measured by the assimila- 
tion of carbon dioxid is greatly reduced, but no corresponding change 
occurs in the transpiration. The result is inevitably a high water 
requirement. 
As a whole, these experiments show clearly that transpiration is not 
a measure of growth, even under the same atmospheric conditions, 
and that the water requirement is profoundly affected by the plant 
food available in the soil. 
Water Cultures, 
woodward's experiments. 
The earliest experiments conducted for the purpose of determining 
the water requirement of plants, so far as the writers are aware, were 
made by Woodward in 1699 with water cultures. Plants of known 
green weight were transplanted to vials. A cork stopper prevented 
loss of water except through the plant, and an accurate record was 
kept of the water consumed. The difference between the initial and 
final green weight was taken to represent the amount of growth. 
Woodward's results, expressed in terms of green weight, are given 
in Table XLII. Although his experiments were made on water cul- 
tures 200 years ago, the results obtained are of fundamental impor- 
tance. He found the water requirement of plants grown in spring 
water and in rain water to be higher than when grown in waters con- 
taining more soluble matter. Moreover, when impure water was 
distilled the water requirement of plants was found to be higher 
when grown in the distillate than when grown in the residue. This 
leads at once to the conclusion that the water requirement of plants 
is, within limits, dependent on the amount of plant food available. 
Table XLII. — Effect of pure and impure water on the water requirement of plants, 
according to Woodward (1699, pp. 200-206). 
Water re- 
, qU gre™n nt ' 
weight. 
Spearmint: 
Spring water 
Rain water 
Thames water 
Hyde Park conduit water 
Hyde Park conduit water and garden earth 
Hyde Park conduit water and garden mold 
Hyde Park conduit water, distilled 
Residue 
Solanum: 
Spring water 
Lathyrus: 
Spring water 
285 
Increase 
Water 
Year. 
in green 
weight. 
transpired. 
Grams. 
Kilograms. 
1691 
0.97 
0.165 
1691 
1.134 
.195 
1691 
1.68 
.161 
/ 1692 
\ 1692 
8.29 
.92 
9.00 
.851 
1692 
10.88 
.696 
1692 
18.4 
.969 
1692 
2.66 
.572 
1692 
6.09 
.281 
1691 
3.695 
.241 
1691 
.227 
.162 
170 
172 
96 
111 
95 
64 
53 
215 
