62 
WATER REQUIREMENT OF PLANTS. 
ditions increased sunlight would, of course, increase the water require- 
ment. 
On the other hand, the use of shade might be expected to lower the 
water requirement, so long as photosynthesis is not interfered with. 
If the shading is carried so far as to prevent carbon fixation at the 
normal rate, then the water requirement would again increase. 
SORAUER'S EXPERIMENTS. 
Sorauer (1880, p. 463) determined the water requirement of bean 
plants in the dark and in the light. The plants were grown for 20 
days under bell jars, four pots being used in each experiment. The 
water requirement of plants grown in the dark was 21 ±3, and of 
plants in the light, 47±1. This experiment does not really afford 
any evidence with respect to the effect of light upon the water require- 
ment, since no elaboration of carbohydrates is possible in the dark. 
Furthermore, no correction was made for the initial weight of the 
seedling plants, and the apparently low water requirement of plants 
grown in the dark is due to this. This experiment illustrates the 
possibility of securing misleading results from the use of seedlings in 
water-requirement measurements. 
HELLRIEGEL S EXPERIMENTS. 
The experiments of Hellriegel (1883) are more conclusive. The 
results are given in Table LII. The water requirement was found 
to be lowest when the plants were grown in full light, and increased 
with increasing shade. The actual light reduction resulting from the 
use of the different screens is not stated. The " medium " screen 
increased the water requirement of the barley approximately 50 per 
cent and of the peas about 75 per cent. 
Table LII. — Effect of different light intensities on the water requirement of barley and 
peas, 1 according to Hellriegel (1883, p. 633). 
Size of pot. 
Full light, 
Shade. 
Plant. 
Dry 
matter. 
Water 
require- 
ment. 
Wide screen. Medium screen. Close screen. 
Dry 
matter. 
Water 
require- 
ment. 
Dry ! Water 
Dry 
matter. 
Water 
require- 
ment. 
Barley 
(65 by 67 centimeters. . . 
\34 by 35 centimeters. . . 
Grams. 
57.3 
28.0 
360 
338 
349 ±9 
Grams. 
15.3 
12.2 
498 
468 
Grams. 
10.8 
11.8 
510 
528 
Grams. 
6^5 
609 
542 
483 ±13 
519±8 
576±29 
34 by 35 centimeters. . . 
/ 38.4 
\ 37.1 
343 
364 
11.4 
624 
354±9 
1 1 
i Garden soil was used. The amount, of soil did not affect the water requirement. Evaporation was 
prevented by waxed-board covers. Each given determination of water requirement is the mean value 
of two measurements, except that the one for shaded pea is a single measurement. 
285 
