THE FREE WATER OF THE SOIL. 209 
water is continually carried from the atmosphere into 
vegetation. 
In a similar experiment, a tobacco plant was employed 
which stood ina soil of humus. This material was also 
capable of supplying the plant with water by virtue of 
its hygroscopic power, but less satisfactorily than the clay. 
As already mentioned, these plants, while remaining fresh, 
exhibited no signs of growth. This may be due to the 
consumption of oxygen by the roots and soil, or possibly 
the roots of plants may require an occasional drenching 
with liquid water. Further investigations in this direc- 
tion are required and promise most interesting results. 
What Proportion of the Capillary and Hygroscopic 
Water of the Soil may Plants Absorb, is a question that 
Dr. Sachs has made the only attempts to answer. When 
a plant, whose leaves are in a very moist atmosphere, wilts 
or begins to wilt in the night time, when therefore trans- 
piration is reduced to a minimum, it is because the soil no 
longer yields it water. The quantity of water still con- 
tained in a soil at that juncture is that which the plant 
cannot remove from it,—is that which is unavailable to 
vegetation, or at least to the kind of vegetation experi- 
mented with. Sachs made trials on this principle with 
tobacco plants in three different soils, 
The plant began to wilt in a mixture of black humus 
(from beech-wood) aud sand, when the soil contained 
12.3°|, of water.* This soil, however, was capable of 
holding 46°|, of capillary water. It results therefore that 
of its highest content of absorbed water 33.7°|, (=46—12.3) 
was available to the tobacco plant. 
Another plant began to wilt on a rainy night, while the 
loam it stood in contained 8°|, of water. This soil was 
able to absorb 52.1°|, of water, so that it might after 
* Ascertained by drying at 212°, 
