Relation of Nonavailable Wafer to Hygroscopic Coefficient !) 
the soil water it does not suffice to know how much water a cer 
tain kind of soil can take up, we must much more know whether 
and how much of this the plant can take up. * * * For this 
purpose one must determine the degree of moistness of the soil at 
which the plants no longer are in position to withdraw from it 
the minimum of their needs, where the roots in the soil no 
longer can take up so much water as may be necessary to re- 
place the smallest evaporation from the leaves. This takes place 
when the leaves in a very moist atmosphere, even by night, re- 
main wilted. Wilting by day and in dry air proves only that 
the leaves give out more than the roots are able to take up in the 
same time, but it does not prove wmether the roots are no longer 
able to take up any ; by strong sunlight the leaves can wilt, even 
when the roots take up very much and stand in very moist soil. 
When on the other hand the leaves wilt in moist air and by the 
absence of light, where their transpiration is almost zero, this 
proves that the roots are unable to take up even this small quan- 
tity and that if the plant should need more it would only suffer 
so much a greater shortage. When the plant is in this condition, 
the soil can still contain considerable quantities of water, more 
or less according to its nature." 
To test the commonly accepted view as to the importance of 
the ability of different soils to attract water vapor from the at- 
mosphere Sachs carried out two experiments, 1 one with a scarlet 
runner plant (Phaseolus mtUtiflorus) and another with a to- 
bacco plant. These experiments indicated that a very hygro- 
scopic soil under certain conditions was able to attract enough 
moisture from an almost saturated atmosphere to maintain the 
life of plants. In the first experiment, that with the scarlet run- 
ner, he took a young plant with three leaves, grown in a very 
heavy soil in a porous earthen flowerpot, and allowed it to stand 
without watering until the soil was fully air-dry and the leaves 
began to wilt. Then the bottom of a high and roomy glass cylin- 
der was covered with a shallow layer of water, an inverted beaker 
placed in the middle of it, and on the bottom of this the flower- 
pot placed. The upper opening of the cylinder was closed with 
a divided glass cover in such a way that only the thin stem 
of the plant projected thru the central opening in the cover. 
Thus the leaves were exposed to the air of the room while the 
soil containing the roots was in an almost saturated atmosphere. 
The wilted lenves became fresh and remained turgescent during 
two months (June and July) without ever wilting. No further 
development took place, the original leaves remaining healthy 
but no new ones anpearino". In the second experiment a tobacco 
a Ibid., p. 236. 
