32 THE HABITAT 
the total amount of water in the soil is divided into the available and the 
non-available water-content. The terms suggested for these are respect- 
ively, Aolard (édos, whole, dpSov, water), chresard (xpijsis, use), and echard (éxw, 
to withhold). 
52. Chresard determinations under control. The determination of the 
chresard in the field is attended with peculiar difficulties. In consequence, 
the method of obtaining it under control will first be described. The 
inquiry may be made with reference to soils in general or to the soil of 
a particular formation. In the last case, if the plants used are from the same 
formation, the results will have almost the value of a field determination. 
When no definite habitat is the subject of investigation, an actual soil, and 
not an artificial mixture, should be used, and the plants employed should be 
mesophytes. The individual plants are grown from seeds in the proper 
soils, and are repotted sufficiently often to keep the roots away from the 
surface. The last transfer is made to a pot large enough to permit the plant 
to become full-grown without crowding the roots. The pot should be glazed - 
inside and out in order to prevent the escape of moisture. This interferes 
slightly with the aeration of the soil, but it will not cause any real difficulty. 
The plant is watered in such a way as to make the growth as normal as 
possible. After it has become well established, three soil samples are taken 
in such a manner that they will give the variation in different parts of the 
pot. One is taken near the plant, the second midway between the plant and 
the edge of the pot, and the third near the edge. The depth is determined 
by the size of the pot and the position of the roots. The holard is determined 
for these in the usual way, but the result is expressed with reference to 100 
grams of dry soil; the average is taken as representative. The soil is then 
allowed to dry out slowly, as sudden drouth will sometimes impair the power 
of absorption and a plant will wilt although considerable available water 
remains. Plants often wilt in the field daily for several successive hot dry 
days, and become completely turgid again during the night. If the drying 
out takes place slowly, the plant will not recover after it has once begun 
to wilt. The proper time to make the second reading is indicated by the 
pronounced wilting of the leaves and shoots. Complete wilting occurs, as 
‘a rule, only after the younger parts have drawn for some time upon the 
watery tissues of the stem and root; by which time evaporation has con- 
siderably deceased the water in-the soil. It is a well-known fact that young 
leaves do not wilt easily, especially in watery or succulent plants. Three 
samples are again taken and the average water-content determined as 
above: This is the non-available water or the echard. The latter is then 
computed on the basis of 100 grams of dry soil, and this result is subtracted 
