'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, holard {o\o<i, whole, apoov, water), chresard (xP^'ts, use), 2lVlA e chard {iyu>, 

 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 wall 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 lOO 

 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 lOO grams of dry soil, and this result is subtracted 



