=158. Field methods.= The conditions which a satisfactory field method of measuring transpiration must fulfill have already been discussed; they are accuracy, simplicity, and normality. These conditions are met only by weighing the plant in its own soil and habitat. This has been accomplished by means of the sheet-iron soil box, already described under the determination of the chresard. The method is merely the familiar one of pot and balance, slightly modified for field use. The soil block, which contains the plant to be studied, is cut out, and the metal plates put in position as indicated in section 53. Indeed, it is a great saving of time and effort to determine transpiration and chresard in the same experiment; this is particularly desirable in view of the close connection between them. In this event, the soil block must be small enough not to exceed the load of a field balance. After the block is cut and encased, all the plants are removed, except the one to be studied. If several individuals of the same species are present, it is an advantage to leave all of them, since the error arising from individual variations of water loss may, in this way, be almost completely eliminated. A sheet of rubber or rubber cloth is carefully tied over the box to prevent evaporation from the soil. A broad band is passed under the box to aid in lifting it upon the scales. The latter must be of the platform type, and should have a capacity as great as consistent with the need for moving it about in the field. Weighings are made in the usual way, care being taken to free the surface of the box from soil. The aeration of the soil block is kept normal by removing the rubber for a few minutes from time to time, or by forcing air through a thistle tube. Water is also added through the latter, when it is desired to continue the experiment for a considerable period. After the study of transpiration is concluded, the rubber cloth is removed, soil samples taken, and the soil allowed to dry out until the plant becomes thoroughly wilted. If the box is weighed again, the difference represents the amount of available water. The per cent of chresard is also obtained in the usual way by taking samples for ascertaining the echard, and subtracting this from the holard. Field determinations of water loss yield the most valuable results when different habitat forms, or ecads, of the same species are used. There is little profit in comparing the transpiration of a typical sun plant, such as _Touterea multiflora_, with that of a shade plant, such as _Washingtonia obtusa_. But the simultaneous study of plants like _Chamaenerium angustifolium_, _Gentiana acuta_, _Scutellaria brittonii_ etc., which grow in several different habitats, furnishes direct and fundamental evidence of the course of adjustment and adaptation.