HYDROHARMOSE II5 



plant to be studied, is cut out, and the metal plates put in position as indi- 

 cated in section 53. Indeed, it is a great saving of time and effort to deter- 

 mine transpiration and chresard in the same experiment; this is particu- 

 larly 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. Tlie 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 transpira- 

 tion of a typical sun plant, such as Touterea rmiUHlora, with that of a shade 

 plant, such as Washingtonia obtusa. But the simultaneous study of plants 

 like Chamaenerium angnstifolhim, Gentiana acuta, Scutellaria brittonii 

 etc., which grow in several different habitats, furnishes direct and funda- 

 mental evidence of the course of adjustment and adaptation. 



Hesselmann^, in his study of open woodlands in Sweden, has employed 

 a method essentially similar to the preceding. Young plants of various 

 species were transferred to pots in the field, where they were allowed to 

 grow for several months before a series of weighings was made to determine 

 the amount of transpiration. Since weighing is the measure used in each, 

 both methods are equally accurate. The one has a certain advantage in that 

 the pots are, perhaps, more easily handled, while the other has the advantage 



^ Zur Kenntnis des Pflanzenlebens schwedischer Laubwiesen. Beih. Bot. Cent., 

 18:311. 1904. 



