HYDROHARMOSE 115 
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 individuai 
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 soi! 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 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 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 
1Zur Kenntnis des Pflanzenlebens schwedischer Laubwiesen. Beih. Bot. Cent,, 
18:311. 1904, 
