WYDROHARMOSE 113 
and most convenient measure of absorption under normal conditions can be 
obtained through transpiration, since the discrepancy between absorption and 
transpiration is scarcely larger than the error of any method applicable 
to the field. Conversely, the measure of absorption obtained by the process 
described in the preceding section serves also as a measure of transpiration. 
The determination of the latter in the field is so much simpler, however, 
that it is rarely desirable to apply the absorption method. 
157. Measurement of transpiration. The water loss of a plant may be 
determined absolutely or relatively. Absolute or quantitative determinations 
are by (1) weighing, (2) collecting, or (3) measuring the water absorbed ; 
relative values are indicated by hygroscopic substances. A number of 
methods have been employed more or less generally for measuring trans- 
piration. The great majority of these can be used to advantage only in the 
laboratory, and practically all fail to meet the fundamental requirement for 
successful field work, namely, that the plant be studied under normal con- 
ditions in its own habitat. The following is a summary of the various 
methods, the details of which may be found in Burgerstein. 
t. Weighing. This is the most satisfactory of all methods for deter- 
mining water loss. It is more accurate than any other, and is unique in that 
it does not place the plant under abnormal conditions. On the score of 
convenience, moreover, it excels every other method capable of yielding 
quantitative results. Various modifications of weighing are employed, but 
none of these have all the advantages of a direct, simple weighing of the 
plant in its own soil. 
2. Collecting the water transpired. This may be done by collecting 
and weighing the water vapor exhaled by a plant placed within a bell jar, 
or by weighing a deliquescent salt, such as calcium chloride, which is used 
to absorb the water of transpiration. The decisive disadvantage of these 
inethods is that transpiration is carried on in an atmosphere far more humid 
than normal. If an excessive amount of salt is used, the air is abnormally 
dry. In both cases, the water loss decreases until it reaches a point much 
below the usual amount. Finally, all methods of this kind are open to-con- 
siderable error, and are inconvenient, especially in field work. They aie. 
of relatively slight value in comparison with weighing. 
3. Potometers. It has already been shown that the amount of water 
absorbed is a close measure of the amount transpired. In consequence, the 
potometer can be used to determine the amount of transpiration provided 
the absorption is not abnormal. It is rarely and only with much difficulty 
that this condition can be met. The use of cut stems and branches does not 
meet it, and even in the case of plants with roots, the results must be 
