HYDROIIARMOSE 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 (i) 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. 



1. 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 zvater 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 

 .methods 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 



