24 Nebraska Agricultural Exp. Station, Research Bui. 6. 
an absorbent salt is used, and a leaf or twig of a growing plant is 
inserted into it. A number of intricate and ingenious modifica- 
tions have been devised. The increased weight of the salt indicates 
the amount of transpiration from a given area. This method 
would appear to be fundamentally wrong in that the leaves in- 
closed are not subjected to the same climatic influences as the 
main portion of the plant tested and consequently will not be 
representative of the transpiration taking place from the plant. 
A slight difference in the climatic factors may have a marked 
effect upon the transpiration rate. If the method were reliable, 
it would be of value in measuring transpiration from plants rooted 
in their natural habitat, and which cannot be transferred to pots 
for weighing. 
THE POTOMETER METHOD. 
The potometer method consists in the use of a vessel filled with 
water, nutrient solution, sand culture, or soil, by which the amount 
of transpiration is determined from contained cuttings or rooted 
plants, either by loss in weight or by measuring the intake of 
water. 
MEASURING THE INTAKE OF WATER. 
Early investigators determined the transpiration extensively by 
measuring the amount removed from the containing vessel. The 
base of a cutting or seedling would be inserted in a sealed bottle 
filled with water or nutrient solution, and having a graduated 
capillary tube by means of which the loss could be read in cubic 
millimeters. The method also had strong opponents who declared 
that the intake of water could not be a measure of the outgo from 
the plant. They were justified in that the water content of the 
plant varies at times according to turgescence, and more water 
may be taken in than is transpired, or somewhat more may be 
transpired than is taken in. Such a discrepancy is, however, 
only serious when the total losses are very small and for only very 
short intervals — as was customary with this method. Error due 
to variations in the amount of water contained by the plant 
caused by differences in turgidity would, in general, be negligible 
in its effect upon the total amount of water transpired in growing 
to maturity. 
LOSS IN WEIGHT. 
Determining transpiration by loss in weight of the potometer 
with its contained plant is correct in principle, and, when healthy 
rooted plants are used, most nearly approaches natural con- 
ditions. If the necessary precautions are taken, it registers quite 
accurately the amount of transpiration under the conditions of 
the test. When the experiment extends over any great length of 
