22 Nebraska Agricultural Exp. Station, Research Bui. 6. 
A number of very ingenious and in some cases intricate modifi- 
cations and combinations of the methods outlined have been 
devised. However, the principles involved are the vital part, 
and for these the outline is inclusive. 
BASIS OF COMPARISON. 
(1) Comparing transpiration rates solely by total water loss 
per plant is of significance only in special cases. During short 
periods in which the leaf -area and thrift of the plant are practically 
constant, the effect of variations in environmental factors on the 
total water requirement for a unit of time may be noted. To 
know the total amount of water used in the normal growth of a 
plant is also of value in calculating the approximate amount of 
water required for a unit of field area of a similar crop, provided 
growth conditions are comparable. The relative total transpira- 
tion cannot be regarded as an index of relative growth. 
(2) Certain interrelationships between the plant and its 
atmospheric environment may be studied by basing the relative 
transpiration rates upon a unit of leaf-area. The effect of dif- 
ference in leaf structure and condition of leaf may also be observed 
by the water loss from a unit of leaf -area. 
(3) From an economic standpoint the most important basis 
for comparison is the amount of water transpired in the production 
of a unit of dry matter. To obtain these data it is requisite that 
the total yield of crop and the total water consumption be secured, 
which data furnish a basis for making all important deductions 
in which the yield of dry matter is concerned. 
The total water transpired, the amount of water used per unit 
of dry matter, and the amount per unit of leaf-area bear no neces- 
sary relation to one another. The relative effect of soil differences 
has very frequently been measured solely in terms of total loss 
per plant or loss per unit of leaf-area during only a fraction of 
the lifetime of the plant. Since no necessary relation exists be- 
tween these characters and the addition of dry matter in the plant, 
the data are inapplicable and are of little importance. 
METHODS OF DETERMINING TRANSPIRATION. 
THE COBALT METHOD. 
The methods which have been employed may be grouped first 
according to whether they are largely demonstrative, or whether 
quantitative measurements of water loss may be made. The 
cobalt test is a case of the former, in which a blue sheet of cobalt 
paper held in close contact with the transpiring surface loses its 
