26 Nebraska Agricultural Exp. Station, Research Bui. 6. 
between transpiration and atmospheric conditions by the use of 
small plants, even in the seedling stage. However, these are 
limited because of the rapid change in the size of the plants and 
must, with a few exceptions, be regarded more as demonstrative 
than as quantitative measurements. 
SUMMARY OF DISCUSSION ON METHOD PRINCIPLES. 
In summarizing the discussion of the principles involved in the 
various methods which have been used for quantitative transpira- 
tion determinations, it is concluded that the use of potometers 
with rooted plants alone can be relied upon to furnish significant 
results. Plants grown beyond the seedling stage are indispensable 
when the relative transpiration is based upon the production of 
dry matter. This basis of comparison is of chief interest from an 
applied agricultural standpoint. However, certain principles of 
response to variable conditions can be satisfactorily determined 
by the loss per unit of leaf -area and the total loss per plant during 
a given length of time. 
DETERMINING THE TRANSPIRATION OF FIELD CROPS. 
To determine the relation of transpiration to production it is 
necessary to secure the ratio of water loss to total dry matter 
produced, and in the case of grain crops also to the yield of grain. 
To attempt any quantitative application to field conditions 
it is desirable also to know the total water transpired per plant, 
the total yield of dry matter per plant, and the normality of growth. 
In comparisons of different kinds of crops these must also be 
grown under conditions comparable to the field conditions of 
the particular crop tested. 
To determine merely the relative effects of two differing factors, 
such as soil moisture, soil fertility, or relative atmospheric humid- 
ity, it is not so essential to have the plants exposed to normal 
field climatic conditions. It is very desirable, however, to have as 
complete a record as possible of all environmental factors which 
may influence the transpiration rate. 
SOURCES OF ERROR IN POTOMETER TESTS. 
The potometer method in which growing rooted plants are 
grown beyond the seedling stage is correct in principle for study- 
ing the water requirements of crops. This method is itself, how- 
ever, subject to sources of error, many of which must be eliminated 
in most part in order to secure significant and applicable results. 
Since we must rely upon this method for securing transpiration 
