Transpiration as a Factor in Crop Production. 21 
number has since been greatly increased. Few phases of the sub- 
ject have been left uninvestigated. This, however, does not nec- 
essarily mean that the problems have been solved. On the con- 
trary, that stage in the work has now been reached when we can 
look upon the past largely as preliminary, preparing the way for 
more significant experiments. This is not to be considered in 
the light of discredit to the earlier workers. Methods of obser- 
vation have simply undergone a natural evolution. The results of 
many experiments have been rendered questionable by a full 
appreciation of the sources of error entering into them. 
The factors influencing transpiration determinations may be 
grouped into two great classes: (1) Factors influencing transpira- 
tion determinations as sources of experimental error, and (2) 
factors influencing transpiration in fact. The significance of the 
results of the numerous investigations which have been pub- 
lished depends upon the reliability of the methods by which 
they were obtained. 
OUTLINE OF METHOD PRINCIPLES FOR STUDYING 
TRANSPIRATION. 
The general principles involved in the various methods of 
observation may be outlined as follows: 
I. Basis of comparison. 
1. Total transpiration per plant. 
2. Transpiration per unit of leaf -area. 
3. Transpiration per unit of weight. (Green or dry weight.) 
a. Weight of grain. 
b. Total weight minus roots. 
c. Total weight including roots. 
II. Manner of determining the transpiration. 
1. Demonstrative. 
a. Cobalt method. 
2. Quantitative. 
a. Field estimates. 
b. Collecting and measuring the water transpired. 
c. The potometer method. 
I 1 . Type. 
a 1 . Measuring the intake of water. 
b 1 . Determining the amount transpired by loss 
in weight. 
2 1 . Character of plant. 
a 1 . Cuttings. 
b 1 . Seedlings. 
c 1 . Mature or intermediate stages. 
