Transpiration as a Factor in Crop Production. 
71 
degrees lower temperature, the buried potometers used 3 per 
cent more water per gram of dry matter than those fully exposed 
in the pit. 
VARIATION IN RESULTS FROM SIMILAR POTOMETERS AND 
REDUCTION OF ERROR BY REPETITION. 
Forty-eight standard potometers were planted to Hogue's 
Yellow Dent corn in 1912 and 80 potometers in 1913. The weight 
of moisture-free soil was 248 pounds in 1912 and 243 pounds in 
1913. The potometers were filled with soil from a productive 
field taken to a depth of 6 inches and no additional fertilizer 
was added. The soil-moisture content was maintained constant 
by adding water to the jars at the base. The conditions were 
similar each year for all plants tested, and the variations between 
the different plants in any one year may be regarded as indicat- 
ing the amount of experimental error to be expected between 
individual potometers due to individuality of the plants. The 
detailed data for 1912 for the individual potometers are given in 
Table 12. The potometers have been averaged in groups of 
four adjacent potometers to correspond to one of the regular 
experiments in which the effects of differences in soil moisture, 
soil fertility, or variety are compared. The 80 plants grown in 
1913 and the ears harvested from them are shown in Figures 
13 and 14. 
The results from the 48 potometers of 1912 and the 80 potom- 
eters of 1913 have been compiled in Tables 13 to 21 so as to show 
the extreme variation and the coefficient of variability when 
single potometers are used and when grouped in sets of two and 
four each. Since variation varies with frequency, these coefficient 
of variability tables have been prepared so that the averages are 
for the same frequency, whether single potometers or the mean 
of two or four potometers is used. 
The coefficients of variability for the two years may be sum- 
marized as in Table 22. 
The data show a consistent reduction in the coefficient of 
variability as the number of potometers averaged together is 
increased. However, considerable variation still exists when 
four potometers are averaged together, and the data will serve 
to guard against attaching too much importance to very small 
differences in the experiments which follow. 
The extreme variation between groups of four is also seen to 
be rather high, and this must be kept in mind when interpreting 
results. 
