1908] FREEMAN—DETERMINATION OF TRANSPIRATION 125 
It will be noted again from Table IT that the transpiration rate was 
greatly reduced during the time the plant was in the cylinder. This 
was of course due to the increase in relative humidity in the air, owing 
to the transpiration from the leaves. It may be suggested, therefore, 
that the plant was thus shown to be under abnormal conditions. 
However, the normal air in the room was at a temperature of 27° C. 
and contained 10.9" water per liter. According to the Smithsonian 
tables, such air at saturation contains 25.4™8 per liter; the relative 
humidity of the normal air was therefore 42 .5 percent. Since 19.5 
liters of air were drawn through the transpiration cylinder, from 
which the P,O, collected 343™ of water, the air in the cylinder con- 
tained 17.5™8 per liter. This would give a relative humidity of 
68.8 per cent. The change in relative humidity from the outside air 
to that inside the cylinder was thus seen to be from 42.5 per cent. to 
68.8 per cent. This cannot be termed abnormal, since much greater 
changes in the outside air take place from day to day, and the range 
In a given day is very often even wider. Since the air was completely 
changed in the cylinder every thirty-six seconds, this condition of 
meidity must have been reached during that time, and have remained 
“onstant for the remainder of the hour. Owing to changes in tempera- 
ture and light a plant in the open will vary much more in the transpira- 
ton rate in the course of a few hours than did the alfalfa plant when 
Placed inside the cylinder. Until the exact effect of light, humidity, 
and temperature are known, therefore, and reduced to formulae 
mieteby, the conditions of the given transpiration experiment being 
known, the transpiration constant for that plant can be estimated, 
fe Parative transpiration experiments must be made at the same 
 . under conditions as nearly identical as possible. The 
to the de = repeuments are moreover valuable only in proportion 
sree in which identical conditions are approached. 
one er to test the practicability of this method in the field, I 
Which tive hapes esas growing within about 12™ of each other, 
ey had :. marked differences in form and texture of the leaves. 
therefore of . previously cut at the same time and the stems were 
to bloom +, or SOU binge OF growth, that is, just asses 
On these va Ja simultaneous comparative experiments were made 
© plants, in order to ascertain whether apy constant 
