Transpiration as a Factor in Crop Production. 
45 
The potometers were 16 inches in diameter and 3 feet deep. 
The amount of soil contents varied somewhat from year to year 
but was approximately 250 pounds of moisture-free soil. The 
exact amount is stated with each experiment. A 3-inch layer of 
gravel served to reduce surface soil moisture evaporation. Rain 
was excluded by means of a loosely fitting galvanized-iron lid 
having a 4-inch opening for the plant. After the plant had 
attained a height of 18 inches a further protection against the 
entrance of rain was added in the form of an oilcloth covering 
over the opening about the plant, which was sealed to the stalk 
by means of a 3-inch collar of plastic modeling clay (plastocene) 
illustrated in Figure 3. 
Fig. 3. — Potometer lid showing oilcloth and plastocene seal about the plant. 
The leaves of a corn plant act as a funnel, and it has been determined 
that without this close seal at the base approximately 6| pounds of water 
will be conducted along the stalk of a full-grown plant into the potometer 
for each inch of rain. The seal is so made that the plant may sway in 
the wind without disturbing the seal. 
1, potometer wall; 2, galvanized-iron lid; 3, oilcloth; 4, oilcloth; 5, corn 
stalk; 6, plastocene; 7, oilcloth girdle. 
Buried in the soil and connected at the top with a covered 
6-quart can, was a spiral coil made of f-inch brass tubing 16 feet 
in length' and perforated every 8 inches. All water was added 
in weighed quantities thru the small can and distributed rather 
uniformly thruout the soil by means of the perforated coil. 
