164 FOUNDATIONS OF BOTANY 
is attached, in eosin solution. Repeat with another leaf and stand 
in water. What do the results teach? 
174. Total Amount of Transpiration. — In order to pre- 
vent wilting, the rise of sap during the life of the leaf 
must have kept pace with the evaporation from its sur- 
face. The total amount of water that travels through the 
roots, stems, and leaves of most seed-plants during their 
lifetime is large, relative to the weight of the plant itself. 
During 173 days of growth a corn-plant has been found to 
give off nearly 31 pounds of water.. During 140 days of 
growth a sunflower-plant gave off about 145 pounds. A 
grass-plant has been found to give off its own weight of 
water every twenty-four hours in hot, dry summer weather. 
This would make about 6} tons per acre every twenty-four 
hours for an ordinary grass-field, or rather over 2200 pounds 
of water from a field 50 x 150 feet, that is, not larger than 
a good-sized city lot. Calculations based on observations 
made by the Austrian forest experiment stations showed 
that a birch tree with 200,000 leaves, standing in open 
ground, transpired on hot summer days from 700 to 900 — 
pounds, while at other times the amount of transpiration 
was probably not more than 18 to 20 pounds." 
These large amounts of water are absorbed, carried 
through the tissues of the plant, and then given off by the 
leaves because the plant-food contained in the soil-water 
is in a condition so diluted that great quantities of water 
must be taken in order to secure enough of the mineral 
and other substances which the plant demands from the 
soil. Active transpiration may also have other causes. 
1 See B. E. Fernow’s discussion in Report of Division of Forestry of U. S. 
Department of Agriculture, 1889. 
