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. 



I See B. E. Femow's discussion in Beport of Division of Forestry of IT. S. 

 Department of Agriculture, 1889. 



