18 PRACTICAL BOTANY 



18. The work of the leaf : transpiration. When a potted 

 plant, so covered that no moisture can escape from the pot 

 or earth, is placed under a dry bell jar, within a few hours 

 moisture is seen to collect upon the inner surface of the jar. 

 After a longer time the amount of moisture may cause 

 streams or large drops of water to run down the inner wall 

 of the jar. 



If a plant that wi]l thrive with its roots in water is planted 

 in a jar of water and carefully sealed around the stem, and 

 the whole apparatus weighed from day to day, a constant loss 

 of water may be demonstrated. Water is ordinarily taken up 

 by the plant in much larger quantities than are used for the 

 work of photosynthesis. Large amounts of water are carried 

 into the air through the leaves. By making careful demon- 

 strations of the weight and volume of this water loss and the 

 area of the leaf surface that is exposed, it is possible to deter- 

 mine the 'amount of water which, on an average, passes through 

 each square inch of leaf surface in a given time. This evapo- 

 ration or loss of water from the plant is known as transpira- 

 tion, and the current of water thus passing through the plant 

 is called the transpiration current. Water evaporates from the 

 stomatal openings or from other parts of the leaf surface. As 

 superficial evaporation occurs, water from the moister portions 

 of the plant must take the place of that evaporated, or there 

 is danger of injury to the plant. Such danger and resulting 

 death often occur, due to great or sudden loss of water. 



The quantity of water loss in transpiration is often surpris- 

 ingly great. It has been estimated in one case that a beech tree 

 110 years old, in one summer transpired approximately 2250 

 gallons of water ; that an oak tree with 700,000 leaves tran- 

 spired about 180 gallons of water daily ; and that an acre of 

 cabbages in their growing season (about four months) tran- 

 spired 500,000 gallons of water. One can scarcely picture in 

 his mind the immense quantity of water that is constantly 

 transpiring from all the vast stretches of forests, grasslands, 

 farm crops, roadside weeds, and swamp plants. 



