96 APPLIED BIOLOGY 



sunflower plant six feet high" have been proved to evaporate 

 a liter (how many pints ?) of water per day ; and it has been 

 carefully estimated that a large birch tree with about 200,000 

 leaves gave off 500 liters of water (2J barrels) on a dry, 

 hot day and probably averaged 60 to 70 liters per day during 

 the summer. 



Estimating Loss of Water from a Plant. (D) Take a potted 

 geranium, or other convenient plant, surround the pot and cover 

 the soil with a sheet of rubber, tin-foil, or waterproof cloth, and 

 tie carefully around the stem of the plant. Or set the pot in a 

 battery-jar and cover the top with sheet rubber tied tightly around 

 both jar and stem of plant. Thus only the upper stem and leaves 

 will be exposed for evaporation. Now place the potted plant on 

 a small platform scale (preferably one with two equal platforms 

 which balance each other), and add the weights until there is an 

 exact balance. Note from day to day the loss of weight from the 

 plant. A glass funnel may be inserted through the waterproof 

 covering into the soil, and a)weighed amount of water [poured in 

 daily to replace the amount lost by evaporation. 



In order to make evaporation more rapid than the slow 

 drying from the surface cells of the leaf, the leaf-pores 

 (stomata) are sometimes opened (as described in 73), al- 

 lowing watery vapor to escape from air-spaces in the leaf 

 (Fig 30). These cavities are surrounded by cells from 

 which evaporation of water takes place rapidly. Thus the 

 giving off of water is more rapid when the leaf-pores are 

 open, and less when they are closed. The leaves of many kinds 

 of plants seem to have a very complete control of the amount 

 of water evaporated, because in these species the covering 

 cells of the leaves are thick and sometimes coated with 

 waxy substances or hairs, and in still other ways are un- 

 favorable for the evaporation of water from the surface of the 

 leaf. In such plants evaporation appears to take place 

 chiefly through the leaf -pores. When we remember that an 

 ordinary cabbage leaf has ten million and a sunflower leaf 

 thirteen million leaf-pores, we can easily understand how 



