I 8 THE LEAF 



a sprig of the dwarf St. John's-wort {Hypericum mutilum) 

 or of wandering Jew into water and notice the silvery 

 appearance of the leaves, especially on the under side. 

 In the iris it is the same on both sides ; why ? Remove 

 the sprig from the water, and the leaves will be perfectly 

 dry. In the wandering Jew, as may be seen with a good 

 hand lens, this is due to the air imprisoned by little mem- 

 branous appendages which surround the stomata and pre- 

 vent the water from entering. In other cases, as cabbage, 

 hypericum, etc., a coating of wax protects the transpiration 

 pores, and it is the reflection of the light from the air 

 entangled in these protective coverings that gives the 

 leaves their silvery appearance under water. 



* 

 19. Amount of Transpiration. — Few people have any 



idea of the enormous amount of water given off by leaves. 

 It has been calculated^ that an oak may have 700,000 

 leaves and that 111,225 kilograms of water (about 244,695 

 lbs.) may pass from its surface in the five active months 

 from June to October, and 226 times its own weight of 

 water may pass through it in a year. If this seems an 

 extravagant estimate, we can easily make one for our- 

 selves. 



Fill three bottles with water, and cover them tightly with 

 rubber cloth to prevent evaporation. Mark the point at 

 which the water stands in the bottles, make a small 

 puncture through the covers, and insert into one bottle 

 the end of a healthy twig of peach or cherry, into the 

 second a twig of catalpa, grape, or any other large-leaved 

 plant, and into the third, one of magnolia, holly, or other 

 thick, tough-leaved evergreen, letting the stems of all reach 

 down well into the water. Care must be taken to select 

 twigs of the same age, as the absorbent properties of very 

 young stems are more injured by cutting and exposure than 

 those of older ones. All the specimens should be cut under 

 water if possible, as even an instant's exposure to the air 

 will greatly diminish the activity of the cut surface. Peach 



1 See Marshall Ward, "The Oak." 



