LEAVES AND THEIR FUNCTIONS 



135 



Pass the petiole of a green leaf through a cardboard cover into a glass of 

 water. Seal the space between the petiole and cardboard with grafting 

 wax (chewing gum will do). Invert another glass over the first. After an 

 hour, examine the upper glass and notice where the moisture has collected. 

 How did it get there? 



The following experiment shows that the amount of water passed off 

 through the leaves by the process called transpiration is great enough to be 

 measured. Cover with a rubber cloth a flowerpot in which a vigorous plant 

 is growing, so that only the stem and leaves are outside of the rubber cover. 

 Before fastening the cloth, water the plant well. Now place the flower pot 

 upon a pan of a balance and put weights in the other pan until the pans 

 balance. Leave the plant in a sunny window for several hours, and after a 

 given time note the position of- the pans. Measure the amount of water 

 lost by removing weights till the pans balance again. 



Amount of Water Lost by Transpiration. — A relatively large 

 amount of water passes off by transpiration every twenty-four 

 hours. A small grass plant on a summer's day evaporates more 

 than its own weight in water. This would make nearly half 

 a ton of water distributed to the air during twenty-four hours 

 by a grass plot, twenty-five by one hundred feet, the size of the 

 average city lot. According to Ward, an oak tree may pass off 

 two hundred and twenty-six times its own weight in water during 

 the season from June to October. 



From which Surface of the Leaf is Water Lost ? — In order to 

 find out whether water is passed out from any particular part of 



Experiment to show through which surface of a leaf water passes off. 



