EVAPORATION OF WATER 



163 



of reserve food into the ears in a single day. The formation of 

 fruit and the growth of grain, potatoes, and other food crops 

 show the economic importance of the work of green leaves. Not 

 only do plants make their own food and store it away, but they 

 make food for animals as well; and the food is stored in such a 

 stable form that it can be kept and sent to all parts of the world. 

 Animals, herbivprous and flesh-eating, man himself, all are depend- 

 ent upon the starch-making processes of the green plant for the 

 ultimate source of their food. When we consider that in 1928 in 

 the United States the total value of all farm crops was about 

 $12,000,000,000, and when we realize that these products came 

 from the air and soil through the energy of the sun, we may under- 

 stand why the study of plant biology is of great importance. 



Practical Exercise 10. Make a table in which you list all the food products 

 obtained in your community from green plants. 



Water is given off from the leaf. Much more water is taken in 

 by the plant than is used by the plant. This water is given off 

 through the leaves. 



Demonstration 12. Take some well-watered potted green plant, as a 

 geranium or hydrangea, cover the pot with sheet rubber, fastening the 



jar. 



Covered 

 vith sheet 

 rubber— 



stare 



24 hours later 



rubber close to the stem of the plant. Next weigh the plant with the 

 pot. Then cover it with a tall bell jar and place the apparatus in the 

 sun. In a short time drops of moisture are seen to gather on the inside 

 of the jar. If after a few hours we weigh the potted plant again, we 

 find it weighs less than before. Obviously the loss comes from the 

 water vapor which has escaped from stem, or leaves, or both. 



