THE LEAF 173 



189. The activities of leaves. As there are only 4 parts 

 of CO 2 to every 10,000 parts of ordinary free air, it has been 

 estimated that in order to supply the leaf factory with the 

 raw material it needs, an active leaf surface of one square 

 meter a little over one square yard uses up, during 

 every hour of sunshine, the CO 2 contained in 1000 liters 

 (1000 quarts, approximately) of ah-. Suppose an oak tree 

 to bear 500,000 leaves, each having a surface of 16 sq. cm., or 

 4 sq. in., and working 12 hours a day for 6 months in the 

 year; you will then have some idea of the enormous quantity 

 of air thafpasses each season through its leaf system. Add 

 to this the almost incredible volume of water transpired in 

 the same time (180), and we may well stand amazed at the 

 tremendous activities of these silent workers that we are in 

 the habit of regarding as mere passive elements in the 

 general landscape. 



190. The economic value of leaves. Besides their im- 

 portance as sanitary and food-making agencies, leaves have 

 a direct commercial value as food products in the hay and 

 fodder they supply for our domestic animals, the tea and 

 salads with which they provide our tables, the aromatic 

 flavors and seasonings contained in them, and the drugs, 

 medicines, and dyes of various kinds for which they furnish 

 the ingredients. 



Practical Questions 



1. Why do gardeners "bank" celery? (Exp. 65.) 



2. Why are the buds that sprout on potatoes in the cellar, white ? (Exp. 

 65.) 



3. Why does young cotton look pale and sickly in long-continued wet 

 or cloudy weather? (Exp. 65.) 



4. Why do parasitic plants generally have either no leaves or very 

 small, scalelike ones? (85, 186, 187.) 



5. The mistletoe is an exception to this; explain why, in the light of 

 your answer to question 4. 



6. Could an ordinary nonparasitic plant live without green leaves? 

 (186, 187.) 



7. Are abundance and color of foliage any indication of the health of 

 a plant? (166, 187; Exp. 65.) 



