THE LEAF 173 
189. The activities of leaves. — As there are only 4 parts 
of CO, 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, contained in 1000 liters 
(1000 quarts, approximately) of air. 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 that passes 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. 
, 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? (186, 187; Exp. 65.) 
