The Work of the Leaves 



together, and perpendicular to the flat surface. There are some- 

 times more than one layer of these cells. 



In the lower half of the leaf's thickness, between the palisade 

 cells and the under surface, the tissue is spongy. There is no 

 crowding of cells here. They are irregularly spherical, and cohere 

 loosely, being separated by ample air spaces, which communicate 

 with the outside world by the doorways mentioned above. An 

 ordinary apple leaf has about one hundred thousand of these 

 stomates to each square inch of its under surface. So the ventila- 

 tion of the leaf is provided for. 



The food of trees comes from two sources — the air and the 

 soil. Dry a stick of wood, and the water leaves it. Burn it now, 

 and ashes remain. The water and the ashes came from the soil. 

 That which came from the air passed off in gaseous form with the 

 burning. Some elements from the soil also were converted by the 

 heat into gases, and escaped by the chimneys. 



Take that same stick of wood, and, instead of burning it in 

 an open fireplace or stove, smother it in a pit and burn it slowly, 

 and it comes out a stick of charcoal, having its shape and size and 

 grain preserved. It is carbon, its only impurity being a trace of 

 ashes. What would have escaped up a chimney as carbonic-acid 

 gas is confined here as a solid, and fire can yet liberate it. 



The vast amount of carbon which the body of a tree contains 

 came into its leaves as a gas, carbon dioxide. The soil furnished 

 various minerals, which were brought up in the "crude sap." 

 Most of these remain as ashes when the wood is burned. Water 

 comes from the soil. So the list of raw materials of tree food is 

 complete, and the next question is: How are they prepared for the 

 tree's use? 



The ascent of the sap from roots to leaves brings water with 

 mineral salts dissolved in it. Thus potassium, calcium, mag- 

 nesium, iron, sulphur, nitrogen and phosphorus are brought to the 

 leaf laboratories — some are useful, some useless. The stream 

 of water contributes of itself to the laboratory whatever the leaf 

 cells demand to keep their own substance sufficiently moist, and 

 those molecules that are necessary to furnish hydrogen and oxygen 

 for the making of starch. Water is needed also to keep full the 

 channels of the returning streams, but the great bulk of water that 

 the roots send up escapes by evaporation through the curtained 

 doorways of the leaves. 



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