The "Work of the Leaves 



Starch contains carbon, hydrogen and oxygen, the last two 

 in the exact proportion that they bear to each other in water, 

 H2O. The carbon comes in as carbon dioxide, CO2. There is no 

 lack of this familiar gas in the air. It is exhaled constantly from 

 the lungs of every animal, from chimneys and from all decaying 

 substances. It is diffused through the air, and, entering the leaves 

 by the stomates, comes in contact with other food elements in the 

 palisade cells. 



The power that runs this starch factory is the sun. The 

 chlorophyll, or leaf green, which colours the clear protoplasm of the 

 cells, is able to absorb in daylight (and especially on warm, sunny 

 days) some of the energy of sunlight, and to enable the protoplasm 

 to use the energy thus captured to the chemical breaking down of 

 water and carbon dioxide, and the re-uniting of their free atoms 

 into new and more complex molecules. These are molecules of 

 starch, CeHioOs. 



The new product in soluble form makes its way into the cur- 

 rent of nutritious sap that sets back into the tree. This is the 

 one product of the factory — the source of all the tree's growth — for 

 it is the elaborated sap, the food which nourishes every 

 living cell from leaf to root tip. It builds new wood layers, 

 extends both twigs and roots, and perfects the buds for the 

 coming year. 



Sunset puts a stop to starch making. The power is turned 

 off till another day. The distribution of starch goes on. The 

 surplus is unloaded, and the way is cleared for work next day. 

 On a sunless day less starch is made than on a bright one. 



Excess of water and of free oxygen is noticeable in this making 

 of starch. Both escape in invisible gaseous form through the 

 stomates. No carbon escapes, for it is all used up, and a con- 

 tinual supply of CO2 sets in from outside. We find it at last in 

 the form of solid wood fibres. So it is the leaf's high calling to 

 take the crude elements brought to it, and convert them into food 

 ready for assimilation. 



There are little elastic curtains on the doors of leaves, and 

 in dry weather they are closely drawn. This is to prevent the 

 free escape of water, which might debilitate the starch-making 

 cells. In a moist atmosphere the doors stand wide open. Evapo- 

 ration does not draw water so hard in such weather, and there is 

 no danger of excessive loss. "The average oak tree in its five 



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