8 TREES 



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 continual supply of CO^ 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. Evaporation does not draw water so 

 hard in such weather, and there is no danger of excessive 

 loss. "The average oak tree in its five active months 

 evaporates about 28,000 gallons of water" — an average of 

 about 187 gallons a day. 



In the making of starch there is oxygen left over — just 

 the amount there is left of the carbon dioxide when the 

 carbon is seized for starch making. This accumulating 

 gas passes into the air as free oxygen, "purifying" it for 

 the use of all animal life, even as the absorption of carbon 

 dioxide does. 



When daylight is gone, the exchange of these two gases 

 ceases. There is no excess of oxygen nor demand for 

 carbon dioxide until business begins in the morning. But 

 now a process is detected that the day's activities had 

 obscured. 



The living tree breathes — inhales oxygen and exhales 

 carbonic-acid gas. Because the leaves exercise the func- 



