182 Our Field and Forest Trees 



a twig to drink some water containing a strong dye, 

 and then he could follow the travels of the sap as 

 it mounted through the wood. He found that 

 when he kept bending the twig to and fro with his 

 fingers the sap was forced to rise much faster. 



The tree sways with every breath of wind. 

 Every time it bends, the wood-cells are squeezed 

 and the sap is forced out of them. Every time it 

 straightens again, the little tubes and cells fill with 

 sap from below. 



But no one can really understand or fully ex- 

 plain the rise of the sap. 



From the lowest root-tip to the topmost twig 

 of a gigantic tree, water may have to travel a 

 distance of three or four hundred feet. For much 

 of this distance it must climb straight up, and all 

 the journey is through tubes as fine as a hair. We 

 must remember, too, that these tubes are not con- 

 tinuous, like the water-pipes of a house. The sap 

 goes up through a long series of wood-cells like 

 little oblong boxes piled end to end. 



Later in the year, when many leaves are spread 

 wide in the sunshine, water Is breathed away from 

 the upper part of the tree. Some of the cells up 

 there part with so much moisture that their sap 

 becomes thickened, and this causes a suction which 

 draws more watery sap up through the wood from 

 below. 



When summer is here, and growth is active, 

 there are, furthermore, little bubbles of gas in the 



