48 UNDER THE OPEN SKY 



the sap up the trunk. Certain it is that 

 many agencies unit in this work. The tips 

 of the roots of the trees are covered with 

 a fine plush-like coating of delicate hairs. 

 Into these the water soaks from the ground, 

 and once there and mixed with the sub- 

 stances inside, it cannot soak out nearly so 

 readily. This is just what happens when, 

 to use a familiar illustration, prunes are put 

 to soak in water over-night. By morning 

 the water has filled the fruit until it is 

 plump, and if then a tube were inserted 

 through the skin from this a stream of prune 

 juice would slowly flow. 



The watery sap taken up by the root- 

 plush is passed by the hairs to pipes be- 

 ginning near the heart of the root, and then 

 up the stem. The very fact that these 

 pipes are small makes the water rise in 

 them just as ink soaks into a blotter or oil 

 rises in the wick of a lamp. 



Meanwhile the water is evaporating from 

 the upper part of the tree, and is in this way 

 producing a diminished pressure which 

 serves to draw the sap to higher levels. The 



