78 THE GARDEN OF EARTH 



We know that things are so. But how does it come 

 about ? That is no easy question to answer. 



Water and all liquids pour downward, never upward. 

 Not once in our lives have we ever seen a stream run- 

 ning uphill. If we tried to any extent to make it travel 

 thus we should fail. For the strong attraction of our 

 Earth, perpetually at work, draws everything towards 

 the ground, from higher to lower levels; and water 

 because of its fluid make responds quickly to that pull. 

 So it always flows downward to lower levels. 



Except in this case ! And here we find just the 

 opposite. Here we have water in large quantities, 

 taken in by the roots, mounting steadily to the tops 

 of the highest trees, going as it seems to us right in the 

 teeth of that tremendous earthward pull which we call 

 " Gravitation " or " Gravity." 



No easy matter this to account for. Many explana- 

 tions have from time to time been given, but even 

 when taken all together they cannot be said to explain 

 fully what goes on. 



It was long believed that the rise of sap was due to 

 a certain form of attraction which is found in very 

 small tubes. If you hold upright in water a tiny glass 

 tube, you will see that the water inside rises just a 

 little higher than the water outside. For the sides of 

 the tube, attracting the water inside, draw it slightly 

 upward. With a very minute tube indeed, the water 

 would rise much higher. 



Now a tree-trunk contains millions of most slender 

 tubes. Some of them may be as much as two or three 

 feet in length, and they lie up and down the trunk, 



