loo THE TREE BOOK 



tubes. Nor are these tubes continuous, like 

 water-pipes in a house; they are a series or 

 chain of wood cells, "like little oblong boxes 

 piled end on end. ' ' A thin membrane separates 

 each tiny cell from its neighbor, adding just that 

 much more to the complexity of the situation. 

 How does the sap ever get through? 



The sap in the cells is much denser than the 

 soil-water in the root hairs. So, by a natural 

 law which governs fluids, the soU-water seeps 

 through the tMn cell wall and mingles with the 

 sap. This extra fluid distends the tiny chanl- 

 ber, and it swells and expands until its walls 

 will give no more. Now the soil-water has di- 

 luted the sap-fluid in the cell until it is much 

 thinner than that in the cell above ; so following 

 the same rule of attraction, the sap is drawn 

 up into the denser cell, and so on and on, until 

 it has mounted to the very topmost twig. 



Some think that the sap is carried through 

 trunk, branch, and twig by the action of the cell 

 walls, much the same as food is conveyed 

 through the cells in animal bodies. Then again 

 some scientist upsets all theories by proclaim- 

 ing that poisonous solutions, strong enough to 

 kill all cell life, are regularly carried in large 

 quantities to the tops of the tallest trees. So, 

 speculate as we may, the mounting sap is in a 



