20 THE TREE BOOK 



The watery parts of the wood, which pass off 

 as vapor when it burns, come into the tree by 

 way of the tiny root hairs which reach out in 

 every direction to find plant food. Much of the 

 gases which go up in flame and smoke have en- 

 tered by the same lowly door, as elements of 

 the soil water. All must be worked over in 

 the leaf laboratories before it can be used to 

 build new tissue. There are regular avenues 

 leading from the roots to the leaves and back 

 again. Raw material travels in one: elabor- 

 ated, life-giving sap in the other. The upward 

 route is by way of the youngest fibers of the 

 sap wood. The return trip is made through the 

 inner bark or cambium, by way of slender, thin- 

 walled tubes, of most delicate tracery. 



Given the proper elements, leaves can make 

 "not only wood, but cork, the tender petals of 

 flowers, the flesh of fruits, and a large number 

 of gums, oils, essences, and perfumes, which 

 have become indispensable in art, manufactures, 

 and medicine." How wondrous must be the 

 machinery compressed between the thin leaf 

 walls ! Let us see what we can find out about 

 it, with the aid of a compound microscope. 



First: We note that the leafy skeleton — ^the 

 veins and arteries — divides and subdivides into 

 the tiniest capillaries, a perfect network of 



