STRUCTURE AND GROWTH 13 



pounds known as carbohydrates (sugar and 

 starch), which, dissolved in water, make the di- 

 gested sap. It is this digested sap which alone 

 can produce growth and can furnish the living cells 

 with the nourishment they need while they are 

 growing and multiplying. Even the roots depend 

 for the material of their growth upon this digested 

 sap from the leaves, the crude fluid they absorb 

 from the soil being of no nutritive value. 



There are thus two streams normally flowing 

 through the trunk. One, flowing up, is of crude 

 sap from the roots, and the other, flowing down, 

 is of digested sap from the leaves. These streams 

 flow through separate concentric regions in the 

 tree's trunk. What has already been said about 

 the wood of the trunk being added to from the out- 

 side ought to give us a hint that the region carrying 

 the digested, growth-producing sap, is the outer — 

 and such it is. 



With these general principles in mind, let us 

 glance at a cross-section of a young oak trunk. 

 Three concentric rings are at once noticeable, the 

 dark corky bark on the outside, the heartwood in 

 the center, and the lighter sapwood between them. 

 But more important to us than any of these is a 

 fourth ring, a very inconspicuous, soft, moist layer 

 between the bark and the sapwood. This is the 

 cambium layer. 



The cambium layer is the great fact to the man 



