THE STRUCTURE OF WOOD. 



13 



(3) Tangential, a longitudinal plane not including the organic axis. 



If a transverse section of the trunk of a conifer or of a broad- 

 leaved tree is made, it is to be noted that it consists of several distinct 

 parts. See Fig. 4. These, beginning at the outside, are : 



(1) Rind or bark 



( a ) Cortex 



(b) Bast 



( 2 ) Cambium 



(3) Wood 



( a ) Sap-wood 



(b) Heart- wood 



(4) Pith. 



(1) The rind or linrh is 

 made up of two layers, the 

 outer of which, the "cortex," 

 is corky and usually scales or 

 pulls off easily ; while the in- 

 ner one is a fibrous coat called 

 ■'bast" or "phloem." To- 

 gether they form a cone, wid- 

 est, thickest, and roughest at 

 the base and becoming nar- 

 rower toward the top of the 

 tree. The cortex or outer 

 bark serves to protect the stem 

 of the tree from extremes of 



heat and cold, from atmospheric changes, and from the browsing of 

 animals. It is made up of a tough water-proof layer of cork which 

 has taken the place of the tender skin or "epidermis" of the twig. 

 Because it is water-proof the outside tissue is cut off from the water 

 supply of the tree, and so dries up and peels off, a mass of dead 

 matter. The cork and the dead stuff together are called the bark. 

 As we shall see later, the cork grows from the inside, being formed 

 in the inner layers of the cortex, the outer layers of dry bark being 

 thus successively cut off. 



The characteristics of the tree bark are due to the positions and 

 kinds of tissue of these new layers of cork. Each tree has its own 

 kind of bark, and the bark of some is so characteristic as to make 

 the tree easily recognizable. 



Vig. 4. Diagram of Cross-section of Three 

 Year Old Stem of Basswood. 



