STEM STRUCTURE 



77 



" green layer " (cortex), and within this the bast fibers and tubes, 

 which transfer liquids downward and give toughness to the bark. 



FIG. 16. Diagram of maple stem show- 

 ing the development of wood and bark 

 through first and second years. At the tip 

 is a mass of living formative material 

 (shown unshaded) from the sides of which 

 arise protrusions that become leaves. Also 

 arising from the formative region, just 

 above the base of the very young leaves, 

 are protrusions which develop into forma- 

 tive regions like those of the main tip, 

 and, as growing-point, produce leaf-bearing 

 branches of the main stem. In the center, 

 around the axis, the formative material as 

 it grows older becomes pith (shown as 

 dotted) and this pith is continuous with 

 that of the branches. The surface becomes 

 changed into a skin or epidermis (coarse 

 shading) covering both stem and leaves. 

 Parts of the formative material between 

 the epidermis and the pith become vari- 

 ously hardened into "bundles of fibrous ma- 

 terial; around the central pith arise strands 

 of wood (fine shading) ; near the epidermis 

 arise corresponding strands of bast (shown 

 by black) surrounded by more or less pith- 

 like material which may become green or 

 corky, called cortex (shown dotted like the 

 pith); and between the rings of wood and 

 bark is a layer of formative material which 

 is continuous with the tip and is called the 

 cambium. From this cambium in successive 

 years new wood is added to that within 

 and new bark to that on its outer side, and 

 thus both wood and bark increase in thick- 

 ness by annual layers. But on the outside 

 the epidermis, and then the older bark, is 

 pushed off or worn away so that the total 

 thickness of the bark is limited. Both 

 wood and bark are continued into the 

 leaves, but not the cambium. The strands 

 of wood and those of the bark are so connected as to form a sort of net- 

 work through the meshes of which extend radially the plates of pith called 

 pith-rays. 



From Sargent. 



