254 



INDUSTRIAL PLANTS 



Fiu. 233. — Diagram of maple sten 

 showing the clcvclopmcnt of wood 

 and bark through first and second 

 years. At tiie tip is a mass of 

 living formative material (shown 

 unshaded) from the sides of which 

 arise protrusions that finally be- 

 come leaves. Also arising from the 

 formative region, just above the 

 base of the very young leaves, are 

 protrusions which develop into 

 formative regions like that of the 

 main tip, and, as growing-points, 

 produce leaf-bearing branches of 

 the main stem. In the center, 

 around the axis, the formative ma- 

 terial 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 

 (shown by coarse shading), cover- 

 ing both stem and leaves. Parts 

 of the formative material between 

 the epidermis and the pith become 

 variously hardened into bundles of 

 fibrous material : around the central 

 pith arise strands of wood (shown 

 by fine shading) ; near the epidermis 

 arise corresponding strands of bark 

 (shown by black), surrounded by 

 more or less pith-like material 

 which may become green, corky, 

 or otherwise peculiar (shown dotted 

 like the pith) ; and between the 

 rings of wood and bark is a layer 

 of formative material which is con- 

 tinuous with that of the tip and is 

 called the catnbium. 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 in- 

 crease in thickness by annual lay- 

 ers. But on the outside the epi- 

 dermis, and then the older bark, is 

 pushed oH or worn aw^ay 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 bark are so 

 connected as to form a sort of network through the meshes of which 

 extend radially the plates of pith called pith-rays. (Original.) 



a layer of cambium between. These strands connect with 

 similar ones in the leaves, and are continuous below with the 

 ring of strands forming between the Avood and the bark 

 which was fully formed when the season began (Fig. 233). 



