254 
a layer of cambium between. 
INDUSTRIAL PLANTS 
Fig. 233. — Diagram of maple stem 
showing 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 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 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 in- 
crease in thickness by annual lay- 
ers. But on the outside the epi- 
dermis, 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 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.) 
These strands connect with 
similar ones in the leaves, and are continuous below with the 
ring of strands forming between the wood and the bark 
which was fully formed when the season began (Fig. 233). 
