Tree growth and wood production
The above ground part of a tree essentially grows in two ways.
Elongation of the main stem and branching is a result of cell
division by the apical meristem at the growing tips. The corky
remnants of this growth, known as the pith, can be seen in
the stem. Thickening of the branches and trunk results from
cell division by the cambium, a thin layer of cells hidden
just below the bark.
Both wood and bark form from cambium growth. The newly created
bark cells form the phloem through which the carbohydrates
and hormones generated in the leaves flow down the trunk to
the root system feeding cambium growth on the way. As new
phloem cells are formed the old ones dry out adding to the
protective bark. On the inside of the cambium, newly fashioned
wood cells add to the sapwood through which the water and
nutrients flow up the tree. The sapwood is also a place to
store starch that is later used to sustain the tree through
periods of slow growth or dormancy. Usually, as each new growth
ring of sapwood is formed, an inner ring of older sapwood
is retired with the cells filled with crystals or resins,
The type and number of wood cells produced by the cambium
is determined by the concentration of carbohydrates and auxin
(which is an important hormone for plant growth) both generated
by the leaves. Following rapid shoot growth, high levels of
carbohydrate and auxin cause the formation of large earlywood
cells. When the levels drop off during dry or cool conditions,
or as the trees enters a dormant phase, small thick-walled
latewood cells are formed. It is the dense latewood cells
that form the growth rings.
The different parts of a tree trunk.
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