How Does a Tree Grow?
grow in two ways. In a young shoot, bundles of cells
form. These are a primary kind of wood known as provascular
tissue. As the shoot grows, a layer of
cambium forms across and between the primary bundles where
each year, cells in this layer divide and grow. As the cambium
divides, wood and bark cells form. Cells pushed outward form
which eventually splits and falls off and is replaced. A
tree's upward growth occurs at the tip of each twig. (See
diagram to right).
The inward growth of the cambium forms the main part of the
trunk and is called
xylem. Tiny tubes which transport water and minerals
from the roots up the trunk and branches to the leaves make up the
xylem. Leaves need this water to help them make food from
sunlight. The outward growth is protected by a layer of
phloem. The phloem is made up of tiny tubes that
transport the sugars from the leaves to the rest of the tree.
If the phloem is damaged the tree will die.
Roots may not go down deep but they can spread outward as
far as the tree is tall. Roots anchor trees to the soil and absorb
water and soil minerals needed for growth. Some trees have
deep tap roots; others have a spreading system of roots.
Roots as they push through the soil are aided by a cap that
forms over the tender growing point of each root. Beyond this
point, myriads of
root hairs extend into the soil, increasing increasing the
surface area of the root and increasing the amount of water the
tree can take up.
In some trees a difference in color exists between the outer
part of the wood, the sapwood
or alburnum, and the inner part, the heartwood
or duramen. The sapwood is the light part of the wood
as seen in cross section. It is the most recently formed
wood, the circle farthest from the central cylinder. Sapwood
is the physiologically active part of the wood through which the
sap rises and has a high level of humidity. Thus, it is
susceptible to rot and vulnerable to attack by fungi and
The heartwood is the part of the trunk nearest the center, the
older part, which makes up the greater part of the trunk. It
is simply a form of the sapwood modified by aging, and differs from
it in its greater compactness and darker, more intense color.
Humidity is lower in heartwood and its cells walls are thickened by
deposits of tannins, resins, starchy substances, coloring matter
and oils. This part of the plant is already dead, performing
only the functions of support and storage.