Plants have a very effective internal set up for moving things around.
It is a liquid transport system that begins with the water that enters
the root through thin-walled cells at the surface called root hairs. The
water and dissolved substances pass from cell to cell into the center
of the root where a complex of tubes begins that connects with every part
of the plant.
The tubes are formed from dead,
hollow cells stacked end to end. Not perfectly enough joined to be as
continuous as a pipe in the plumbing of your house, but with very little
in the end connections to interfere with easy flow. These cells make up
a tissue called the xylem. In a plant that grows for more than a few weeks,
new cells are continually added to the xylem on the outside of the core
which already exists, and the new tissue increases the capacity of the
system to carry liquids to meet the needs of the enlarging plant body
that has to be served.
The oldest parts of the
xylem are eventually taken out of service as liquid carriers of
liquid, and may be plugged and used by the plant to store waste
products that cannot otherwise be eliminated.
The transport system extends all the way into the leaves, branching
into smaller and smaller divisions, so that no cell in the leaf
is more than a few cell-widths away from a vein, as the divisions
are now called. Picture a city expanding into its suburbs, with
the main water supply being pumped through a network of pipes that
change in size to match the consumption needs of progressively more
distant areas. The xylem is the corresponding system in each plant.
The sewage disposal from
our houses in the suburbs takes place in pipes that increase in
size as more and more consumer's wastes flow into the system and
back to the treatment plant. So it is with the collecting tissue
in the veins, leading to the progressively larger transport system
in the twigs and branches and tree trunks. This collecting tissue
(called the phloem) carries not only waste from the cells of the
leaf, but all the simple sugars that are formed by the leaves, using
the energy of sunlight, which is their reason for being.
the trunk of the tree, the mature xylem forms the wood that
we use as lumber. The cells that produce the new xylem are
larger in the spring when growth is fast and smaller in the
fall.The familiar "annual" rings formed in this
way let us estimate the age of the tree, and are one of the
elements that contribute to the characteristic grain pattern
of certain tree species.Since many of the waste products stored
in the disused xylem are toxic, the heartwood is often more
resistant to fungus and insect attack.
The phloem connects the leaves
with every other part of the plant, including the roots where some of
the sugars cross back into the xylem to be carried up again to reach any
cells not served by the downward flowing phloem. The sugars are broken
down as an energy source for all the living processes in each cell of
the plant, and are also used in conjunction with the minerals that come
in with the water from the soil solution to make the myriad of compounds
that make up the plant body and all the chemicals found in it.
The stems or trunks of the
plant, then, are the connecting pieces between the roots and the leafy
canopy. They are largely made up of cells that carries water and dissolved
materials upwards, and another type of tissue that transports manufactured
and waste products downwards.
They are also connecting or
supporting pieces in the sense of holding the leaves up where they can
intercept the energy of sunlight that the plant needs. Competition for
light between crowded plants probably favoured the evolution of taller
and taller species.
The internal makeup of the trunks is largely determined by the need to
contain the conducting tissues, but the details of where exactly these
lie depend on the type of plant. Forest trees are cylinders of woody tissue,
while vines often have several separate strands running though them, or
have a core of wood that is deeply ridged, which allows them to be more
flexible. Palms never make wood. Their trunks have large numbers of separate
strands of conducting tissue. The system works well for them, and is just
the product of a different evolutionay pathway from the one that regular
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