Maximum Yield Australia/New Zealand March/April 2020 | Page 56

The
Mechanisms
Behind Vascular
Plants
From the Casparian strip to the phloem, the vascular
system is crucial to nutrient uptake and plant health.
Grubbycup puts on his biology hat and explains the
mechanisms of vascular plants. | by Grubbycup
V
ascular plants include trees, plants with fruit, plants with
flowers, dicots, monocots, land plants, and pretty much any
plant that is isn’t a moss, liverwort, hornwart, or algae. They
all have a vascular system (hence the name) including some
form of xylem and phloem. This system allows for the plant to
absorb moisture and nutrients from the growing medium and
move essential resources around the interior of the plant.
The Casparian Strip
Water and nutrients enter through the root hairs. Once inside
the root hair, the liquid can move either by going along the
spaces between the cells (apoplastic pathway), by entering
a cell and moving from cell to cell through the adjoining
cell walls (symplastic pathway), or a combination of the two
(transcellular). Regardless of the type of pathway taken, they
all lead to a layer of cells known as the Casparian strip.
The Casparian strip blocks the pathways between cells
(which ends the apoplastic paths), and forces liquids to
enter through a plasma membrane (in symplastic fashion)
to cross the barrier. This plasma membrane can not only
regulate the flow of liquid, but filters out a variety of
contaminates and microbes. It selectively allows desired
nutrients and water to pass through, while (mostly)
preventing undesirable substances a pathway to the plant’s
sapstream. The filtered nutrient solution passes through the
Casparian strip to vascular bundles in the stele.
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Maximum Yield
Trundle over to the Vascular Bundles
Vascular bundles include both xylem and phloem, which
are the main fluid pathways. In most dicots (plants with two
seed leaves) there is a layer separating the two called the
vascular cambium.
The xylem and phloem allow the plant to transport internal
fluids. There are different arrangements for the xylem
and phloem within the bundles, but in general the xylem
tends to be toward the centre of the stem while the phloem
is closer to the bark. In leaves, the xylem side tends to be
closer to the top or “face” of the leaf, while the phloem tends
to be closer to the undersides of leaves.
Xylem from Start to Exit
The filtered water and nutrients pass through the Casparian
strip to the xylem. The xylem is a series of long, connected
dead cells (tracheids or vessel elements) that form “pipes” that
carry water and nutrients from the roots up through the plant
to the undersides of leaves where most of the water exits the
plant through openings called stomata.
Each stoma opening has a pair of guard cells that can open
and close the opening as needed. The opening and closing of
the stomata resemble a pair of lips, which are closed when
relaxed, and when swollen create pursed or “duck” lips to
create an opening between them.