Behind the root cap is the cortex, which makes up the bulk of
the rest of the root. The cortex is loosely packed with cells and
empty space, both of which store water and allow it to flow
into the xylem vessels, sending water and dissolved nutrients
to the above-ground shoot systems. Roots lack openings such
as the stomata found in leaves. Rather, roots are covered in
thin-walled cells, known as parenchyma cells, which act as a
water-absorbing membrane. These are primarily found in the
cortex of the root. This entire outer wall of the cortex is known
as the epiblema and the main conduit for this transport at the
center of the cortex is known as the endodermis. Unlike the
rest of the cortex, the cells in the endodermis are more tightly
arranged so water does not escape back into the cortex but
can be sent on its merry way upwards through passage cells.
However, this does not happen until the water and nutrients
have passed the root system’s smell test. To that end, the
endodermis is a plant guardian. If the plant has inadvertently
absorbed any toxic material, the endodermis filters it out and
rejects it. A waxy barrier known as the Casparian strip is the
gatekeeper at play here. Proteins within the cells allow for
the good compounds and molecules to pass, while toxins are
weeded out and removed.
Root hairs are an extension of the endodermis. These fine,
long, and narrow projections grow out from mature roots,
helping increase a root system’s ability to absorb moisture
and nutrients by increasing the root system’s surface area,
which increases contact between roots and soil.
The stele, pericycle, conjunctive tissue, and vascular bundles
round out some of the lineup that make up the typical root.
If you really, really want to know more about the intricate
functions of roots, or have a severe case of insomnia, look up
“histogen theory” and/or “Quiescent Center.” For our purposes
here, these go way beyond the realm of useful knowledge for
the layman horticulturist, this author included.
38
Maximum Yield
o n c e
inside the roots or root cells, water
and dissolved nutrients travel through
different root pathways in one of two
ways — either symplastic movement
or apoplastic movement.”
The Secret World of Roots
Root systems are a network of connected botanical append-
ages whose entire mass can sometimes dwarf the above-
ground portion of the plant it is supporting. It takes a lot of
work behind the scenes to feed and care for all the flowers,
fruits, and leaves getting most of the accolades. They don’t,
however, work alone. The root (mostly below ground) and
shoot (mostly above ground) systems work in unison in a
positive feedback loop. Though an oversimplification of the
process, it’s safe to say the photosynthesis work the leaves
and greens do sends nutrients below to the root system, which
is then able to expand and grow, sending more food and water
upward so more leaves can be produced. Then more photosyn-
thesis can occur, and more roots can develop and so on.
Roots are constantly attracting and transporting water and
dissolved nutrients from the medium they are in. Deep down
in the root cells, a pressure builds. This root pressure creates
a siphon-like action which forces water and nutrients up into
the above-ground portions of the plant while water and nutri-
ents from the surrounding soil are drawn into the root. This
is due to the higher concentration of nutrients and minerals
inside the root cells than in the soil environment around the
root system. In addition to this force, moisture from the soil
is continually being absorbed into the roots by the negative
water potential within the root cells.