Maximum Yield Australia/New Zealand March/April 2020 | Page 58

Once enough of the chains have
“
been broken,
the plant will go into ‘terminal wilt,’
will no longer respond to watering, and will die.”
Can’t Cross the Cambium
The stomata
not only allow
water vapour
and oxygen to
escape, but they
let in carbon
dioxide. In times
of high heat or
in the absence of
light (at night),
they may close
to prevent excess
water loss. When the guard cells have closed, they cut off the
supply of CO 2 , which retards plant growth.
Movement of water through the xylem is passive and does
not require energy from the plant. The xylem tubes are
small enough to take advantage of capillary action (with
the help of some of water’s more unusual properties) to draw
water up from the roots to replace what has been lost due to
evaporation through the stomata.
This effectively creates “chains” of water that lead from
the roots to the leaves. If an insufficient amount of water is
available, at first these chains will shrink, causing the plant
to droop. This can be seen in a slightly underwatered plant
that starts to wilt. In the early stages, this can be corrected
by watering the plant, which should respond fairly quickly
(within an hour or so) by replacing the lost water and
swelling back to normal appearance.
If the plant continues to dehydrate due to lack of water, the
water chains will get thin enough to begin to break, causing
air pockets to develop. Once enough of the chains have been
broken, the plant will go into “terminal wilt,” will no longer
respond to watering, and will die.
While it is usual to consider the xylem as carrying water,
hormones, and nutrients, and the phloem as transporting
water, hormones, and sugars, in some cases, such as with
maple trees, sugars stored in the roots can use the xylem as a
pathway to lift them up. Maple syrup is collected by piercing
the xylem to collect this sugar sap, which is then boiled to
condense and thicken.
Vascular land plants can grow taller than their non-vascular
counterparts such as mosses and algae in part because the
xylem gives strength and structure to the plant stems.
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Maximum Yield
The vascular cambium layer often forms as a cylinder
along the stem (or trunk), with the xylem (wood) on one
side and the phloem (bark) on the other. The vascular
cambium has a concentration of meristem (building block)
cells which are used to increase both the xylem to the
inside and the phloem on the outside.
As an example, tree trunks add a yearly layer of wood
(tree rings) formed from xylem, and the cambium grows
a new layer of xylem to replace the old one. As the inner
wood diameter increases, the cambium on the phloem
side adds additional cells as needed.
Xylem
Cambium
Phloem
Go with the Phloem
While the xylem carries water and nutrients in only one
direction (from the roots to the leaves), the phloem carries
sugars, hormones, amino acids, and relocating nutrients
around the plant from stores (where the resources are
in the plant) to sinks (where the resources are needed).
For example, in the spring, sugars stored in the roots
are moved to new growth sinks to plant growth. During
photosynthesis the sugars made in the leaves act as
stores that can be transported to sinks such as the roots
(to prepare for the following spring) or to flowers and fruits.
When growing vascular plants, it is helpful to be at least
acquainted with vascular systems. By understanding
that guard cells close at night, it becomes apparent
that supplemental CO 2 is better used during the day.
By understanding that guard cells close in extreme
heat, it is easier to comprehend why plant growth
“stalls” during a heat wave. Each piece of the puzzle
makes it easier to get the big picture.