The inside of a plant is just as mysterious for most of
us as our own insides: Complicated processes are carried out that somehow
result in-for plants-roots, leaves, flowers, and fruit. How does it all work?
Fortunately for all
of us, I'm no doctor, but I can
explain to you how plants live and grow. And when we understand how plants do
that, how they fare in our gardens makes much more sense.
Water and food travel between the roots
and the leaves of
a plant; water travels up from the roots in a system of tubes called xylem,
while sugars made during photosynthesis travel down from the leaves through
phloem.
In herbaceous plants, those that die back to the ground
each winter, the vascular system above ground is replaced every year. In woody
plants, new layers of xylem and phloem are added to old. This leads to
heartwood in the center of a tree trunk-older layers of the vascular system-and
a band of active xylem and phloem protected by a crusty bark.
Plants need oxygen-that's always
a surprise to gardeners
who begin to learn about the vascular system of plants and the uptake and
output of various minerals, gases and water. The roots of a plant use oxygen,
and so it needs to be available to them in the various sizes of pores in the
soil structure. Oxygen is lost from the soil through compaction and when it is
waterlogged. That is one of the reasons that the swamp maple, Acer rubrum, is
such a popular choice as a street tree. It is native to wet soils and so has adapted
to low levels of oxygen.
One of the functions of roots is to take up water from
the soil and send it up to the leaves to be used during photosynthesis. Tiny
root hairs absorb water, not the thick, woody anchor roots. Root hairs are worn
out fast and are constantly being replaced.
Water is pulled up through a plant to the leaves through
transpiration; it is used in photosynthesis and then released, along with
oxygen, to the atmosphere through tiny pores called stomata that are on the
undersides of leaves. Stomata create their own tiny moist atmosphere when open.
As water is released through stomata, more is pulled up from the roots like
liquid through a straw.
More water is lost from a plant during hot, dry or windy
weather. Instead of water molecules hanging around each stomate opening, the
moisture dries up or is whisked away. As water is lost, more water is pulled up
from the roots.
Some plants have the ability to shut the stomata during
such stressful times, and thereby reduce water loss. But they can't stay shut
forever, or photosynthesis cannot take place. Of course, there are exceptions
to this rule. In the desert, some plants have developed the ability to absorb
light during the day, but carry out part of the photosynthetic process at
night, when it's cooler and they won't lose as much water.
Gray-leaved plants have developed their
own ability to
reduce stress. Many, such as Artemisia and Senecio, are native to hot-summer
areas. The silver-gray color of their leaves reflects much of the light and
heat, while still absorbing enough light to carry out photosynthesis.
At the very least, we all
remember the term
photosynthesis from some science class somewhere in our past. And we probably
also remember that photosynthesis keeps us all alive-no photosynthesis, no
plants, no food, no us. Let's go a step deeperwithout having to rely on any
memories of your organic chemistry classand see how photosynthesis works.
Chlorophyll is the green
pigment we see in leaves. It is
located in cells called chloroplasts. Here, photons of light are captured and,
through a process involving water and carbon dioxide, sugars are made. The
sugars are sent down to the roots of the plant through the phloem.
In a fast-motion sequence film,
we could see how leaves
orient themselves throughout the daylight hours in order to capture as many
photons as possible. They look as if they are doing a leafy dance to the sun.
But even in normal time we can spot the particular arrangements that help a
plant's ability to photosynthesize.
Deciduous shade plants, for example, often have thin
leaves that are spaced so that they overlap as little as possible; in this way
they can capture more photons. Our native vine maple (Acer circinatum) growing
in light shade is just such a plant. Evergreen plants, such as Leucothoe
fontanesiana, also arrange their leaves in such a manner, but their leaves are
thicker and have a waxy cuticle, which helps them last for more than one
season.
Although photosynthesis is usually a leaf activity, some
plants have adapted to harsh environments by shifting the light-gathering
process elsewhere. Thin leaves wouldn't last long in the desert, and so
cactuses have swollen stems called cladodes where photosynthesis takes place.
Photosynthesis is
carried out and energy is made during
the growing season, although these activities slow as the summer progresses.
Because the low light of winter is not the best time for active above ground
growth, most plants slow or shut down shoot and leaf growth then.
Deciduous plants shed their leaves,
because it cost too
much energy to keep them. The death of a leaf (senescence) occurs when the
cells in vascular path break down. As chlorophyll dies off, other pigments can
be seen better, such as the reds of anthocyanin and the oranges of carotene.
Autumn colors are
a showy manifestation of what goes on
inside a plant. But even those mostly unseen events have an effect on a plant,
its health, and ultimately, our gardens.
