Their contribution to the plant world
is to hold the above-ground parts steady - picture trying to balance
a 2 x 4 on end, then mentally nail it to the center of a cross composed
of other timbers and see how much more stable it becomes. Now fasten
the arms of the cross to the ground securely and you are starting
to approximate the support which the roots give to the plant. The
actual spread of many plants' roots has been measured, and it is
commonplace for a tree to have roots that extend outwards from the
base of the trunk to the same length as the height of the trunk.
The roots may or may not go deep into the ground, but they certainly
spread out through the top several inches.
That is the physical side of where roots
fit into the complex system that makes up a plant. Their other functions
have to do with the flow of water and dissolved materials into the
plant body. For most plants you would not be far wrong to say that
if the roots are not working properly the plants have no chance
of survival. As always in life there are exceptions, and some of
the modifications make interesting reading, but for most of the
plants that we grow, keeping the roots healthy and vigorous is essential
to keeping the plant alive.
The roots are made up of living cells,
which means that they must be supplied with food. Each cell is a
regular little furnace, down there in the darkness, needing oxygen
to break down the food to make energy available for all of the cell's
activities, and also having to get rid of carbon dioxide, which
is the waste product from burning the food. The living cells allow
water to pass in and out through their walls, and the flow is from
moist soil into the outside layers of the root, particularly special,
short-lived cells called root hairs, and through various other layers
in to the center of the root where specialised cells allow it to
pass rapidly up into the rest of the plant. Chemicals dissolved
in the water are carried along this same pathway, and may be simply
a passive presence in the water, or may be actively drawn in by
the plant. The chemical elements carried in the water include those
needed by the plant to form all the structures and the controlling
substances that make up the living organism.
So, the two very important functions of
the root portion of the plant system are to give physical support
to the above-ground part of the plant, and to take in water and
dissolved chemicals that all parts of the plant need. There is a
third function in some plants, which is to store food for use later
by the plant. It is this activity that gives us carrots and parsnips
and yuca for our tables, as well as sugar beet and sweet potatoes
How does knowing what the roots do, and
what they need, make us better growers? More than anything it points
out the need to understand how the physical structure of the soil
or artificial growing medium affects what happens when we water.
The environment that the roots encounter is made up of solid particles
with spaces between them. The roots grow in the spaces that are
filled either by liquid or by air. When we water a soil heavily
the spaces fill with water, driving out the air. As the soil drains,
air flows back into spaces that open up. Since the living cells
of the roots need both air and water, the ideal situation is to
have both present, and, in fact, if one or other is missing, the
roots are in trouble.
It is easy to understand that the plant
dries out if the soil contains no water, but it is very important
to remember that the roots die if they are in a soil that stays
full of water with no air for too long. Fortunately, as a soil drains
and opens spaces into which air flows, there will still be a period
when there is a film of water around the soil particles which is
available to the roots.
The second part of this article deals
with the things that make up a soil or a growing medium, and how
various mixtures react to wetting and drying.
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