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From Mensagenda - June 2002
Phytophilia
John Masengarb
[John Masengarb's Phytophilia
column was a regular feature in Mensagenda from December 1990 through May
1993. This is a reprint of the June 1992 article.]
It All Stems from This
or
Going Out on a Limb
Between the root and the leaf is the shoot. On
trees, the largest plants, the main one is called the trunk. The trunk
holds limbs, limbs produce branches, and branches, twigs. Foresters often
refer to large, unbranched trunks as boles before harvest, logs afterward.
The term “stem” is used more on herbaceous plants, as in “flower stem.”
“Stalk” is often used for the stem of cereal grains, such as “corn stalk.”
Whatever it’s called, its main functions are to provide
support, conduct sap, and produce new living tissue.
For illustrative purposes, let’s look primarily at the
trunks of trees — the largest stems on earth. Within trunks are two types
of conducting tissues: xylem and phloem. We know them as wood and bark.
Xylem transports water, with its dissolved minerals, up
from the roots. The transportation is passive. The xylem vessels
themselves are dead. They are just thin capillary tubes through which the
sap is drawn by the leaves, or pushed up by the roots.
The walls of the vessels are thick with cellulose and
lignin. These tough materials provide most of the structural support.
(The main structural support of nonwoody, herbaceous plants is water.
Without it, they go limp.)
On the outside of the xylem, the phloem tissue
actively conducts (purists say “translocates”) the thicker sap from the
leaves down to the roots. It can also transport sap up, and can even
go both ways at once!
Phloem cells must be alive to work, but they can live
only about one to three years. No matter how old a tree is, it depends
on young cells for its continued existence.
These two types of tissues are separated by a thin,
usually green layer that produces them both. It remains unchanged and
active (except for dormant periods) throughout the life of the tree.
This cambium, only a cell or two thick, every year produces new wood on its
inner surface, new bark on its outer. Thus, annual rings are formed.
Eventually, old phloem is pushed so far from the
younger cells that it lacks the moisture and nourishment it needs to live.
Stretched ever thinner by the ever enlarging trunk, now drier and less
elastic, it breaks and cracks until it sloughs off or weathers away.
Until then, it protects the vital inner living tissues from the outside
world.
The xylem can’t be pushed anywhere, but eventually the
xylem vessels that were first created for the young sapling lie deep in the
center of the trunk. This old tissue becomes the tree’s toxic waste
dump. The old, hollow vessels get filled with metabolic byproducts and
residues of all kinds: resins, gums, tannins, pigments. These are
often toxic when concentrated in living tissue, so they have to be excluded
from the working part of the tree. But these various substances make
the old xylem harder, heavier, more durable, thus becoming a strengthening
spine for the tree.
This old inner xylem we call heartwood. The
younger, functioning, paler xylem we call sapwood.
Herbaceous plants have vascular bundles of both xylem
and phloem, rather than rings of wood and bark. In dicots, like
alfalfa, the bundles are arrayed in a circle. In menocots, like corn, they
are scattered in the stem.
At the tip of a stem is an area of potentially
immortal tissue. This area, called the meristem, produces new cells
and extends the length of the stem. The cambium is a specialized type
of meristem, and indeed is attached to it because it was created by it.
At the very tip, the meristem produces cells that differentiate into phloem,
cambium, and xylem. The meristem may cease activity during winter or
drought, or produce growth continuously where and when conditions are
favorable, such as in tropical rain forests.
In most plants, only the stem tips grow longer.
But in members of the grass family, there are meristematic regions between
the nodes of stalks. Thus, wheat stalks can really “shoot” up when all
these growing regions along the stem grow at the same time.
Stems also produce more than just more stem.
They also produce leaves, flowers, and roots, too. They can do this in
an orderly way along the stem at nodes, when injured, near the wound.
Sticking with trees as primary examples, one notes
that nodes are slightly swollen places along the stem where new growth
starts. That is where buds form. Buds are just dwarf, dormant
shoots that usually have protective covers called scales. There may be
one, two, or three buds at a node. Some open up into leafy shoots;
some, flowers; some, both. However, buds never produce roots.
Having nodes, and buds that form at nodes, trees
develop an orderly branching pattern — unlike the more random pattern of
roots. Thus, trees of a certain kind form characteristic shapes that
knowledgeable people can recognize at a distance.
The less-orderly growth that stems produce arises from
embryonic buds buried in the bark. When the tree is broken in a storm,
for instance, these hidden reserve buds break their years of dormancy to
help the tree make up for the part lost. These latent buds produce
long, slender, straight leafy shoots. When they form on apple tree
branches, they are called water sprouts. Around the base of the tree,
they are called suckers. They often grow just below where a limb is
cut off, or around an injury to the bark. They help to give growth
where it is needed. But arising as they do, they are weakly attached
to the tree, may soon get top-heavy with large leaves, and often break off
later. “Topped” trees produce a flurry of these; properly thinned
trees don’t.
Stems can also produce roots at injury sites.
Thus, we can propagate many plants from pieces of stems. Roots will
also form at nodes on some plants, especially herbaceous ones whose stems
grow laterally along or even under the ground. After roots grow from
these nodes, leaves often follow. Once both roots and leaves form at a
node, it can be severed from the mother plant and live on its own. If
not severed, an ever expanding clonal colony can grow outward forever.
So stems help to propagate the plant, too.
Stems serve other functions, as well. They can
store food and water, such as in cactus. They can produce thorns to
protect the plant from animals. They can produce prickles, such as on
roses, that serve primarily as grappling hooks to aid the slender, weakened
plant to clamber over its rivals in the wild. At some nodes, grapes
produce highly modified stems that are branched tendrils. These aid
its climb into the light by holding on to larger, stiffer supporting
vegetation.
Lastly, stems can serve as leaves, as in prickly pear
cactus and in mature asparagus “ferns.”
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