Saturday, December 3, 2022

The long and short of stems


The 3 major internal tissues of a plant stem are: (1) the nursery cells in root or shoot tips that divide into, (2)  tubes that grow to transport water and dissolved minerals up to leaves, flowers, and fruits, and (3) tubes that move sugars created by photosynthesis down around the plant.  Leaf veins are extensions of this ductwork.

In case you’re interested, the stem tissues’ fancy names respectively are cambium, xylem and phloem.

Plants that germinate with a single leaf, grass for example, produce up and down tubes in stringy side-by-side pairs inside the stem. Zoom in on a cross cut stem, and the string pairs look a dotted starburst. Bite a celery stem, and see its strings arranged on the outer edge. In plants that germinate with two leaves, trees for example, the xylem, cambium and phloem form distinct rings of tissue around a pithy central cylinder.

Commercial herbicides are designed to travel in one or the other tube system which explains why a concoction that turns out the lights for broadleaf weeds won’t bother Bermuda grass.

Xylem or tubes that transport up, form when the cells of a root or shoot’s tip divide. As the plant grows, the initial xylem cells die and empty out. Hard layers of these dead cell walls form heartwood, a woody plant’s skeleton. In a large tree only the outermost wood brings water and dissolved minerals into the plant.

Phloem or tubes that transport down, form in the same stem location as xylem, and differentiate into either thin-wall filler tissue, sieve tubes, leaf veins or fibers like flax or hemp.

A plant part may be called a stem only if it has buds or leaves. Strawberry runners, for example, are stems.

The typical stem is visible above ground, but grass rhizomes, potato and cyclamen tubers, gladiola corms and onion bulbs are all subterranean stems. Some roots look like underground stems others do not.  If it has “eyes” that will produce buds then leaves, it’s a stem not a root.

Stems produce new plants with ease: think rose canes, grass plugs, potato pieces, and garlic cloves.

What do shoots, twigs, branches, and trunks all have in common?  They’re stems with different birthdays.

A shoot is an infant with some leaves.  A twig is a dormant toddler less than a year old, and without visible leaves.  A branch is over a year old, and bears lateral stems.  Tree and shrub trunks are main stems with a few years under their proverbial belts.

The difference between a tree and a shrub is usually the number of main stems and the height.  A tree averages 12 feet or taller, whereas a shrub averages 12 feet or shorter.  Vines are trailing stems, and some, like English Ivy have aerial roots.

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Noelle Hood
Noelle Hood
Noelle is our Resident Green Thumb and a Texas Agri-Life Master Gardener.

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