Understanding Plant Growth And Development

Plants grow from the tips of shoots or branches upwards and the tips of roots downwards. This means there is always a newly formed area at the growing tip of each root and shoot.

Flowering ClematisFlowering Clematis

This embryonic area, a tiny patch of sugar-cube-shaped cells, is responsible for having produced all the cells that make up the branch or root behind it.

The little cluster of embryonic cells is called a meristem (pronounced MER-i-stem).

In the shoot, the meristem is enclosed within the wrappings of a bud. The meristem of a root is protected by a “bumper” of loose cells that pad the tip. This protective structure is called the root cap.

Step outside and pick a few buds from your garden plants. Try to get elm, lilac and buckeye. These will be winter buds, covered with a heavy protective overcoat of dried scales. As you peel these scales away, you will find all of this year’s new growth neatly packed away!

This is the original dehydrated “add water and serve” package. Your buds may vary inside. They may be flower buds, leaf buds, or mixed buds with both flowers and leaves in the same bud.

Notice how the tiny leaves are all folded together, and the stem on which they are attached is just a short stub. At the tip of this stubby stem is the meristem. It was the meristem activity of last summer that produced the millions of cells in that bud that made the stem, leaf, and flower tissues.

The activities of plant meristems have interested botanists for a long time. Think of a germinating seed, say, a kidney bean. You may want to sprout a few beans on a wet blotter in a saucer covered with another saucer, inverted, to refresh your memory. As the bean sprouts, first, its root breaks through the seed coat, and shortly the shoot follows.

This initial activity is largely hydration growth. That means that cells already there fill up with water, swell, and the whole embryo seems to grow. Actually, it is just swelling, not making new cells.

Now that hydration is complete, the cells are awake and ready to resume normal activities, and true growth begins. It begins in two places – the tip of the root and the tip of the shoot. The shoot tip is wrapped in a soft bud of tiny leaves. From these two embryonic areas will come the entire plant: several yards of roots, and many yards of branches, if we have a pole bean.

How Does This Come About?

Remember that the meristem areas always are at the most distant tips. As they make tiny new cells, these grow and mature, pushing the root tip deeper and the shoot tips higher into the air. Occasionally some of the cells left behind by meristems remain capable of dividing.

In the shoot, these are the side buds. When the meristems here become active, a plant develops side branches. In many cases, these lateral meristems are stopped from growing by a hormone sent down by the active tip-end meristem. This substance keeps the side buds from breaking.

A good gardener knows that if they pinch off the tip of a shoot, removing the source of the inhibiting hormone, their plants will branch and become bushy. To revert to an old theme, this is another of nature’s ways to insure the continuation of the species. If the uppermost branch, destined to bear the first flower, is lost, side branches will develop, flowers and seeds will come, and the species will be saved.

The Root A Different Story

In the root, the story is a little different. The potentially embryonic tissue lies deep inside the root, rather than as a bud on the surface. Branch roots break out from almost the center of older roots.

As cells are left behind by the meristems, they mature, grow larger, and often become specialized. Some go to work conducting sap, water, and minerals. These mature as long tubes, the pipe-line system of the plant. Others become thick-walled and make the plant-strong. They are called supporting or mechanical tissues.

A third general group fills up space between supporting tissues and conducting tissues. These are the “excelsior packing” of the plant and may become involved in storing food, holding chlorophyll, or acting as a source of cork cells. Still another group forms the “skin” of the plant.

Look at the diagrams of the stem and root cross-sections and visualize these groupings in your garden plants.

Hint: the “strings” of a celery stalk are veins of conducting tissue wrapped in a protective sheath of supporting (mechanical) tissue.

Functions of Stem Tissues

Cross section of a young sunflower stem. These tissues are all “primary.” that is, they originated in the bud of the stem.

  • A – Epidermis
  • B – Cortical parenchyma
  • C – Pericycle
  • D – Phloem
  • E – Cambium
  • F – Xylem
  • G – Stelar Parenchyma
  • H – A vascular bundle

Epidermis (epy DER mis) – an outer covering for the plant, prevents excessive loss of water.

Parenchyma (par ENKa ma) – a filler tissue, relatively unspecialized. The part outside the vascular bundles is called cortical parenchyma and generally is green (having chloroplasts); The stelar parenchyma, inside the ring of bundles, sometimes acts as storage tissue and is sometimes called pith.

Pericycle (PERT cycle) – differs in form and function between stem and root. In the stem, its cells are thick-walled and give rigidity, padding the vascular bundle from outside bruising.

Phloem (FLOW um) – a vascular tissue of long, slender cells, a sort of pipeline to carry sugar solution and other foods (sap) up and down the stem.

Cambium (CAM be um) – a latent tissue, dormant in young stems, but later becoming active. When the cambium divides it snakes phloem cells and xylem cells. Thus it is a lateral meristem, and the tissues it produces are called secondary tissues.

Xylem (ZILE um) – another conducting tissue made of thick-walled cells that are elongated. Xylem carries water and minerals from the soil to the upper parts of the plant. Movement in the xylem is almost always upward.

Functions of Root Tissues

Cross section of a young buttercup root. The various cells making up this tissue all started in the root meristem.

  • A – Epidermis
  • B – Cortical parenchyma
  • C – Endodermis (thick-walled)
  • D – Pericycle
  • E – Phloem
  • F – Cambium
  • G – Xylem
  • H – Endodermis (thin-walled)

Epidermisfunctions in the root much as it does in the stem.

Parenchymain the root is frequently storage tissue. In the buttercup root the parenchyma cells are filled with starch grains which are reserve food for the plant during early spring, late fall and winter when the stems are dead and the roots must be self-reliant.

Endodermis (END o der inis) – a tissue seen only in underground plant organs, Apparently has to do with the radial movement of dissolved foods and minerals. Notice that, in this root, it has thick-walled cells opposite the phloem and thin-walled ones opposite the xylem.

Pericyclebranch roots arise deeply within the root, originating from actively dividing cells of the pericycle.

The vascular tissues, phloem, and xylem, function in the root just as in the stem. The cambium, too, is responsible for the production of secondary xylem and phloem in the root as it is in the stem.

By Dr. John P. Baumgardt | Edits by PlantCareToday Staff

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