Stems

Introduction

The stem is the major aerial support system in most plants. Sometimes called a stalk or trunk, it holds up the plant into the air and provides a pathway for fluid transport between the shoot and the root. The stem also may represent a huge volume for storage of materials. Sometimes the stored materials are for retrieval and later use; sometimes the stored materials are toxins put into the trunk as permanent waste or as material to prevent trunk rot by microorganisms.

The stem consists of the hypocotyl that leads up through the soil to the cotyledons (seed leaves). These leaves attach to the stem at its first node. The stem continues from this first node with an internode leading to the second node. The internode is a part of the stem that elongates considerably in some plants...especially in vines! The node is just the part of the stem where leaves attach. Obviously the node is an area where elongation growth cannot be extensive...it would tear up the connections to the leaf! The stem continues internode-node, internode-node, up to its very tip (apex).

Along the stem, just above each node, and in the axil (angle) of the leaf you can find lateral buds. These are miniature branches waiting to grow out and become limbs...if conditions are right. In some plants, and in other conditions, the lateral buds are destined to produce a flower or a whole collection of flowers (inflorescence).

At the tip of the stem is the apical bud. This bud adds new internodes and nodes to the tip of the plant. This means, of course, that most plants grow from their tips rather than from the base. If you hammer a nail into a tree trunk at a level 1 meter above the ground, and then come back ten years later, the tree may be many meters taller, but the nail will still be just one meter above the ground.

Stem Origin and Structure

The stem and its shoot system originate at the shoot apical meristem in the apical bud. The young stem has three fundamental tissues: epidermis, ground parenchyma, and vascular. The epidermis forms an outer covering for the stem, the vascular tissues (xylem and phloem) are typically in vascular bundles, and they are surrounded by ground parenchyma (cortex and pith). Here is a view of a slice (cross section) of a stem:

Quite obviously the outer layer is the epidermis. This layer may have a waxy cuticle to prevent water loss; the water-proofing substance is cutin. Some epidermal cells may grow outward to form hairs or surface glands. Other epidermal cells may be specialized as guard cells surrounding a stoma, for gas exchange.

The vascular bundles are typically arranged into one or more concentric rings. Toward the outside of the ring is the ground parenchyma region called the cortex. Here we may two different layers. In many species the layer of cortex closest to the epidermis is composed of collenchyma cells. These cells have elongated tremendously and have specially thickened primary walls...thickened extensively where three or more cells meet, but typically thin between just two cells. The extra thickening gives some support against gravity for a young and growing stem. Just inside the collenchyma layer the cells are typical parenchyma; they carry out a range of biochemical functions from photosynthesis to synthesis of very special defense molecules.

To the inside of the ring of vascular bundles is the pith. This region is made up of almost exclusively parenchyma cells. These are too far on the interior to carry out much photosynthesis or other interesting biochemistry. But these cells may store water (as in cacti) or storage polymers for later use. For example, the pith of a potato tuber (a kind of stem) holds vast quantities of starch. In some stems, the pith becomes quite hollow at maturity...the cells have died...a process commonly called apoptosis. Bamboo is an excellent example of that.

Each vascular bundle consists of four layers:

Toward the cortex the bundle may be a layer of fibers. These are sometimes called phloem fibers and the individual cells are highly elongate having expanded by intrusive growth. Some individual fiber cells can be in the range of meters in length. These fibers have been removed from stems, spun into thread, and woven into fabric. The fibers of flax were treated this way right here in Willimantic. At one point our little town was the world's leading supplier of linen!

Moving interior to the fibers, we find the functional primary phloem. Here sieve tube elements conduct water and sugars and amino acids from the leaves to the rest of the plant. The sieve tube cells are alive, but lack nuclei and other organelles. They are comparable to red blood cells in humans. However, unlike RBCs, phloem sieve tube elements can live for years. This ability is provided by adjacent companion cells. The companion cells have complex cytoplasm with mature organelles and provide the metabolic and homeostatic needs of the sieve tube elements through plasmodesmata...cytoplasmic strands that connect between these two types of cells. The presence of two cell types with vastly different appearance gives the functional phloem area a superficially "messy" look. A closer examination reveals some regular patterns in the distribution of the various cell types in phloem tissue.

The next layer toward the pith is the vascular cambium (aka cambium). These cells are arranged in regular rows and columns like an accountant's spreadsheet. These cells are meristematic and divide mitotically to produce additional secondary xylem and secondary phloem in woody plants. This layer produces the huge trunk diameter in common trees. This layer makes all the wood we use in building homes and furniture, and in paper manufacturing. This layer may be absent in some non-woody species...such as grasses.

The innermost layer of the vascular bundle is the primary xylem. This tissue conducts water and soil minerals up the stem from the roots. Like human skin cells, most of these xylem elements are dead at functional maturity. The tracheids and possibly vessels of the plant stem serve as plumbing. They lack cytoplasm altogether at maturity...the last act of the cytoplasm in the developing tracheary element is to digest its own cytoplasm and sometimes also its end walls to form the pipes through which the water and minerals are conducted.

Stem Functions

Support of Leaf Display The stem must be strong enough and flexible enough to position the leaves in the sunlight to optimize the photosynthesis of the plant.

Support of Flower Display The stem must put the flowers into a correct (usually visible) position to attract the appropriate pollinators. The stem must be able to support the weight of some perching pollinators. In the case of wind pollination, the flower must be positioned properly for male parts to be upwind from the female parts.

Support of Fruit Display A plant must be able to support its load of developing fruit. For wind-dispersed fruits and seeds, getting the fruits up to the wind exposure is critical. For animal-dispersed fruits and seeds, the stem must position the fruit for best display or best attachment to the animal parts. Some animal dispersed fruits are incredibly heavy and numerous; the plant stem must support the whole load against the forces of gravity, wind, snow, etc.

Conduct water and minerals up from soil The flow of xylem continues from root to the leaves via the stem. The stem must not only conduct minerals and water up through the plant, but it must distribute it to all elements in the shoot system (leaves, flowers, fruits, etc.). In the spring, the xylem sap may also include carbohydrates that were stored the previous year in the trunk and roots. We tap this xylem sap and boil it down to concentrate the sugars to make various plant syrups...the best example is maple of course.

Conduct water and nutrients Sugars and amino acids from photosynthesis in the leaf are conducted by the phloem in the stem to the rest of the plant. This flow will be upwards from leaf to apical bud, to flower, or to fruit above the leaf. The flow will also be downwards from leaf to root to supply the root's fuel for respiration. This flow is achieved by loading the phloem sieve cells with sugar by active transport from the bundle sheath cells in the leaf veins. The transport of sugar causes water to enter the sieve cells by osmosis. The developing pressure pushes the phloem sap from the leaf to the rest of the plant. At the root and shoot tips, and at points along the transport pathway, the phloem sieve cells are unloaded by active transport into the bundle sheath. The water again follows by osmosis, reducing the pressure at this end of the phloem. The pressure gradient, highest at "source" leaves and lowest at "sinks," causes the flow of the materials in the phloem.

Photosynthesis In some species, cacti are good examples, the stem is the primary site of photosynthesis. In cacti, the leaves are converted to non-green spines, and the branches to non-green areoles. So in these plants virtually all the photosynthesis occurs in stems. In other herbaceous plants and in young shoot ends of woody plants, the stems are often green and contribute in at least a minor way to the photosynthetic output of the plant.

Storage of water, etc In the stems of cacti, in underground stems such as in potato, and in the swollen hypocotyl of radish, water can be stored. A plant with stems that are loaded with water and dissolved materials is called succulent. The cacti and succulent plants hold tremendous quantities of water in their stems. Of course this water needs to be protected from herbivory in desert situations.

Defense Desert plants have evolved spines (from leaves) and glochidia (from lateral buds) to mechanically protect the stems from herbivory. The stem must position these appendages and support t hem sufficiently for them to function. In other species, such as Lophophora williamsii, the peyote cactus, the stem posesses mescaline or other potent chemistry to deter and/or kill herbivores. For big herbivores, such as humans, the dose is not sufficient to kill if the animal eats small amounts of stem tissue. Sometimes in these cases, the side effects of a small dose are quite pleasant...even recreational! Of course there may be longer-term side effect of chronic use, so user beware!

Anchorage Especially in vines, stems provide anchorage functions. This anchorage can be accomplished by: twining as in morning glory, by the formation of adventitious roots as in poison ivy, or by the formation of "suction cups" as in Virginia creeper. With these specializations, stems serve to hold the plant in place.

 

This page © Ross E. Koning 1994.

 

 

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