Plant Stems

Cronodon    Plant stems     Stem growth    Woody stems    Wood    

The diagram above shows a cross-section through the young green and fleshy stem of a dicotyledonous angiosperm. Dicotyledons (or dicots) include many of the flowering plants - broad-leaved flowering trees, herbs and shrubs. The stem of a young tree or woody shrub is similar in structure to that of a green herbaceous plant, such as a daffodil or dandelion. Only after its first year of growth, does the stem of a woody plant begin to develop wood and take on a very different structure. Monocotyledonous angiosperms are the other main division of flowering plants and include grasses, palm trees, and bamboo. Monocots have a different stem structure also. Here we shall restrict our attention to dicot stems.

If you slice a young green stem in two then you will see the structures indicated above. You will see that the centre of the stem contains either whitish pulpy tissue called pith (made of parenchyma cells) or that it has become hollow (the pith is present initially but may disintegrate) and may contain liquid. The pith acts as storage - its cells are enlarged and bloated with food reserves and water.

Covering the stem you will see a thin layer of, usually green tissue, that peels off in strips. This is the epidermis. Just beneath the epidermis is greenish tissue (the subepidermal layers) which may contain parenchyma cells or these parenchyma cells may be modified into thick-walled collenchyma or even further into tough and very thick-walled fibres (sclerenchyma). Collenchyma makes up the chewy fibres in a celery stem, just beneath the epidermis. The stem may be ribbed and then the collenchyma is concentrated under the ribs, or it may be square or triangular, with collenchyma concentrated under the corners. This collenchyma or fibrous tissue strengthens the stem, especially against bending.

Beneath the collenchyma there will be ordinary parenchyma, together the parenchyma and collenchyma beneath the epidermis but outside the vascular bundles, makes up the cortex. The epidermal and cortical cells usually contain chloroplasts and are photosynthetic. The pith cells are not photosynthetic.

Embedded in the cortex, between the subepidermal layers and the pith there is a cylinder of vascular bundles. These bundles contain vessels and the whole bundle is enclosed in a bundle sheath (of starch
containing parenchyma cells). Towards the outside of the stem, the bundle contains phloem vessels which transport sugary sap from the leaves (where sugars are made by photosynthesis) or from sugar stores in the roots (such as in early growth or early spring when the leaves are not yet ready) to other parts of the plant (shaded leaves, roots, growing fruit etc.).

Toward the inside of the plant the vascular bundles contain xylem vessels which transport water and minerals from the roots to the aerial parts of the plant. In between the xylem and phloem, each vascular bundle contains a vascular cambium. A cambium is a region of cells that grow and split into two new daughter cells, in other words a region where new cells and tissues are produced. The vascular cambium produces new xylem cells toward the inside and phloem toward the outside.

This gives rise to the situation shown below. Here we look at the vascular bundles in more detail. We can see that the oldest or first formed xylem is innermost and is called the protoxylem, whilst the newer xylem outside of this is called the metaxylem. Similarly the newest phloem cells produced by the cambium are called metaphloem and the older or first-formed protophloem is nearest to the outside of the stem. The protophloem vessels often get crushed and cease conducting as the stem grows and new phloem pushes it outwards, but it develops many thick and strong sclerenchyma fibers that give the stem its toughness.

Why are the vascular bundles all toward the outside of the stem?

We can picture the vascular bundles as forming a cylinder of vascular tissue near the outside of the stem. As well as transporting materials, vessels provide support for the stem - they are tough and strong. As explained in the section on biomechanics a wide hollow cylinder resists bending better than a narrow solid stem - in other words for the same amount of strengthening material, adding this material to the outside of the stem makes the stem stiffer and stronger.

The parenchyma in the cortex and pith also help to support the stem. When filled with water, these cells swell like little balloons or pressurised cells and push against one another, this arrangement is similar to polystyrene. Polystyrene foam contains tiny balls of polystyrene filled with air. When you squash the foam the air resists compression and the foam springs back. Parenchyma works like this, except each cell is pressurised with water rather than air. All these pressurised water-filled cells push against one another and make the stem stiff like polystyrene foam and so aid support. If, however, the plant is short of water it wilts as the cells lose water and deflate. This might cause the stem to lose its stiffness, but it does lower the stem and leaves out of the searing sun and drying wind and helps conserve water. When a stem wilts, the vessels and sclerenchym and collenchyma still gives the stem toughness so it does not break apart. When the plant is watered it soon returns to its stiff and upright posture (we say the swells are swollen and turgid and so call this supporting water pressure, the turgor pressure).

The stem below is angular. Many plants have stems like this. Each angle is reinforced by a larger vascular bundle and a thicker layer of collenchyma or sclerenchyma cells. This also gives the stem additional stiffness. Many stems have a gutter running along one side, as well as stiffening the stem, this gutter conducts rain water from the leaves down to the roots. Parsley has stems like this, for example.

Many herbaceous plants are annuals - they grow, seed and die away in one year. Others will grow each year from a bulb and then die away after setting seed. In these plants the stem changes little beyond the scheme outlined so far. However, in other plants there is a second phase of growth (the first being the production of the stem from the seed or bud). During this secondary growth the stem will grow wider. (New stem tissue will be added at the tip as the plant grows in length, and this growing shoot will be like the primary green stem we have seen so far). This increase in girth is accompanied by the formation of wood in woody plants.

Turning a primary green stem into a woody stem - secondary growth

Initially what happens is that some of the parenchyma cells between the vascular bundles become cambial cells - that is they start dividing to produce new cells. (Cells multiply by splitting in two or dividing and then each daughter cell expands and may divide again or mature into a final cell type). In this way the cambium forms a continuous cylinder inside the stem as shown below (the dotted lines represent cambium between the bundles):

How a cambium affects growth depends firstly on the plane of cell division. If a cell divides in one direction (along a radial plane) then it will increase the girth of the stem, and if it divides in another direction, it will add to the thickness of the stem.

The diagram below shows part of the stem, whose surface is drawn in green and five cells inside the stem in (1). The dotted lines are imaginary radii - a radius is the line joining the center of a circle to any point on the edge of the circle. The middle cell, shown in yellow, divides into two cells along a radius, what we call a radial division, and so adds to the circumference and diameter of the stem. (In this instance the radial plane is also the anticlinal plane, which is the plane at right angles to the surface of the stem and so we can also call this an anticlinal division).

The diagram below shows the same stem. This time the dashed lines are parallel to the surface perimeter of the stem and the cell in yellow in (1) is going to divide along one of these periclinal planes, in what we call a periclinal division. (Periclinal means parallel to the nearest surface of an organ, in this case the surface of the stem and coincides in this instance with the tangential direction, which is at right angles to the radii). Both cells may go on to divide again. This also adds to the thickness of the stem.

In a thickening stem both radial and periclinal divisions occur. Cells produced by periclinal divisions mature into phloem on the outside and xylem on the inside. Periclinal divisions fill in the gaps as the stem widens, > gaps that would result if only radial divisions occurred! Some cells do not mature but remain as cambium to produce new cells.

In the growing shoot tip, cells also divide vertically (in the plane of the screen) which makes the stem longer. This is primary growth, but secondary growth which occurs in the oldest parts of the stem near the base consists of radial and periclinal divisions that increase the girth of the stem.

The end result is not only that the stem gets wider, but that a cylinder of xylem, or wood, is produced inside the cylinder of dividing cambial cells and a cylinder of phloem cells is produced on the outside. This produces the young woody stem shown below:

Above: a young woody stem. The cambium has been dividing for several years, producing wood (xylem)
plates, called rays, that extend through the xylem (xylem rays) and through the phloem (phloem rays). The rays store and transport materials across the stem, whilst the wood and xylem transport materials mostly up or down the stem. The pith remains in the centre of the stem. The cortex remains on the outside. The epidermis may remain for a time, and will divide radially (anticlinally) to increase in circumference (otherwise it would rupture). The cortex may also divide radially and so persist. How long the epidermis and cortex persist for depends on species, usually, sooner or later, they fall away and are replaced by the periderm. The primary xylem (not shown) remains as a thin ring of tissue around the pith.

Bark

The periderm derives from a layer of cambial cells that develops, usually in the first year of secondary growth, from a cylinder of parenchyma cells. These cambial cells (called the phellogen or cork cambium)
divide periclinally and radially, producing cork cells (phellem) toward the outside and parenchyma cells inside. The cork cambium and the cork together constitute the periderm. The periderm usually develops just beneath the epidermis, as shown here, but may arise in the cortex or primary phloem. These cork cells cut off the cortex and epidermis, which die and eventually fall away, leaving the cork layer outermost. These outer layers of the young woody stem constitute the early outer bark. The phloem constitutes the inner bark layer. The cork cells are protective - they resist concussion, dehydration and attack from grazers, insects and micro-organisms.

As a tree grows older more periderms originate at successively deeper levels within the phloem - cutting off some of the phloem from the rest of the tree, killing this phloem. The result is a multi-layered structure of dead phloem cells sandwiched between dead cork cells. This thick multi-layered outer bark is called rhytidome and is protective. In redwood trees (which are conifers and not dicots) the bark may be two feet thick and resists fire attack. In shrubs, the outer bark peels off easily and so a thick rhytidome does not develop.

Bark is porous, it has to be to allow oxygen in to the living cells underneath. These pores may be inconspicuous or hidden among the rough ridges, or they may be obvious elongating slits, as in the birch
and hazel. These pores are called lenticels. The lenticels are visible in wine corks, from the cork oak, as the brown lines, and the corks are cut such that these pores run across the cork and so prevent the liquids leaking out through the cork.

The outer bark of some trees, like lime (Tilia) and citrus contains some phloem rays that expand and flare outwards due to cambial activity in cells of the ray that undergo radial divisions. This enables the phloem to expand to accommodate the increasing circumference as the tree grows. In beech (Fagus) the outer bark is made up almost entirely of such expansion tissue. Beech has a single cork cambium (a single layer of periderm) and produces only small amounts of cork and so has thin bark which flakes off in dust-like fragments, creating a smooth bark. Similarly, birches, cherries, hornbeams, firs and some maples have thin smooth bark resulting from a single thin periderm. However, this may change later in life, for example after about 100 years, cherry trees develop rough bark. Old birches also have rough bark, especially toward the base of the tree.

Some trees produce no periderm and so no cork, but maintain the cortex and epidermis throughout life, resulting in very thin bark, for example some holly species, maple, acacia, eucalypt and citrus.

Oak trees have much faster growing bark and form new periderms every few years, arranged in vertical plates or scales, strengthened by bundles of sclerenchyma fibres, which produce vertical elongated ridges. The inner periderms cut off the outer bark layers, which subsequently die, which means that expansion tissue cannot form in these outer layers and so the outer dead bark splits as the tree thickens. Oaks younger than about 30 years of age have smooth bark.

In the cork oak there is only one cork cambium (one periderm) but this produces a very thick cork layer over several years. This bark can be stripped away, because the inner layers of bark do not come away with the cork (in contrast stripping a ring of bark from the trunks of most trees will kill the tree if the ring goes all the way around the trunk, since the phloem is pulled away and the roots cannot then receive sugars from the leaves and the roots die). When the cork is peeled from a cork oak, the production of even more cork is stimulated. After 3-6 peelings the cork is good enough for use as wine corks.

In birch and cherry trees the cork cambium produces alternating layers of thin-walled and thick-walled cork cells. The bark ruptures along the weaker thin-walled layers and so peels away in thin papery sheets. Similarly in the plane tree (Platanus) the bark scales detach along planes of thin-walled cork cells. The plane tree sheds its bark very rapidly, maintaining quite a thin smooth bark with a patchwork of different colours according to the freshness of the exposed surfaces. This is probably helps explain the high resistance of these trees to atmospheric pollution - pollution blocks lenticels and makes it hard for trees to breathe, but the plane tree sheds its bark rapidly, along with the pollution blocked pores. Consequently, plane trees are often planted in polluted inner cities as few other trees grow well in such atmospheres.

One of the disadvantages of having thick bark with dead outer layers, is that the tree loses the green epidermis and cortex of the young plant and so loses some photosynthetic tissue. However, the loss is tiny, since the surface area of the trunk is small compared to that of the leaves, and in nay case the trunk is usually shaded by the leaves. However, in cherry and poplar trees, the bark is thin and the living inner cork cells have chloroplasts and so can photosynthesize.

The bark of most trees is rich in tannins - a dark brown / black chemical that gives tea its colour and is toxic to micro-organisms. This chemical is also toxic to grazing animals and insects and protects the tree from fungi, bacteria and grazing animals.

When lightning strikes a tree it takes the path of least resistance. For a wet tree, this might be the outer bark. For example, beech trees have thin bark that wets easily and so they are less likely to be damaged by lightning than oak. In oak trees the thick bark takes considerable wetting and so tends to be drier. This encourages lightning to flow through the water-conducting xylem vessels in the wood instead. This can boil the sap in the vessels, causing them to explode and so do more damage to the tree.


Click the link below to understand the origin of tree rings and other amazing properties of woody stems.

Click here to see a more detailed exhibit on wood and woody stems

Click here to learn about how some plants grow woody stems

Structure of wood