Stems

Stems are usually above ground organs and grow towards light (positively phototropic) and away from the ground (negatively geotropic), except in the case of certain metamorphic (modified) stems.

The main stem develops from the plumule of the embryo, while lateral branches develop from auxillary buds or from adventitious buds. In normal stems clearly defined internodes and nodes can be distinguished, the latter being the regions where the leaves are attached. In younger stems stomata are found in the epidermis while in the mature stems lenticels are evident. Depending on the hardness of the stem one can also distinguish between herbaceous and woody stems.

In this section we will discuss the internal structures of young dicotyledonous and monocotyledonous stems, secondary thickening in the stems of dicots, and differences in the internal structures of dicots and monocots.

Stem of a Dicotyledonous Plant: Internal Structure

From the study of the transverse section of the dicotyledonous stem you will identify the following three regions of tissues: epidermis, cortex, and vascular cylinder or stele.

Epidermis

The epidermis consists of a single layer of living cells which are closely packed. The walls are thickened and covered with a thin waterproof layer called the cuticle . Stomata with guard cells are found in the epidermis. In some stems either unicellular or multicellular hair-like outgrowths, trichomes, appear from the epidermis.

Functions

• The epidermis protects the underlying tissues.
• The cuticle prevents the desiccation of inner tissues and thus prevents water loss .
• The stomata allows gaseous exchange for the processes of respiration and photosynthesis.

This region comprises the collenchyma, parenchyma and endodermis. It is situated to the inside of the epidermis.

• Collenchyma

  These cells lie under the epidermis and constitute three to four layers of cells with cell walls thickened at the corners. The collenchyma cells contains chloroplasts

  Functions of the Collenchyma

  • This tissue serves to strengthen the young stem.
  • The chloroplasts are responsible for the synthesis of organic food during photosynthesis.

• Parenchyma

  Beneath the collenchyma cells are a few layers of thin-walled cells, parenchyma, with intercellular spaces. The parenchyma cells make up the bulk of the cortex.

  Functions of the Parenchyma

  • The synthesized organic food (mainly starch) is stored here.
  • The intercellular air spaces are responsible for gaseous exchange.

• Endodermis or Starch Sheath

  The endodermis or starch sheath forms the innermost layer of the cortex. This is a single layer of tightly-packed rectangular cells bordering the stele of the stem.

  Functions of the Endodermis

  • The cells of this tissue store starch.
  • It allows solutions to pass from the vascular bundles to the cortex.

This region comprises the pericycle, vascular bundles and pith (medulla).

• Pericycle

  The pericycle is made up of sclerenchyma cells which are lignified, dead fibre cells . These cells have thick, woody walls and tapering ends.

  Functions of the Pericycle

  • It strengthens the stem.
  • It provides protection for the vascular bundles.

• Vascular Bundles

  The vascular bundles are situated in a ring on the inside of the pericycle of the plant. This distinct ring of vascular bundles is a distinguishing characteristic of dicotyledonous stems. A mature vascular bundle consists of three main tissues - xylem, phloem and cambium. The phloem is located towards the outside of the bundle and the xylem towards the center. The cambium separates the xylem and phloem which bring about secondary thickening.

  Functions of the Vascular Bundles

  • The xylem provides a passage for water and dissolved ions from the root system to the leaves.
  • The xylem also strengthens and supports the stem.
  • The phloem transports synthesized organic food from the leaves to other parts of the plant.
  • The cambium, divides to produce new xylem and phloem cells, making secondary thickening possible.

• Pith (Medulla)

  The pith occupies the large central part of the stem. It consists of thin-walled parenchyma cells with intercellular air spaces. Between each vascular bundle is a band of parenchyma, the medullary rays, continuous with the cortex and the pith.

  Functions of the Pith or Medulla

  • The cells of the pith store water and starch.
  • They allow for the exchange of gases through the intercellular air spaces.
  • The medullary rays transport substances from the xylem and phloem to the inner and outer parts of the stem.

A diagrammatic representation of a dicot stem with the different tissue systems of the stem.

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Secondary Thickening in Dicotyledonous Stems

In herbaceous dicots a limited amount of secondary thickening occurs, while it is more evident in perennial, woody dicots. The stem increases in thickness as it grows older. In the vascular bundle of a young dicot stem the xylem and phloem are separated by cambium.

Secondary thickening begins when mature parenchyma cells in the medullary rays which lie between the adjacent vascular bundles become meristematic and form the fascicular cambium. The fascicular cambium forms a continuous ring of cambium as it joins up with the fascicular cambium. This cambium ring undergoes division to form secondary phloem to the outside and secondary xylem to the inside. The secondary xylem and secondary phloem are laid down in the form of concentric cylinders on either side of the cambium ring. At certain points the cambium forms parenchyma, which radiates from the middle of the stem through the secondary xylem and secondary phloem to form vascular rays. As a result of these changes the stem increases in thickness. The primary xylem and phloem are pushed further and further apart. The pith remains alive.

Some of the parenchyma cells between the vascular bundles continue to exist to form radially directed vascular rays. Annual rings develop in the secondary xylem, each consisting of a layer of spring wood and a layer of autumn wood. A cylindrical meristem develops in the cortex, the cork cambium (phellogen). The cork cambium gives rise to the cork cells (phellem) on the outside and the secondary cortex (phelloderm) on the inside. Together this is known as the periderm. Opposite the stomata the cork cells (phellem) give rise to lenticells for gaseous exchange.

A diagrammatic representation of how secondary thickening takes place in the stem of a dicotyledonous plant.

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Stem of a Monocotyledonous Plant: Internal Structures

The tissues of dicots and monocots are basically the same as you will see. However, there are essential differences in the arrangement of the epidermis, ground tissue and vascular tissue .

Epidermis

The structure and functions of this tissue are the same as those of the epidermis of the stem of a dicotyledonous plant. The epidermis consists of a single layer of living cells which are closely packed. The walls are thickened and covered with a thin waterproof layer called the cuticle. Stomata with guard cells are found in the epidermis. In some stems either unicellular or multicellular hair-like outgrowths, trichomes, appear from the epidermis.

Functions

• The epidermis protects the underlying tissues.
• The cuticle prevents the desiccation of inner tissues and thus prevents water loss .
• The stomata allows gaseous exchange for the processes of respiration and photosynthesis.

This region is composed of small, thick-walled sclerenchyma on the inside of the epidermis. These layers of cells are followed by larger thin-walled parenchyma cells. Intercellular air spaces are found in the parenchyma. A cortex or pith is absent.

Functions

• Sclerenchyma tissue strengthens the stem.
• Parenchyma tissue stores synthesised organic food such as starch.
• Intercellular air spaces allow the exchange of gases.

The vascular bundles are found scattered throughout the ground tissue. The vascular bundles occurring nearer the rind of the stem are smaller and are closer to one another. The vascular bundles contain no cambium and consequently secondary thickening does not occur. The vascular bundle is composed of the following parts:

• Sclerenchyma sheath

  Thick-walled sclerenchyma fibres surround the vascular bundle.

  Function of the Sclerenchyma sheath

  • Sclerenchyma sheaths protect the vascular bundles and give strength to the stem.
• Xylem

  Large xylem vessels are found within an irregular intercellular air space called the lysigenous cavity. This space is surrounded by thin-walled parenchyma cells.

  Functions of the Xylem

  • Xylem transports water and dissolved ions from the root system to the stem.
  • The lignified thick-walled xylem cells strengthen the stem.
• Phloem

  Phloem is composed of thin-walled cells, viz. sieve tubes and companion cells.

  Function of the Phloem

  • The phloem transports synthesized organic food such as carbohydrates from the leaves to other parts of the plant.

A diagrammatic representation of the tissues of the stem of a monocotyledonous plant.

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Differences between Monocot and Dicot Stems

Monocotyledon	Dicotyledon
1. A large number of vascular bundles.	1. A limited number of vascular bundles.
2. The vascular bundles are scattered in the ground tissue.	2. The vascular bundles are arranged in a ring.
3. No cambium occurs between the xylem and phloem.	3. Cambium occurs between the xylem and phloem.
4. There is no distinction between the cortex and pith.	4. The cortex and pith can be clearly distinguish.
5. No Secondary thickening.	5. Secondary thickening can occur.
6. No annual rings are formed.	6. Annual rings are formed due to secondary thickening.

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