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.

Cortex

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

Vascular Cylinder or Stele

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

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.

Ground Tissue

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.

Vascular Bundles

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:

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


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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