Dose--Permanent Tissues

Permanent Tissues

The cells of permanent tissues do not have the ability to divide. These cells are already differentiated in different tissue types and is now specialized to perform specific functions. They are subdivided into two groups, viz, simple tissues consisting of cells which are more or less similar, e.g. epidermis, parenchyma, chlorenchyma, collenchyma, sclerenchyma and complex tissues consisting of different kinds of cells, e.g. xylem and phloem.

Simple tissues are:

Epidermis
Parenchyma
Chlorenchyma
Collenchyma
Sclerenchyma

Complex tissues are:

Xylem
Phloem

Sclerenchyma

Mature sclerenchyma cells are dead and have secondary cell walls thickened with cellulose and usually impregnated with lignin. In contrast to collenchyma, which is pliable, sclerenchyma is elastic. The cell cavity orlumen is very small or it may disappear completely. There are two types of sclerenchyma cells, namely sclereids and fibres.

1. Sclereids: The cells are irregular in shape. The cell walls are thick, hard and lignified which makes the lumen very small. Simple pits (canals) are found in the thickened cell walls and link adjacent cells. Sclereids are commonly found in fruit and seeds.
2. Fibres: The cells are needle-shaped with pointed tips, thick walls and rather small lumen. Secondary cell walls, impregnated with, are formed. Simple pits are also present. Fibres are abundant in the vascular tissue of angiosperms, i.e. flowering plants.

Functions:

• sclerenchyma is an important supporting tissue in plants,
• sclereids are responsible for the hardness of date seeds and the shell of walnut,
• fibres probably play a role in the transport of water in the plant,
• starch granules are stored in the young, living fibres.

A line drawing of a sclerenchyma cell.

Epidermis

The epidermis is the outermost cellular layer which covers the whole plant structure, i.e. it covers roots, stem, leaves, flowers and fruit. It is composed of a single layer of living cells, although there are exceptions. Epidermis is usually closely packed, without intercellular spaces or chloroplasts. The outer walls, which are exposed to the atmosphere and usually thickened, and may be covered by a waxy, waterproof cuticle which are made up of cutin. Apart from the normal epidermal cells there are also stomata in the epidermis of leaves and stem. A stoma is an opening (pore) which is bounded by two beanshaped cells called guard cells . The guard cells differ from normal epidermal cells in that they have chloroplasts and the cell walls are thickening unevenly; the outer wall is thin and the inner wall (nearest the opening) is thick. The thin-walled epidermal cells of roots give rise to root hairs. Hair- like outgrowths may also be found in the epidermis of leaves and stems.

Functions:

• the epidermal cells protect the underlying cells,
• the waxy cuticle prevents the loss of moisture from the leaves and stems,
• the transparent epidermal cells allow sunlight (for photosynthesis) to pass through to the chloroplasts in the mesophyll tissue,
• the stomata of leaves and stems allow gaseous exchange to take place which is necessary for photosynthesis and respiration,
• water vapour may be given off through the stomata during transpiration,
• the root-hairs absorb water and dissolved ions from the soil.

A diagrammatic representation of a stomata, when open and close.

Parenchyma

Parenchyma is the most common plant tissue. It is relatively unspecialized and makes up a substantial part of the volume of a herbaceous plant and of the leaves, flowers and the fruits of woody plants. The thin-walled parenchyma cells have large vacuoles and distinct intercellular spaces.

Functions:

• the most important function of the parenchyma cells of roots and stem is the storage of food (e.g. starch) and water,
• the intercellular air spaces permit gaseous exchange.

A line drawing of a parenchyma cell.

Chlorenchyma

Chlorenchyma cells are actually parenchyma cells, but they contain chloroplasts, e.g. the parenchyma cells of leaves and stems. The mesophyll cells of leaves can thus be regarded as chlorenchyma.

Functions:

• the chlorenchyma are the main photosynthetic cells of the plant and manufacture carbohydrates during photosynthesis.

Collenchyma

Collenchyma tissues are mainly found under the epidermis in young stems in the large veins of leaves. The cells are composed of living, elongated cells running parallel to the length of organs that it is found in. Collenchyma cells havethick cellulose cell walls which thickened at the corners. Intercellular air spaces are absent or very small. The cells contain living protoplasm and they sometimes contain chloroplasts.

Functions:

• the collenchyma serve as supporting and strengthening tissue,
• in collenchyma with chloroplasts, photosynthesis takes place.

A line drawing of a collenchyma cell.

Xylem

Xylem is a complex tissue composed of xylem vessels, xylem tracheids, xylem fibres and xylem parenchyma.

1. Xylem vessels: Xylem vessels comprise a vertical chain of lengthened, dead cells known as vessel elements. The cells are arranged end to end and the cross-walls dissolve completely or have simple or complex perforation plates between successive cells. The secondary walls of vessels are impregnated with lignin and are thickened unevenly. The walls of the vessels may be thickened in different ways, e.g. annular, spiral and pitted thickening may be observed.
2. Xylem tracheids: A tracheids is an elongated cell, the contents of which are non-living. The cell walls are thickened, impregnated with lignin and the lumen is smaller. As in the case of vessels, there is a differentiation between annular, spiral and pitted tracheids again caused by the type of thickening of the secondary walls. Tracheids have no perforation plates.
3. Xylem fibres and xylem parenchyma bear a strong resemblance to normal fibres and parenchyma. Xylem fibres are sometimes separated by thin cross walls and the walls of xylem parenchyma are sometimes thicker than those of normal parenchyma.

Functions:

• xylem is an important strengthening tissue,
• xylem vessels and tracheids transport water and mineral salts,
• starch is sometimes stored in the xylem fibres and xylem parenchyma.

A line drawing of the different xylem cells.

Phloem

Phloem is a complex tissue composed of sieve tubes, companion cells, phloem fibres and phloem parenchyma

1. Sieve Tubes: A sieve tube, like xylem vessels, is a series of cells (sieve elements) joined end to end. The cross walls between successive cells (sieve elements) become perforated forming sieve plates. The cell walls arethin. Although the cell contents are living, the nucleus disintegrates and disappears. The lumen is filled with a slimy sap which is composed mainly of protein.
2. Companion Cells: Companion Cells are specialized parenchyma cells which always appear with the sieve tube element. They are also elongated, thin-walled and there is a distinct nucleus in the cytoplasm of the companion cell. Companion cells are linked with the sieve tubes by small canals filled with cytoplasm, which are smaller than pits.
3. Phloem Fibres: These cells are elongated tapering cells, found particular in the stem. They have thickened walls.
4. Phloem Parenchyma: Phloem Parenchyma is living and has thin cell walls. These cells form the packing tissue between all the other types of cells.

Functions:

• sieve tubes transport organic compounds,
• companion cells helps to regulate the metabolic activities of the sieve tube elements,
• the phloem fibres give the plant mechanical strength,
• the phloem parenchyma stores compounds such as starch.

A line drawing of the different phloem cells.

Courtesy---http://www.botany.uwc.ac.za/SCI_ED/grade10/plant_tissues/index.htm