There are no special excretory organs or systems in plants. The plants as living things…
SUPPORTING TISSUES IN PLANTS
Plants generally are known to possess supporting tissues which give them definite shape, strength, rigidity and resistance against external forces such as wind and water to which they are continuously subjected.
Types of Supporting Tissues in Plant
The main supporting tissues in plants are parenchyma, collenchyma, sclerenchyma (fibre) and wood (xylem).
Location: Parenchyma tissues are found in the cortex of stem, phloem, root, leaf, mesophyll, storage tissues and xylem.
Structure: They are composed of cells with large vacuoles and relatively thin wall. They are living cells with cellulose and many air spaces within them. Parenchyma tissue is the most common and abundant plant tissues.
- When the vacuoles are filled with sap, parenchyma tissues gives firmness and turgidity to the stems of herbaceous plants.
- They can also store food and water.
- They are found in the leaf and mesophyll, functioning mainly in the synthesis of food.
Location: Collenchyma cells are usually located in the cortex of stems, roots and in the hypodermis just beneath the epidermis.
Structure: The cells of collenchyma tissues are living, elongated and unevenly thickened at the corners. The cells are flexible and thus allow the bending and twisting strains to which the stem, roots and leaves of plants are often subjected.
- Collenchyma cells provides strength and supports in young growing plant parts such as stems, petioles and leaf blades.
- They gives flexibility and resilience to plants, i.e ., enable plants to bend without breaking.
Location: Sclerenchyma cells are found mainly in the pericycle in the vascular tissues and cortices of roots and stems.
Structure: Sclerenchyma have cells which are thicked walls containing lignin in addition to cellulose and other substances. There are two types of sclerenchyma. These are fibres and sclereids. Fibres are elongated cells with tapering ends. These help to provide strength and flexibility to plants. Sclereids unlike fibres are not too elongated but have great strength like fibres.
- The sclerenchyma (fibres) give flexibility to plants and prevent them from breaking up easily.
- They provide strength, rigidity, hardness and support to plants.
Wood or Xylem Tissues
Location: Wood or xylem tissues are found mainly in the vascular tissues of stems, roots and leaves.
Structure: The wood or xylem tissue is made up of many cells. These are tracheids, vessels, fibres and xylem parenchyma.
- Tracheids: Tracheids are non-living, elongated, tapering cells with thickened, lignified walls which have piths that aid the passage of water and dissolved mineral salts.
- Vessels: Vessels are long tabular structures that are formed by the fusion of several elongated cells stacked one on top of another.
- Fibres: Fibres are similar to sclerenchyma fibres. They are narrow, elongated cells with very thick walls and tapering end walls.
- Xylem parenchyma: These are similar to the parenchyma tissues. They are composed of cells with large vacuoles.
- The wood or xylem tissue provides support, strength and shape to the plant.
- It is also a conducting tissue as it helps to conduct water and dissolved mineral salts from the roots to the leaves.
Location: Closely associated with the major supporting tissues are the phloem tissues. The tissues are located within the vascular bundles of all plants be it in the roots, stems and leaves.
Structure: The phloem tissues are made up of four cells. These are sieve tubes, phloem parenchyma, companion cells and phloem fibres.
- Sieve tubes: These are made of elongated rows of cylindrical cells arranged vertically. The cells are living and they conduct mainly food.
- Phloem parenchyma: These are similar to the parenchyma cells earlier discussed. They provide strength and support to the plant. The cells also help in food storage.
- Phloem fibres: These are special cells which are concerned with the strengthening of the organs in which they are found.
- Companion cells: They are small and short cells which are vertically elongated like the sieve tube. They assist in the conduction of food substances.
- The general function of the phloem is to conduct manufactured food from their area of synthesis to areas where they are needed.
- Secondly, they assist to provide support to the entire plant.
Note: The vascular tissues (xylem and phloem) are found mainly in the roots, stems and leaves of plants.
MECHANISMS OF SUPPORT IN PLANTS
The entire plant body be it internal or external parts provide the necessary support to plants. The knowledge of the internal structure of the leaves, stem and roots will assist in the understanding of the mechanism of support in plant.
Epidermis or Piliferous layer: The epidermis is the outer covering of the leaves and stem while that of the root is piliferous layer. The epidermal layer is one-cell thick. Their function is protection. They prevent the inner cells from injury, infection and loss of water. In some cases only the guard cells of the leaves that has chloroplast are the only cells that can carry out photosynthesis.
Cortex: The cortex is found between the epidermis and vascular bundles of a dicotyledonous stem. The cortex is made up of three tissues which are collenchyma (on the outside), a middle parenchyma and an inner endodermis. The collenchyma is about three or four cells thick. The parenchyma is made of large, thin-walled cells with many air spaces. Both cells provide strength and support. The endodermis is a single layer which is also referred to as the starch sheat. It stores starch hence when stained with iodine solution will turn blue black.
Sclerenchyma: This layer is found on the outer part of vascular bundles. It consists of dead, lignified cells. It aids to strengthen the stem.
Vascular bundles: Vascular bundles are found in the inner part of the stem. It consists of xylem, phloem and cambium.
Xylem: Xylem is responsible for the conduction of water and dissolved mineral salts from the soil to the leaves through the roots and stems.
Phloem: Phloem is responsible for the conduction of manufactured food from their area of synthesis, e.g. leaves to the areas where they are required.
Cambium: Cambium is found between the phloem and xylem. The cambium cells are constantly dividing cells. This increase in size of cells which is called secondary thickening is responsible for the increase in size of the trunks of many trees.
Pith: This is the central part of the stem. It is large and made of parenchyma and extends between the vascular tissue.
In flowering plants, strength and rigidity are achieved by a combination of tugor pressure and supporting tissues. The parenchyma cells of the pith when fully turgid, push outside and this force is restrained by the inelastic epidermis.
Hence when the cells of the parenchyma tissue are fully expanded with water (turgid), they give rigidity and strength (hydrostatic support).
In the vascular bundles, the xylem vessels and fibres which are lignified add mechanical strength to the stems and roots of plant. The function of the cambium which contributes to the growth of tree trunks in width or girth(secondary growth or thickening) provides the necessary support and strength to plants. The wood fibres generally make stems strong and rigid. Other supporting tissues such as parenchyma, collenchyma and sclerenchyma provide the necessary strength needed by plants.
USES OF FIBRES TO THE PLANTS
Sclerenchyma fibres known simply as fibres provide flexibility and strength -two of the special functions of the strengthening tissues in plants. The fibres give mechanical functions, that is, the necessary strength, rigidity, flexibility and elasticity to the plant body and also enable it to withstand various strains. Plants like Hibiscus, jute and sisal are known to contain some of the strongest plant fibres hence they are used for making cloth, sacks, mats and ropes.
FUNCTIONS OF SUPPORTING TISSUES IN PLANTS
Supporting tissues provide the following functions to plants:
- Strengthening: Two major supporting tissues namely sclerenchyma and collenchyma are two distinct forms of strengthening cells found in plants. They provide the necessary strength required by plants.
- Rigidity: The supporting tissues like collenchyma, sclerenchyma and wood fibres provide the necessary materials to make the plant strong against any external forces.
- Resilience and Flexibility: The supporting tissues also provide the necessary materials which make the plants resilient and flexible thereby preventing the plants from being broken by the bending and twisting movements caused by strong winds.
- Protection: Some supporting tissues are known to protect the delicate parts of the plants body, e.g. cambium and phloem vessels.
- Distinct shape: Supporting tissues generally give distinct shapes to different plant species.
- Conduction: Some supporting tissues especially xylem and phloem tissues are known to also conduct water and manufacture food respectively within the plant.
How is support provided for in Herbaceous and Woody plants
- By turgidity, water is preserved.
- Due to the presence of water, vacuoles inside parenchyma cells/living cells;
- Thickening of walls of collenchyma cell (at the corners).
- Sclerenchyma in the cortex.
- By the xylem strengthened by deposits of ligning in their walls;
- Secondary growth/thickening provides extensive wood/secondary xylem and bark.