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Here are answers to Frequently Asked Question about supporting tissues in plants.

Plants do not have to move about to obtain food. So they do not need a highly developed skeletal system like animals. However, plants do not need supporting tissues to remain upright, spread out their branches and hold their leaves in the best positions for trapping sunlight.

What Are The Types Of Supporting Tissues?

The important types of supporting tissues in plants are;
  • turgid parenchyma (unspecialized plant cells)
  • collenchyma
  • sclerenchyma
  • xylem (wood). 
What Are The Arrangement And Location Of Supporting Tissues?

Most terrestrial plants have an aerial shoot system and an underground root system. The roots are surrounded by soil which they grip, and which in turn presses on them. The movements of the shoot also exert a pulling force on the roots. The upright aerial stems are subjected to the bending force of the winds. As a result, the internal structure of stems and roots are designed to withstand bending and pulling forces respectively. 

What Are The Internal Structure Of Herbaceous Roots?
Internally, a root consists of an outer cylinder and an inner central cylinder or stele. A transverse section of a root (image below) shows the following arrangement of tissues from the circumference to the centre: 
• piliferous layer, {outer cylinder}
• cortex, including the endodermis {outer cylinder}
• pericycle, {inner cylinder or stele}
• vascular tissues, {inner cylinder or stele}
• pith {inner cylinder or stele}
Dicotyledonous root and Monocotyledonous root

  The outer cylinder of a wide zone of loosely packed, thin-walled parenchyma bound on the outside by a single-cell thick piliferous layer. Root hairs arise from young cells of the piliferous layer. The endodermis is the innermost layer of the cortex. It is made up of a single-cell thick layer of barrel-shaped cells. Each cell is encircled by a thick waxy band. 
      The stele consist of vascular tissue, made up of alternate phloem and xylem bundles arranged in a ring. Usually there are more vascular bundles in a monocotyledonous root than a dicotyledonous one. The pericycle which bounds the vascular tissue on the outer side is a one or two-cell thick layer of thin-walled cells. The pith which is large in monocotyledonous roots is composed of thin-walled parenchyma. In most dicotyledonous roots, the xylem fills up the centre of the stele, forming a centrally supporting column.
   The main supporting tissues in roots are xylem and the turgid parenchyma which makes up the cortex.

Note : Cambium, a meristematic tissue which gives rise to secondary growth, appears in older dicotyledonous roots but it completely absent in monocotyledonous roots.

What Are The Internal Structures Of Herbaceous Stems?
A stem has more supporting tissues than a root. The arrangement of tissues in dicotyledonous stems differs markedly from that in monocotyledonous stems. A transverse section of a dicotyledonous stem (image below) shows the following arrangement of tissues from the circumference to the centre:
• epidermis, Cortex including the endodermis are outer cylinder

• pericycle, vascular bundles, pith and medullary rays are inner cylinder or stele.

well labeled Dicotyledonous stem diagram

 The epidermis is single-cell thick and made up of closely packed rectagular cells. The cells are thickened on the outer walls by cutin, a waterproof material which forms a  outer skin or cuticle. Stomata and lenticels are found between the epidermal cells. The cortex is made up of collenchyma and parenchyma. The collenchyma forms a strengthening hollow cylinder down the length of the stem. The innermost layer of the cortex is bound by the endodermis. The pericycle, found just above the vascular bundles, is composed mainly of sclerenchyma. It is usually several cells thick, and forms of solid strengthening strands that run down the length of the stem. The vascular bundles are arranged in a ring within the pericycle. Each bundle is composed of phloem, cambium and xylem. The pith consist of thin-walled parenchyma, which extends between the vascular bundles as medullary rays.

What Are The Main Features Of Supporting Tissues?

Turgid parenchyma : Parenchyma is a tissue made up of living unspecialized plant cells that are roughly spherical in shape. A parenchymatous cells has a thin but fairly rigid cell wall composed mainly of cellulose, and a large vacuole containing cell sap. Cell sap is a concentrated solution and has a high osmotic pressure. Water (absorbed by the roots) enters the cell and passes into the vacuole by osmosis. 
Collenchyma : This tissue consist of living cells that are thicker at the corners by the deposition of extra cellulose, to provide support and mechanical strength. They are usually polygonal elongated cells with tapering ends. 
  Collenchyma is found in the primary tissues of the plant. It is, therefore, an important supporting tissue in young plants, herbaceous plants and plants or plant organs, such as leaves in which secondary growth dies not occur. Collenchyma is usually found below the epidermis in stems and in the midrib of leaves. 
Xylem : This is the water conducting tissue which also has a strengthening function, especially in plants that undergo secondary growth. Four types of cell make up the xylem tissue; the tracheids, vessels, fibres and parenchyma. Of these, the first three are lignified and so provide mechanical support. 

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