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Friday, 14 December 2018

Location and Arrangement 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 roots are designed to withstand bending and pulling forces respectively.

Internal structure of herbaceous roots : internally, a root consists of an outer cylinder and an inner central cylinder of stele. A transverse section of a root shows the following arrangement of tissues from the circumference to the centre:

• Piliferous layer, 
• Cortex, including the endodermis,  } outer cylinder
• Vascular tissue,
• Pericycle, and 
• Pith. } inner cylinder or stele

    The outer cylinder consists of a wild 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 consists 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 : Cambuim, a meristematic tissue which gives rise to secondary growth, appears in older dicotyledonous roots but is completely absent in monocotyledonous roots.

Internal structure 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 shows the following arrangement of tissues from the circumference to the centre:

• Epidermidis,
• Cortex, including the endodermis, } Outer cylinder
• Pericycle,
• vascular bundles,
• Pith and medullary rays  }  inner cylinder or stele

    The Epidermidis is single cell thick and made up of closely packed rectangular cells. The cells are thickened on the outer walls by cutin,  a waterproof material which forms an 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 inner most later of the cortex is bound by the endodermis. The pericycle found just above the muscular bundles is Composed mainly of sclerenchyma. It is usually several cells thick, and forms 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, cambuim and xylem. The pitch consists of thin Walled parenchyma, which extends between the vascular bundles as medullary rays.

    A transverse section of a monocotyledonous stem shows an epidermidis and a ground tissue with vascular bundles scattered in it. The vascular bundles are surrounded by sclerenchyma which forms hollow, strengthening cylinders that run down the length  of the stem cortex, pith and cambium are absent. Usually there is a hypodermis which is a two or three cell thick layer of sclerenchyma below the epidermis. This forms a strengthening hollow outer cylinder.

   The collenchyma hollow cylinder are the main supporting tissues in the stem. Turgid parenchyma is an important supporting tissue in herbaceous stems. In woody stems, however, secondary xylem or wood is the main supporting tissues.

Note : Ground tissues include all plant tissues , except the dermal and vascular tissues. Thus, the cortex, pith and medullary rays are part of the ground tissue. In monocotyledons. The cambuim  is absent in the vascular bundle.

Leaves : leaves have a large surface area to carry out the  photosynthesis. In dicotyledons, a network of vascular tissue (veins) support the leaf blade from the forces of the wind. In monocotyledons, the vascular tissue is arranged in a parallel pattern.
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