Dicotyledonous Stem – Class 11 | Chapter – 6 | Short Notes Series PDF

Dicotyledonous Stem: Dicotyledon plants, or simply dicots, are those whose seed include two cotyledons or embryonic leaves. This section will teach you about the properties and anatomy of the dicot stem. Dicot stems feature a circular arrangement of vascular tissues, whereas monocot stems contain vascular-tissue bundles distributed throughout. In monocots, the vascular bundles are also found on the outside of the stem.

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Structure of Dicotyledonous Stem

Dicotyledonous Stem Structure

Parts of Dicotyledonous Stem

  • The epidermis is the outermost layer of the (dicot) stem, including multicellular epidermal stem hairs. The cells are alive, barrel-shaped, and compactly packed, with no intercellular gaps or chloroplasts. They may have stomata for gas exchange. The epidermis is outwardly protected by a thick cuticle. The epidermal multicellular stem hairs aid in protection and heat loss.
  • Cortex is present in multiple levels under the epidermis. It is classified as follows:
  • The hypodermis is made up of 3-5 layers of collenchyma with no gaps between them. It comprises thickenings from the deposition of additional cellulose with pectin and provides mechanical support. It also aids in photosynthesis because of the presence of chloroplasts.
  • The general cortex encloses intercellular spaces with resin ducts, which are bordered by a layer of small thin-walled spherical or oval cells. It is loosely organised parenchymatous. Cortical cells may contain chloroplasts and undergo photosynthesis. Some oil conduits are lined with epithelial cells. It also aids in gaseous exchange and the storage of food items.
  • Endodermis is a unilayered innermost cortex composed of barrel-shaped, compactly packed parenchymatous tissues. It is often referred to as a starch sheath because it stores starch. Casparian stripes are plainly apparent in the endodermis. It is the deposition of lignin and suberin. Several dicot stems may lack separate endodermis.
  • The pericycle is heterogeneous, with sclerenchyma and intervening masses of parenchyma. They have thick wall sclerenchymatous tissue located in patches above the phloem or bast, which is why they are also known as hard bast or bundle cap. The hard bast aids in mechanical support, whereas the parenchyma aids in the storage of food supplies.
  • Medullary rays are parenchymatous, radially elongated or polygonal cells that exist between vascular bundles. It aids in the radial conduction of water and food items. Because it is an extension of pith, it is also known as pith rays.
  • Vascular bundles are conjoint, collateral, open, and organised in a ring (eustele). Each bundle is made up of outer phloem and inner xylem on the same radius, with a strip of cambium in between (open type).

Outside of the vascular bundle, phloem is made up of sieve tubes, companion cells, phloem parenchyma (all of which are alive), and a few bast fibres. Companion cells are linked to sieve tubes.

  • Xylem is found at the pith or centre of the plant and is made up of xylem vessels, tracheids, xylem fibres (wood fibres), and xylem parenchyma (wood parenchyma). The xylem is an endarch (i.e. protoxylem lie towards the pith, while metaxylem lies towards periphery). Protoxylem is narrower with annular, reticulate, or spiral thickenings, but metaxylem is wider with pitted vessels. The xylem parenchyma is usually located near the protoxylem.
  • Cambium is a thin strip of dramatically organised two or three-layered cells. It is found between the xylem and the phloem. In a transverse section, the cells are nearly rectangular and have a thin wall. It is also known as intra fascicular cambium due to its location. Cambium is a form of lateral meristematic tissue that enhances plant thickness through secondary growth.
  • The pith is well developed and takes up much of the ground tissue. It is composed of spherical or oval, thin-walled parenchymatous cells with a considerable number of intercellular gaps. Pith aids in the storage of food materials.

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By Team Learning Mantras