structure and function of stem

Epidermis

The epidermis consists of a single layer of living cells which are closely packed. The walls are thickened and covered with a thin waterproof layer called the cuticle . Stomata with guard cells are found in the epidermis. In some stems either unicellular or multicellular hair-like outgrowths, trichomes, appear from the epidermis.

Functions
• The epidermis protects the underlying tissues.
• The cuticle prevents the desiccation of inner tissues and thus prevents water loss .
• The stomata allows gaseous exchange for the processes of respiration and photosynthesis.

Cortex This region comprises the collenchyma, parenchyma and endodermis. It is situated to the inside of the epidermis.

• Collenchyma These cells lie under the epidermis and constitute three to four layers of cells with cell walls thickened at the corners. The collenchyma cells contains chloroplasts
  
  
  
  Functions of the Collenchyma
  • This tissue serves to strengthen the young stem.
  • The chloroplasts are responsible for the synthesis of organic food during photosynthesis.

• Parenchyma Beneath the collenchyma cells are a few layers of thin-walled cells, parenchyma, with intercellular spaces. The parenchyma cells make up the bulk of the cortex.
  
  
  
  Functions of the Parenchyma
  • The synthesized organic food (mainly starch) is stored here.
  • The intercellular air spaces are responsible for gaseous exchange.

• Endodermis or Starch Sheath The endodermis or starch sheath forms the innermost layer of the cortex. This is a single layer of tightly-packed rectangular cells bordering the stele of the stem.
  
  
  
  Functions of the Endodermis
  • The cells of this tissue store starch.
  • It allows solutions to pass from the vascular bundles to the cortex.

Vascular Cylinder or Stele This region comprises the pericycle, vascular bundles and pith (medulla).

• Pericycle The pericycle is made up of sclerenchyma cells which are lignified, dead fibre cells . These cells have thick, woody walls and tapering ends.
  
  
  
  Functions of the Pericycle
  • It strengthens the stem.
  • It provides protection for the vascular bundles.

• Vascular Bundles The vascular bundles are situated in a ring on the inside of the pericycle of the plant. This distinct ring of vascular bundles is a distinguishing characteristic of dicotyledonous stems. A mature vascular bundle consists of three main tissues - xylem, phloem and cambium. The phloem is located towards the outside of the bundle and the xylem towards the center. The cambium separates the xylem and phloem which bring about secondary thickening.
  
  
  
  Functions of the Vascular Bundles
  • The xylem provides a passage for water and dissolved ions from the root system to the leaves.
  • The xylem also strengthens and supports the stem.
  • The phloem transports synthesized organic food from the leaves to other parts of the plant.
  • The cambium, divides to produce new xylem and phloem cells, making secondary thickening possible.

• Pith (Medulla) The pith occupies the large central part of the stem. It consists of thin-walled parenchyma cells with intercellular air spaces. Between each vascular bundle is a band of parenchyma, the medullary rays, continuous with the cortex and the pith.
  
  
  
  Functions of the Pith or Medulla
  • The cells of the pith store water and starch.
  • They allow for the exchange of gases through the intercellular air spaces.
  • The medullary rays transport substances from the xylem and phloem to the inner and outer parts of the stem.