T.S. of dicot stem shows following internal features:

Epidermis:

• It is the outermost layer and has a single layer of parenchymatous cells.
• It possesses stomata and large number of multicellular hairs (trichomes).
• The outer walls are greatly thickened and cutinized.
• The cells are compactly arranged and do not possess intercellular space.
• The epidermis has following functions:
• Minimize the rate of transpiration owing thick cuticle
• Protects the underlying tissues from mechanical injury
• Prevents the entry of harmful organisms
• Helps in the exchange of gases through stomata.

Hypodermis:

• This layer lies below the epidermis and is composed of 4 or 5 layers of collenchymatous cells.
• These cells are specially thickened at the corners against the intercellular spaces due to deposition of cellulose and pectin.
• The cells are living in nature and may contain few chloroplasts.
• It provides mechanical strength and elasticity to the peripheral portion of the stem particularly the young and growing organs.
• They perform photosynthesis and also acts as storage of food.

Cortex:

• It lies below the hypodermis.
• It consists of a few layers of thin-walled, large, rounded, or oval, living parenchymatous cells, having intercellular spaces.
• Cells of cortex may contain some chloroplasts which may function to manufacture of food materials.
• They serve for storage of food.

Endodermis:

• It is the single innermost layer of the cortex which separates the cortex from vascular bundles.
• Cells are somewhat barrel shaped and compactly arranged, having no intercellular spaces and are parenchymatous.
• Usually, the cells contain starch grains and thus the endodermis maybe termed as starch sheath.
• They serve as food reserve.
• The radial and the transverse walls are thickened due to the deposition of lignin forming casparian strips.

Pericycle:

• It lies in between the endodermis and vascular bundles.
• It is generally composed of sclerenchymatous and parenchymatous cells.
• The sclerenchyma is in the form of semilunar patches above the vascular bundles which give mechanical support to the plant parts.
• Similarly, parenchymatous pericycle is present outside the medullary rays which serves to store food.

Vascular bundles:

• These are arranged in a ring around the central pith and inner to the pericycle.
• These are conjoint, collateral, open and wedge-shaped.
• The size of the bundles varies in different species.
• Each bundle has a patch of xylem towards the center, a patch of phloem towards the periphery and a strip of cambium in between them.
• Xylem:
  • It lies towards the pith of vascular bundles.
  • It consists of tracheids, vessels, xylem parenchyma, xylem fibers.
  • Tracheids and vessels consists of smaller protoxylem and larger metaxylem.
  • Protoxylem is first formed that lies towards the center but metaxylem is later formed that lies towards the periphery.
  • This type of xylem is called endarch xylem. It helps in conduction of sap.
• Phloem:
  • It lies just below the sclerenchymatous patch of pericycle and is composed of following elements such as sieve tubes, companion cells, and phloem parenchyma.
  • It conducts the foods.
  • Cambium:
  • It lies in between xylem and phloem.
  • It consists of a narrow strip of meristematic cells having large nuclei and dense cytoplasm, called fascicular cambium.
  • It is responsible for secondary growth in thickness of the plant body.

Pith:

• It occupies the central portion of the stem.
• It is composed of thin walled parenchymatous cells which are rounded or polygonal, with or without intercellular spaces.
• Food is stored in this region.
• Medullary rays:
• These are the thin-walled, radially elongated parenchymatous cells present in between vascular bundles.
• These store food materials and help in internal translocation of water.

Internal structure of dicot stem

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