Anatomy of Primary Dicot Stem

Welcome to MBC Biology!

A runner bean (Phaseolus coccineus) with young beans 

This post describes the internal structure of the primary dicot stem.

Primary dicot stem consists of epidermis, hypodermis, cortex, endodermis, pericycle, vascular bundles, medullary rays or pith rays, and pith or medulla (see Figure 14a & 14b.).

Epidermis

Epidermis is the outermost single layer of stem. It is made of compactly arranged, elongated, barrel-shaped parenchymatous cells without intercellular spaces. It is covered externally by cuticle. Stomata and several unbranched multicellular hairs or trichomes are also present. Each stoma has a pair of specialized kidney-shaped cells, guard cells. Epidermis helps in protection of internal tissues, exchange of gases through stomata, reduce the rate of surface transpiration by cuticle, prevents the entry of harmful organisms and heat insulation by epidermal hairs.

Hypodermis

Hypodermis lies below epidermis. It is made of 3- to 4-layered collenchyma. Collenchyma is green due to chloroplasts in it. It helps in mechanical support, cell elasticity, storage of food, and photosynthesis.

Cortex

Cortex lies below hypodermis. It is made up of a few to several layers of thin-walled, angular, oval or rounded parenchymatous cells with intercellular spaces. Its main function is to store food.

Endodermis

Endodermis is a single wavy innermost layer of cortex. It is made of barrel-shaped, compactly arranged, parenchymatous cells without intercellular spaces. Cells in it contain large number of starch grains. Due to presence of starch grains in cells, endodermis is also called starch sheath.

Pericycle

Pericycle lies between the endodermis and vascular bundles. It is generally heterogeneous or rarely homogeneous. Heterogeneous pericycle contains both the parenchyma and sclerenchyma. Sclerenchymatous pericycle is just outside the vascular bundles in the form of semicircular patches, called bundle caps. The parenchymatous pericycle is made either of only parenchyma or only of sclerenchyma. The sclerenchymatous pericycle gives mechanical support while the parenchymatous pericycle stores food.

Vascular bundles

Vascular bundles in dicot stem are present as eustele form i.e. ring of vascular bundles present inner to the pericycle around the central pith. They are wedge-shaped structures. Each vascular bundle consists of phloem at outer side, xylem towards inner side, and a strip of cambium in between phloem and xylem. So, the vascular bundle is conjoint, collateral, and open.

Phloem

Phloem lies towards pericycle. It consists of sieve tubes, companion cells, phloem parenchyma, and some phloem fibres. The sieve tubes are main conducting element of phloem that transport food.

Xylem

Xylem lies towards the pith or inner side of the vascular bundle. It consists of tracheids, vessels, xylem parenchyma, and xylem fibres. Tracheids and vessels form small protoxylem and large metaxylem. Protoxylem is the first formed xylem and lies towards centre whereas metaxylem is the later formed xylem and lies towards periphery. This type of xylem is called endarch xylem. Protoxylem has annular thickenings whereas metaxylem has pitted thickenings. Vessels remain in chains or rows. They are with angular outline. Xylem helps in conduction of sap and mechanical support.

Cambium

Cambium lies between xylem and phloem of same vascular bundle. It is a narrow strip of primary meristematic tissue. It is called intrafascicular cambium or fascicular cambium. In transverse section, cambial cells appear rectangular. Cambium helps in increasing the girth of stem by producing secondary phloem towards outer side and secondary xylem towards inner side.

Medullary rays or pith rays

Medullary rays lie between two adjacent vascular bundles. They are parenchyma with radially elongated cells. There may or may not be intercellular spaces. Medullary rays help in radial conduction of food and water. They also transport gases from pith to cortex and vice-versa.

Pith or Medulla

Pith is present at the centre of stem. It contains oval, rounded or polygonal parenchyma cells with or without intercellular spaces. Pith cells help in storage of food.

Figure 14a T. S. of dicot stem (diagrammatic view)

Figure 14b T. S. of dicot stem (detailed view)