Anatomy of Dicot Stem: The Primary Structure

The present post discusses the Primary Structure of Dicot Stem studied under microscope. The anatomy of dicot stem is studied by a T.S. (transverse section) took through the internode of the stem.

Primary Structure of Dicot Stem

Ø  The components of cortex and stele are together known as Ground Tissue.

Ø  Anatomically the dicot stem has the following regions:

(1). Epidermis

(2). Cortex

a).  Hypodermis
b).  Outer cortex
c).   Inner cortex
d).  Endodermis

(3). Stele

a).  Pericycle
b).  Vascular bundles
c).  Medullary rays
d).  Pith

(1). Epidermis

Ø  Epidermis is the outermost layer, composed of parenchymatous cells.

Ø  Usually, epidermis composed of single layer of cells.

Ø  Cells are closely packed without any intercellular spaces.

Ø  The outer tangential wall of epidermal cells is thicker than other walls.

Ø  This wall area is deposited with fatty substances called cutin.

Ø  The cutin over the cell wall occurs as separate layer called cuticle.

Ø  The epidermis of young stem also contains few stomata.

Ø  Multicellular hairs (called trichome) are usually present in the epidermis.

Ø  In herbaceous plants, where secondary growth is absent, the epidermis remains throughout the life cycle.

Ø  However, in woody plants, the epidermis is replaced after the secondary growth due to back formation.

Ø  Functions of epidermis:

o   Protection

o   Cuticle prevent water loss

o   Stomata in stem facilitate gaseous exchange.

o   Trichomes and hairs provide protection from fungal spores and insect pests.

(2). Cortex

Ø  Cortex is the tissue occupied just inner to the epidermis.

Ø  In some plants, the cortex is simple and undifferentiated.

Ø  In majority of plants, the cortex is differentiated into many zones.

Ø  Usually the cortex in dicot stem composed of FOUR zones.

a.       Hypodermis
b.      Outer cortex
c.       Inner cortex
d.      Endodermis

(a). Hypodermis

Ø  Hypodermis is the layer of tissue just below the epidermis.

Ø  Cells of hypodermis are collenchymatous and with thick primary wall.

Ø  Cells are compactly packed without any intercellular space.

Ø  In very young stem, the collenchyma is poorly developed.

Ø  In stem with ridges and furrows, the collenchyma mainly occurs below the ridges.

Ø  Usually, chloroplasts absent in the hypodermis.

Ø  Rarely collenchymatous cells of hypodermis do contain chloroplasts.

Ø  In xerophytic plants, the hypodermis is sclerenchymatous.

Functions of hypodermis:

o   Provide mechanical support.

o   In plants with secondary thickening, hypodermal cells give rise to cork cambium which produces the bark.

(b). Outer cortex

Ø  Outer cortex consists of the tissue occupied just inner to the hypodermis.

Ø  Cells of this region are chlorenchymatous (parenchyma with chloroplasts).

Ø  The green colour of young stem is due to his region.

Ø  The cells are loosely packed with plenty of intercellular spaces.

Ø  In xerophytes, the outer cortical cells forms palisade like tissue for photosynthesis, since these plants usually lack leaves.

Function of outer cortex: photosynthesis

(c). Inner cortex

Ø  This is the tissue inner to outer cortex.

Ø  Composed of loosely packed parenchymatous cells.

Function inner cortex: storage of carbohydrates.

Special features of cortex in some plants:

Ø  In hydrophytes, the cortex is with plenty of air cavities (aerenchymatous).

Ø  The Aerenchyma helps in gaseous exchange and provides buoyancy of to plants.

Ø  Sclerenchymatous patches occur in the cortex of Eucalyptus, Eugenia, Ficus.

Ø  Secretory cavities occur in the cortex of Eucalyptus.

Ø  Resin canals occur in the cortex of Anacardium.

Ø  Laticifer cells occur in the cortex of latex producing plants.

Functions of cortex

Ø  Hypodermal layer provides mechanical support.

Ø  During secondary growth, the hypodermal cells give rise to the cork cambium (phellogen) for the bark formation.

Ø  Chlorenchymatous cells in the outer cortex can do photosynthesis.

Ø  Parenchymatous cells of inner cortex can store carbohydrates.

Ø  Cortical cells also store ergastic substances.

Ø  Resin canals, latex canals etc. occurs in the cortex.

(d). Endodermis

Ø  Endodermis is the innermost layer of cortex.

Ø  The endodermis is very distinct in lower plants such as Pteridophytes.

Ø  NOT distinct in the stem of Gymnosperms and Angiosperms.

Ø  Cells of the endodermis accumulate plenty of starch as grains. Thus, the endodermis is also called starch sheath or starch band or starch layer.

Ø  If distinct, the endodermis is uniseriate (single layer) with barrel shaped cells.

Ø  Cells paranchymatous and they compactly arranged.

Ø  Endodermal cells have characteristic thickness in radial and inner tangential walls.

Ø  This thickening is called casparian thickening (casparian band, casparian layer).

Ø  The casparian band is composed of suberin and lignin, both of them are impervious to water.

Ø  Due to the presence of casparian thickening, they block the passage of water and solutes through the protoplasts of endodermal cells.

Functions of endodermis

Ø  The exact function of endodermis is not known.

Ø  They do not allow the passage of water from cortex to stele, thus may have specific role in the conduction of water.

Ø  They can store food material as starch grains.

(3). Stele

Ø  Stele is the central vascular cylinder of the stem.

Ø  The stele of stem composed of four components.

a)     Pericycle
b)     Vascular bundle
c)      Medullary rays
d)     Pith

(a). Pericycle

Ø  Pericycle is the outermost layer of the stele.

Ø  It is located next (just inner) to the endodermis.

Ø  The nature of pericycle in stem shows wide variation.

Ø  Pericycle is absent in some plants.

Ø  If present, it usually multilayered composed of 3 or more layers of cells.

Ø  The pericycle in the stem of different plants may be:

o   Completely parenchymatous

o   Completely sclerenchymatous

o   Mixture of parenchyma and sclerenchyma (alternating bands)

Ø  Sclerenchymatous pericycle forms the bundle sheath of the vascular bundle in most of the dicot plants.

(b). Vascular bundle

Ø  Vascular bundles (VB) are also called as fascicles.

Ø  They are located inner to the pericycle.

Ø  VB are developed from the pro-cambium.

Ø  The number of vascular bundles is limited in dicot stem.

Ø  Usually, 6 to 8 vascular bundles are present and they are arranged as broken ring in the ground tissue.

Ø  Vascular bundles of a typical dicot stem are:

o   Conjoint: (= xylem and phloem together as bundle)

o   Open: (= vascular bundles with cambium)

o   Collateral or Bicollateral

Ø  Collateral: the usual type of vascular bundle composed of once patch of xylem and one patch of phloem and a strip of cambium between them.

Ø  Biocollateral: a special type of vascular bundle composed of a median patch of xylem laying in-between two phloem patches.

Ø  Bicollateral VB is characteristic of Cucurbitaceae family (Example: Cephalandra, Cucurbita).

Learn more: Vascular bundles: Structure and Classification

Ø  The vascular bundles composed of (I) Xylem placed inner to cambium; and (II) Phloem placed outer to cambium.

(I). Xylem

Ø  Xylem is the water and minerals conducting tissue of vascular bundles.

Ø  It is a complex tissue, composed of tracheids, vessels, fibres and parenchyma.

Ø  Xylem in the VB is differentiated into:

o   Protoxylem

o   Metaxylem

Protoxylem

Ø  Protoxylem is the first formed part of xylem in the VB.

Ø  It is arranged towards the centre of the stem.

Ø  Protoxylem composed of very less amount of tracheary elements and large amount of parenchyma.

Ø  Tracheary elements are with very narrow lumen.

Ø  They show annular or spiral thickening in their secondary wall (primitive type).

Metaxylem

Ø  Metaxylem is the xylem part formed after the protoxylem.

Ø  It is arranged towards the exterior of the stem.

Ø  They composed of more tracheary elements then protoxylem.

Ø  The cells of the tracheary elements are with large lumen than that of protoxylem.

Ø  They show reticulate or pitted thickening (advanced type).

Ø  Functions of xylem:

o   Conduction of water

o   Conduction of minerals

o   Provide mechanical support

o   Xylem parenchyma store food materials

(II). Phloem

Ø  Phloem is the food conducting tissue of vascular bundles.

Ø  Similar to xylem, phloem is also a complex tissue composed of sieve tubes, companion cells, phloem parenchyma and phloem fibres.

Ø  The primary phloem is differentiated into:

o   Protophloem: first formed phloem, arranged towards periphery.

o   Metaphloem: differentiated after protophloem, located near to cambium.

(III). Cambium

Learn more: Characteristics of Meristematic cells

Learn more: Difference between meristem and permanent tissue

Learn more: Classification of Meristems

Ø  Cambium is a layer of meristematic tissue present between xylem and phloem.

Ø  Cambium present in the VB is called as fascicular cambium or vascular cambium.

Ø  It is the remnant of original pro-cambium.

Ø  The cambial cells are parenchymatous and thin primary cell wall.

Ø  Cells with dense cytoplasm and prominent nucleus.

Ø  Secondary growth in dicots occurs due to the activity of cambium.

Ø  Vascular bundle with cambium is called ‘open vascular bundle’.

(c). Medullary Ray

Ø  It is also called pith ray.

Ø  Medullary ray is a layer of tissue occurs between vascular bundles.

Ø  It is composed of loosely packed parenchymatous cells with plenty of intercellular spaces.

Ø  The cells of the medullary ray are radially elongated.

Ø  During secondary growth, cells of the medullary rays give rise to inter-fascicular cambium.

Ø  The fascicular and inter-fascicular cambium fuse together to form a complete ring of cambium and this produce secondary xylem and secondary phloem.

Functions of medullary ray

Ø  Allow radial transport of water.

Ø  Provide inter-fascicular cambium during secondary growth.

(d). Pith

Ø  It is also called as medulla.

Ø  Pith is the exact central portion of the stem.

Ø  It is located towards the inner side of vascular bundles.

Ø  Usually, the pith composed of parenchymatous cells.

Ø  Parenchyma may be loosely arranged with many intercellular spaces.

Ø  Sometimes the parenchymatous cells undergo secondary wall thickening.

Ø  Cells of outer region of the pith are smaller whereas, those in the inner region larger.

Ø  In some plants, the pith is replaced by a large air filled cavity called Pith Cavity.

Function of pith: storage of food materials

Identification reasons of Dicot Stem Primary Structure (Practical exam)

Stem

Ø  Vascular bundle conjoint, open, collateral or bicollateral.

Ø  Xylem endarch (protoxylem arranged towards the centre).

Ø  Endodermis not distinct.

Dicot stem

Ø  Collenchymatous hypodermis.

Ø  Ground tissue differentiated to hypodermis, cortex and stele.

Ø  Limited number of vascular bundles, usually 6 to 8

Ø  Vascular bundles are arranged as a broken ring

Ø  Vascular bundles, conjoint, open, collateral or bicollateral.

Ø  Pith large and well developed.

Reference:

Ø  Prakash J.J., 2000, Test Book of Plant Anatomy,  Ed. 2, Emkay Publications, New Delhi

Ø  Esau K, 1965, Plant Anatomy, Ed. 2, Wiley Eastern Private Limited, New Delhi

Key Questions

Ø  The primary structure of a typical dicot stem

Ø  Tissue differentiation in dicot stem

Ø  Structure of vascular bundle in dicot stem

Ø  How dicot stem is different from the monocot stem?

Ø  How stem is different from root?

Ø  What are the functions of medulla and pith?

Ø  Differentiate collateral and bicollateral vascular bundles

Ø  What is the importance of casparian thickening?

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