Anatomy of Dicot Stem: The Primary Structure – Key Points with PPT


Stem Structure Anatomy

Anatomy of Dicot Stem
Anatomy of the Primary Structure of Dicot Stem

“Biology is the study of complicated things that have the appearance of having been designed with a purpose….”
Richard Dawkins

Ø  The anatomy of dicot stem is studied by a T.S. (transverse section) took through the internode of the 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.

Dicot stem structure

Plant Anatomy Diagram Dicot Stem

Ø  Cells are closely packed without any intercellular spaces.

What is cuticle / cutinØ  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

  Cuticle prevent water loss

o   Stomata in stem facilitate gaseous exchange.

  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

(2). Cortex: (a). Hypodermis

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

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

what is hypodermisØ  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.

(2). Cortex: (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

(2). Cortex: (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.

what is casparian thickeness(2). 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.

what is stele?(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

(3). Stele: (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.

Open Vascular Bundle Structure

(3). Stele: (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.

difference between collateral and bicollateral vascular bundle(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.

open vascular bundle 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’.

(3). Stele: (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.

Plant Anatomy Diagram Dicot Stem

(3). Stele: (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.

what is pith?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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