INTERNAL STRUCTURES OF PLANT ROOTS, STEMS AND LEAVES

Objectives           

This blog post provides readers with the following objectives. The reader will be able to:

o     Describe the internal structure of root, stem and leaf.

o     Distinguish between the different types of tissues of the root, stem and leaf.

o     Relate the structure of the types of cells to their functions.

o     Compare T.S. of root and stem of monocotyledonous and dicotyledonous plants. 

o     Distinguish between monocotyledonous and dicotyledonous plants. 

Internal Structure of Root

The longitudinal section through root shows different zones or regions.

Root cap

The outermost protective tip of the root. It protects the delicate apical region. It also eases the movement of the root through soil. It slimy substance, mucigel, which assist the movement of the root tip trough the soil

Meristematic region or Cell Division)

It consists of meristematic cells. The cells actively divide to form new cells which differentiate to form specialized root tissues. The new cells replace the worn-out cells of the root cap and also increase the length of the root

 

Region of elongation

Cells produced in the meristematic have vacuoles that absorbs water, swells and elongate. This causes the root to elongate. The meristematic and elongation zones are referred to as the region of growth

Region of Maturation or Differentiation

The enlarged cells undergo changes to become the specialized primary tissues. Primary tissues include epidermis, cortex and vascular system.

Primary Tissues of Root

The transverse section of root consists of the following structures  

Epidermis

Epidermis is the outer layer of the cells of the young root. The cells are closely- packed, thin-walled parenchyma cells with no cuticle, chloroplasts or stomata. The outermost of cells is called piliferous layer. Root hairs arise from the piliferous layer.

Cortex

The cell walls are thickened and contain suberin (fatty substance). The cortex consists of thin-walled parenchyma cells with numerous intercellular spaces.

Endodermis

Endodermis forms the innermost layer of the cortex. The cells are rectangular shape with thickened side walls called casparian strips. The casprin strip is made up of suberin. The casparian strips control the movement of water from cortex into the xylem.

Vascular Cylinder or Stele

Vascular Cylinder or Stele comprises all the tissues enclosed by the endodermis. It consists of the pericycle and vascular tissues (xylem and phloem).

Pericycle

Pericycle is a single cell thick, tightly-packed and meristematic. It gives rise to Lateral roots and vascular cambium. 

Vascular tissue

Vascular tissue is conducting tissue in the root. It consists of xylem and phloem, which are separated from each other by parenchyma.

□    Xylem: consists of non-living, thick- walled cells. It transports water and dissolved minerals from the roots to the leaves. It also provides structural support for plants.

□ Phloem: alternates between the arms of the xylem and consists of living thin-walled cells. It transports organic substances such as sucrose from the leaves to the roots.

Difference between Dicot root and monocot root   

Monocot Root	Dicot Root
Xylem is not star-shaped	Xylem is star-shaped
Wide pith present	Pith is absent
Cambium is absent	Cambium is present
Xylem has few vessels	Xylem has many vessels
Secondary growth usually does not occur.	Secondary growth takes place
Vascular bundles are many	Vascular bundles are few

Internal Structure of Stem

A longitudinal section through the stem reveals:  

1. Apical meristem or promeristem: the cells at the tip of the stem. The cells constantly divide to produce new cells, which contribute to an increase in length of the shoot. 

2. Region of elongation: Cells undergo rapid elongation and enlargement, by taking in water and developing vacuoles, and as result cause growth in length of shoot.

3. Region of maturation: is where the cells of the shoots acquire specific shapes and functions. They are therefore said to be differentiated or specialized.

A transverse section through dicot stem reveals the following:

Epidermis

Epidermis is the outermost covering of the stem. It is a single layer of tightly packed, brick-shaped parenchyma cells with no air spaces. There are several unbranched multicellular projections called trichomes. The outer wall is covered with a thin transparent waxy material called cuticle. The cuticle prevents excessive evaporation of water. It also prevents the entry of harmful organisms  

Cortex

It lies next to the epidermis. The main function is to store food.

 It is distinguished into three layers. 

1. Collenchyma cells:  small cells, the corners are thickened with pectin. This area carries out photosynthesis. It provides mechanical support to the stem. 

2.  Parenchyma cells: the cells are rounded or angular with intercellular spaces. It provides support to stem due its turgidity.

3.  Endodermis: the innermost layer of the cortex. The cells of endodermis are rectangular shaped and closely arranged without intercellular spaces.

Stele (Vascular cylinder)

Stele consist of pericycle, vascular bundles and pith.

Pericycle

Pericycle is the outermost covering of the stele. It’s a layer of compactly arranged sclerenchyma cells. Pericycle strengthens the stem. It also provides protection for the vascular bundles.

Vascular Bundle

Vascular Bundle consists of phloem, xylem, and cambium, arranged in the form of a ring around the central pith.

a. Cambium

Cambium is present between phloem and xylem. It contains thin walled, brick shaped, meristematic cells i.e., they divide to produce new cells. Cells produced to the outside differentiate as phloem tissue and those to the inside as xylem tissue. The division of the cambium cells results in secondary thickening. 

b. Xylem 

Xylem transports water and dissolved nutrients from the roots to all parts of the plant. It contains four conducting cells; tracheids, vessel elements, fibers and xylem parenchyma. All the cells have thick lignified walls and are dead at maturity

i.  Tracheids:  elongated, thin-walled cells with tapered ends. Cells are dead with empty lumen when mature. They have lignified cell walls with their end walls pitted.    

ii.  Vessel Elements:  elongated, hollow tubes arranged end to end. The walls are lignified in variety of ways forming annular, spiral, or reticulate thickenings. The vessel elements transport water more rapidly.   

iii.  Xylem Fibers: are long cells with much lignified wall. They provide support. 

iv.  Xylem parenchyma: are living cells, thin-walled, and their cell walls are made up of cellulose. They store metabolites.

c. Phloem 

Phloem transports manufacture food throughout the plant. Phloem is composed of sieve tube elements, companion cells and phloem parenchyma.

i. Sieve tube elements: are elongated, tube-like cells, stacked in vertical rows. They are living with no nuclei and have few cytoplasmic organelles. Their end walls are perforated by large pores to form the sieve plates. 

ii. Companion cells: are closely associated with sieve tube elements. They are narrow cells with nucleus and dense cytoplasm. They regulate the activities of the sieve tube. 

ii. Phloem parenchyma: is made up of elongated, thin-walled cells with dense cytoplasm and nucleus. It stores food material and other substances like resins, latex and mucilage.

Pith

Pith is the innermost part of the stem. It consists of loosely arranged parenchyma cells with intercellular spaces. It stores food. The pith expands among the vascular bundles. These extensions of the pith are referred to as pith rays/medullary rays.

Difference Between Dicotyledonous Stem and Monocotyledonous Stem

Monocotyledonous Stem	Dicotyledonous Stem
Numerous vascular bundles	Few vascular bundles
Vascular bundles scattered	Vascular bundles arranged in a ring
Vascular cambium is absent	Vascular cambium is present
Pith is absent	Pith is present
No Secondary thickening	Secondary thickening occurs
Has medullary rays	No medullary rays

Internal Structure of a Leaf

Epidermis 

The upper and lower surface of leaf is bound by upper and lower epidermis respectively.

Epidermis is a single layer of rectangular shaped cells containing few or no chloroplasts. The cells are transparent and tightly packed. The epidermis is covered with a waxy, waterproof cuticle. There are a large number of pores called stomata present on the lower surface. Each stomatal pore is surrounded by two bean-shaped cells called guard cells which contain chloroplasts. Stomata controls exchange of gases and transpiration.

Functions of the Epidermis

1.  The cuticle prevents water loss.

2.  The epidermis protects the internal tissues from injury.

3.  The stomata allows gaseous exchange for photosynthesis and respiration.

4..  The epidermis is transparent which allows light to reach the mesophyll tissue.

Mesophyll

Mesophyll is the ground tissue which lies between the upper and lower epidermis. It consists of loosely arranged parenchymal cells. It consist of two distinct regions.

Palisade mesophyll: it occurs below the upper epidermis. It consists of one or more layers of thin-walled, cylindrical cells oriented with their long axis perpendicular to the upper epidermis. The cells are filled with chloroplasts (usually several dozen of them).  It is the main site of photosynthesis in the leaf.

Spongy mesophyll: lies beneath the palisade layer. It is made up of loosely packed, irregular shaped cells with large intercellular air spaces. These large intercellular spaces allow free movement of air within the leaf. The air spaces are interconnected and open to the outside through stomatal pores. The cells contain fewer chloroplasts.

Functions of the Mesophyll

1. It contains numerous chloroplasts which absorbs light for photosynthesis.

2. Intercellular air spaces allow exchange of gases.

The Veins (Vascular bundles)

The vascular bundles of a leaf extend densely through the mesophyll. Each vein contains two types of vascular tissues: Xylem and Phloem. Veins are surrounded by layers of parenchyma or sclerenchyma cells making up the bundle sheath.

Functions of the Vascular Bundles

1.  The veins strengthen the lamina.

2. The xylem conducts water and dissolved ions to the mesophyll tissue.

3.  The phloem conducts organic food from the mesophyll to other parts of the plant.

Cell Types in the Plant

Parenchyma Cells

1.     cylindrical in shape and have thin wall

2.     living at maturity

3.      usually loosely packed, with large intercellular spaces

4.       help in synthesizing and storage of synthesized food products

5.       controls plant's metabolic reactions

6.       play a vital role in healing and regeneration of plants

Collenchyma Cells

1.    elongated and irregularly thickened at the corners

2.    living at maturity with no intercellular spaces

3.    contain few chloroplasts

4.    provides a support to herbaceous plants 

Sclerenchyma Cells

1.     long, narrow and non-living

2.     thick, lignified cell walls and lack protoplasts at maturity

3.     provide strength and support to plants

4.     Sclerenchyma cells are of two types:

i. Sclereids: short cells with irregular shape, thick, lignified secondary walls

ii. Fibers: are long, slender and are arranged in threads.

Differences Between Dicotyledonous and Monocotyledonous Plant

Monocotyledonous	Dicotyledonous
One cotyledon	Two cotyledons
Narrow leaf shape, wraps around stem	Broad, flattened leaf shape
Leaf venation mostly parallel	Leaf venation mostly net-like or reticulate
Leaves are sessile (absence of petioles)	Leaves have petiole
Fibrous root system	Taproot system
Flower parts usually in threes or multiples of three	Flower parts usually in fours or fives or in multiples of four or five
Usually herbaceous, never woody	Woody or herbaceous
Pollen grain has one furrow or aperture	Pollen grain has 3 furrows or apertures
Vascular bundles in stem are scattered	Vascular bundles in stem arranged in a ring
Vascular tissue in root arranged in a ring	Root xylem usually star-shaped, the phloem between arms of star
Absence of cambium	Presence of cambium
Seed germination is mostly hypogeal	Seed germination is mostly epigeal

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