Parenchyma: Structure, Classification and Functions

The tissue (a group of cells with particular function) composed of single type of cells. Three types of simple tissue system in plants, namely (1).  Parenchyma, (2).  Collenchyma and (3).  Sclerenchyma. The present article discusses the Structure and Types of Parenchyma Cells in Plants.

Characteristics of Parenchyma (Parenchymatous Cells)

Ø  Parenchyma is a simple permanent tissue.
Ø  They are living cells which contains plenty of water.
Ø  Cells are nucleated with prominent nucleus.
Ø  They are thin walled cells.
Ø  Cell wall composed of cellulosic primary cell wall only.

Ø  No lignin deposition in the cell wall of parenchyma.

Ø  Parenchymatous cells are relatively undifferentiated

Ø  Parenchyma is the least specialized along simple permanent tissues in plants.

Ø  Parenchyma usually occupies in the ground tissue of stem, root, leaves, petiole and fruits.

Ø  They are also found in xylem and phloem as xylem parenchyma and phloem parenchyma respectively.

Ø  The parenchyma of ground tissue is originated from the ground meristem.

Learn more: Classification of Meristems

Ø  Parenchyma of the xylem and phloem are originated from the cambium.

Types of Parenchyma

Ø  Different types of parenchymatous cells occur in plants.

Ø  Classification of parenchyma in plants is based on two criterions: (1) Function and (2) Shape.

Ø  Based on functions the parenchyma is classified into Seven categories, they are:

(1).    Chlorenchyma

(2).    Aerenchyma

(3).    Prosenchyma

(4).    Vascular Parenchyma

(5).    Medullary Parenchyma

(6).    Conjunctive Parenchyma

(7).    Armed Parenchyma

(1). Chlorenchyma

Ø  They are the parenchymatous cells with chloroplasts.

Ø  Chlorenchyma usually present in the mesophyll tissue of leaves.

Ø  They are also present in the outer cortex young stem.

Ø  The green colour of young stem and aerial roots are due to the presence of chlorenchyma.

Ø  Chlorenchymatous cells are usually loosely packed with many intercellular spaces.

Ø  Function of chlorenchyma: photosynthesis

(2). Aerenchyma

Ø  They are parenchyma cells with plenty of intercellular spaces.

Ø  The huge intercellular spaces appear as large air cavities.

Ø  Air cavities of aerenchyma can store air.

Ø  Aerenchyma is a characteristic of aquatic plants.

Ø  In aquatic plants, aerenchyma assists in floating.

Ø  They provide buoyancy to the plants to balance in the water.

Ø  Respiratory gases (CO2 or O2) stored in aerenchyma can be utilized by the plants when required.

(3). Prosenchyma

Ø  They are elongated and narrow sized parenchyma.

Ø  They possess interpenetrating tapering (pointed) ends.

Ø  Prosenchyma is usually present in the vascular tissues of higher plants.

(4). Vascular parenchyma

Ø  They are the parenchyma cells occur in the vascular tissue of plants.

Ø  They provide nourishment to the vascular tissues.

Ø  They can also store ergastic substances like resins, tannins etc.

Ø  Axial parenchyma and ray parenchyma of wood are best examples.

Ø  Parenchymatous cells in heart wood (old secondary xylem) forms tyloses which can block the vessels cavities and can prevent the water flow through them.

Lear more: Tyloses formation and its Functions in Wood

(5). Medullary parenchyma

Ø  Medullary parenchyma occurs in the medullary ray of primary vascular tissue of stem.

Ø  They are radially elongated cells with very thin walled cells.

Ø  They allow radial condition of water and minerals.

Ø  They can also store starch grains.

(6). Conjunctive parenchyma

Ø  They form the conjunctive tissue of the stele of roots.

Ø  In most of the monocots, the conjunctive tissue becomes sclerenchymatous during maturation.

(7). Armed parenchyma

Ø  They are star shaped parenchymatous cells with many spiny projections to the interior of the cells.

Ø  Armed parenchyma is found in the mesophyll of some gymnosperm.

Ø  Example: mesophyll cells of Pinus leaf (needle).

Based on shape in cross section (C.S. or T.S.), parenchyma is classified into two categories, they are:

(1).   Angular Parenchyma

(2).   Circular Parenchyma

(1). Angular parenchyma

Ø  In angular parenchyma, the cells are angular and polygonal in outline.

Ø  This parenchyma is usually compactly packed without any intercellular spaces.

(2). Circular parenchyma

Ø  In circular parenchyma, the cells are circular (round) in outline.

Ø  They are usually loosely packed with plenty of intercellular spaces.

Functions of Parenchyma

@.  Parenchyma forms the fundamental ground tissue system in plants.

@.  Parenchymatous cells can store water.

@.  They can store food materials as starch grains, protein grains or oil droplets.

@.  Chlorenchyma of leaves and young stem assist in photosynthesis.

@.  Aerenchyma in aquatic plants provides buoyancy.

@.  Parenchyma can store ergastic substances.

@.  Parenchyma in secondary wood form tyloses.

@.  All meristematic cells are parenchymatous cells

@.  Parenchyma can be easily differentiated into secondary meristem (Eg. cork cambium).

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