Concept of Plant Tissues

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In this post, I am presenting a concise description of the plant tissues and their types.

What is Tissue? 

The term ‘tissue’ was coined by Nehemiah Grew, an English Anatomist and Physiologist, also known as ‘Father of Plant Anatomy’ in 1682. The term ‘tissue’ denotes a group of cells having a common origin and co-operating with one-another to perform a similar function.

Plant tissues and their types

As the plants are multicellular organisms, they contain different types tissues. Based on the capacity of division, their tissues are of two types – meristematic tissues or meristem and permanent tissue.

A. Meristems

In 1858, Nageli derived the term ‘meristem’ from Greek word ‘Meristos’, which means ‘divisible’. So, the meristem is a group of young cells which retain the capacity of cell division. It occurs in the regions where growth occurs. In principle, meristem has following features:

• Meristems are composed of immature cells with capability to divide and grow.
• Intercellular spaces between cells are absent.
• Cells contain thin and elastic cell walls made of cellulose.
• Cells are oval, rounded, polygonal or rectangular.
• They do not store reserve food materials.
• The cells have dense cytoplasm with large, distinct and prominent nuclei.
• Vacuoles are either absent or very small.
• Metabolic activities are very high.
• Colourless proplastids are present.

Meristems are classified on the basis of origin and development, position, and function.

I. Meristems on the basis of origin and development

Based on the origin and development, meristems are classified as promeristems or primordial meristems, primary meristems, and secondary meristems.

1. Promeristems 

These are youngest cells in growing region such as apices of shoots and roots. They give rise to primary meristems. They are also called embryonic meristems as they develop directly from the embryo (See Figure 1 a.).

2. Primary meristems

Primary meristems are derived from promeristem. They are found at the tip of root, stem and appendages. They make primary structure of a plant. Intrafascicular cambium of dicot stem belong to these meristems (See Figure 1 b.).

3. Secondary meristem 
Secondary meristems are formed from the permanent tissues by dedifferentiation of permanent tissues. Examples are interfascicular cambium and cork cambium (See Figure1 c.).

II. Meristems on the basis of position
On the basis of position, meristems are apical meristems, intercalary meristems, and lateral meristems (See Figure 2).

1. Apical meristems
Apical meristems are present at the tips or apex of stem, bud, root, and leaf. They increase the length of plant.

2. Intercalary meristems
Intercalary meristems are derived from apical meristems. They develop when permanent tissues are formed in between apical meristems. They are found at the base of leaves or at the bases of internodes.

3. Lateral meristems
Lateral meristems are present along the sides of stem. Cells of these meristems divide and increase thickness or girth of plant. An example is intrafascicular cambium in dicot stem.

Figure 2 L. S. of shoot showing the positions of meristems

III. Meristems on the basis of function
Based on the function, meristems are protoderm, procambium, and ground meristem (See Figure 3).

1. Protoderm
Promeristem is the outermost layer of meristem. It gives epidermis or epiblema of the developing parts of plants.

2. Procambium
Procambium lies internal to the protoderm. It gives rise to primary vascular tissues.

3. Ground meristem
These are meristematic tissues except protoderm and procambium in primary meristems. They make ground tissues such as hypodermis, cortex, endodermis, pericycle, medullary rays, and pith.

B. Permanent Tissues

Permanent tissues are the tissues that have their growth has stopped either completely or they do not divide till their dedifferentiation happens. Sometimes, they regain meristematic activity partially or wholly. These tissues may be living or dead. The living permanent tissues have thin-walled or thick-walled cells whereas the dead permanent tissues have thick-walled cells. Permanent tissues are of two types – simple permanent tissues and complex permanent tissues.

I. Simple Permanent Tissues
These tissues are made up of one type of cells forming a uniform homogeneous system of cells. Simple permanent tissues are of three types – parenchyma, collenchyma, and sclerenchyma.

1. Parenchyma
Parenchyma is a tissue made of thin-walled living, similar, oval, rounded or polygonal isodiametric cells with or without intercellular spaces. Internally, cells have large central vacuole and peripheral cytoplasm, and nucleus. It is found in the non-woody or soft areas of stems, leaves, roots, flowers, and fruits etc. As they perform special function, they are classified as epidermis, epiblema, simple parenchyma, chlorenchyma, aerenchyma, prosenchyma, idioblast, phloem parenchyma, xylem parenchyma.

a. Chlorenchyma
Parenchyma having chloroplast are called chlorenchyma. They are found in green stem, green fruits, and leaves. In leaves, they are called mesophyll tissues. Mesophyll tissues are differentiated into palisade parenchyma and spongy parenchyma (See Figure 4 a.).

b. Aerenchyma
Aerenchyma are parenchyma with air cavities. They are found in aquatic plants and some land plants (xerophytes). The air cavities are surrounded by thin-walled living oval, rounded, or irregular cells. Air cavities store gases to make aquatic plants light and buoyant (See Figure 4 b.).

c. Prosenchyma
Prosenchyma are slightly thick-walled living fibre-like elongated parenchyma (See Figure 4 c.). They are found in pericycle and conjunctive tissues. They provide mechanical support and storage site of food.

Functions of Parenchyma

• Parenchyma helps in storage of food.
• They provide turgidity to softer parts of plants.
• Epidermis helps in protection of inner tissues.
• Chlorenchyma help in photosynthesis.
• Aerenchyma provide buoyancy and storage of metabolic gases.
• Phloem and xylem parenchyma helps in slow lateral conduction of materials.
• Prosenchyma help in mechanical support.
• Epiblema help in absorption of sap.

Figure 4 c. Prosenchyma tissue

2. Collenchyma

Collenchyma is a simple living permanent tissue with the cells having deposition of cellulose and pectin in specific areas of their wall. They are elongated, circular, oval or angular in transverse section. Internally, they possess large central vacuole and a peripheral cytoplasm along with nucleus. They may have a few chloroplasts. They are generally found in hypodermis of petiole, pedicel, leaf and stem of herbaceous dicot plants in the ridges; absent in woody dicot stem, monocot stem, and roots. Based on the thickening of cell wall, they are angular, lamellate, and lacunate collenchyma.

a. Angular collenchyma
They have wall thickening at the angles (See Figure 5 a.). Examples are collenchyma in stem of tomato, Datura, Salanum, Tagetes, etc.

b. Lamellate collenchyma
The wall thickenings are at tangential walls (plate like thickening) (See Figure 5 b.). Examples are collenchyma in stem of sunflower and Rhombus etc.

c. Lacunate collenchyma
The wall thickening is in the intercellular spaces but with a small hollow cylinder (lacuna) (See Figure 5 c.). Examples are collenchyma in stem of Cucurbita and petiole of Salvia etc.

Functions of Collenchyma

• Collenchyma provides mechanical strength to young stem, leaves, and petioles of herbaceous plant.
• It helps in cell elasticity and support to the growing organs.
• It provides support to delicate leaf margins and prevents tearing of leaves.
• It provides flexibility to organs and allows their bending. So, it prevents lodging of  herbaceous dicot stem.
• It takes part in photosynthesis because it has chloroplast.
• It helps in storage of small amount of food.
• It allows growth and elongation of organs.
• Cells of collenchyma undergo dedifferentiation and can form cork cambium or phellogen.

3. Sclerenchyma
Sclerenchyma is a simple highly thick-walled dead tissue. The wall is made up of cellulose or lignin or both. It is found in hard part of plants. It provides mechanical support and stiffness to plants and their parts. Sclerenchyma is of two types – fibres and sclereids.

a. Fibres
Fibres are highly elongated, narrow, and spindle-shaped thick-walled dead cells with pointed end walls. They are arranged in longitudinal bundles. Fibres occur in mechanical strength requiring parts like leaves, petioles, cortex, pericycle, phloem, and xylem as well as around the vascular bundles (monocot stem). Fibres are of three types – wood fibres (in xylem), bast fibres (in phloem), and surface fibres (present in other than xylem and phloem) (See Figure 6 a.).

b. Sclereids
Sclereids are highly thickened dead sclerenchyma cells with very narrow cavities. They are broader than fibres. They may be isodiametric, polyhedral or cylindrical. The thick cell wall has branched or unbranched simple pits. They occur singly or in groups. They provide stiffness to the plants.

The different types of sclereids are stone cells or brachysclereids, macrosclereids, osteosclereids, astrosclereids and filiform sclereids (See Figure 6 b.). Brachysclereids give gritness to fruits of guava, apples, and pears etc. Macrosclereids are found in epidermal covering of some legume seeds. Osteosclereids are found in sub-epidermal covering of some legume seeds. Astrosclereids are found in tea leaves and petioles of lotus. Filiform sclereids are found in stem of hydrophytes.

Functions of Sclerenchyma

• Fibre type sclerenchyma help in mechanical support to the various parts of plants.
• Fibres allow the plant organs to tolerate bending, shearing, compression, and pull forces by environmental factors.
• Numerous fibres are commercially used e.g. Corchorus (Jute), Linus (Flax), Cannabis (Hemp), Agave, and Musa etc.
• Splitting and coiling of values during dehiscence of some fruits is due to orientation of sclerenchyma.
• Sclereids help in stiffness to the plant parts.
• Sclereids make stony endocarp of drupes, called stone fruits, in almond, and coconut etc.

Figure 6 a. Sclerenchyma tissue (fibre)

Figure 6 b. Sclerenchyma tissue (sclereids)

 II. Complex Permanent Tissues

Complex permanent tissue is made up of different group of cells to perform a common function. So, it is heterogenous tissue. It is also called conducting tissue or vascular tissue, or physicomechanical tissue. It consists of phloem and xylem.

1. Phloem
Phloem is a complex permanent tissue that transports food in plant (See Figure 7 a.). It is also called bast. It consists of four types of cells – sieve elements, companion cells/ albuminous cells, phloem parenchyma, and phloem fibres.

a. Sieve elements
Sieve elements are of two types – sieve tubes and sieve cells.

i. Sieve tubes
Sieve tubes are elongated tubular conducting channels of phloem present in angiosperms (See Figure 7 b.). They are placed end-to-end by transverse or oblique end wall. They have small sieve pores. Each sieve pore is lined by a layer of callose. Internally, a sieve tube has peripheral layer of cytoplasm without any nucleus. The central part of sieve tube is occupied by a network of canals which contain fibrils of protein. Sieve tubes take part in the conduction of organic food.

ii. Sieve cells
Sieve cells are the only conducting elements of phloem in pteridophytes and gymnosperms (See Figure 7 a.). Internally, a sieve tube or sieve cell has peripheral layer of cytoplasm without any nucleus. Sieve cells take part in the conduction of organic food.

b. Companion cells or albuminous cells
Companion cells are narrow, elongated and thin-walled living cells associated with sieve tubes (See Figure 7 b.). They are present in the phloem of angiosperms. They are square or rectangular in shape. Each companion cell has dense cytoplasm and nucleus. In pteridophytes and gymnosperms, companion cells are replaced by albuminous cells, the modified parenchyma. They are associated with sieve cells.

c. Phloem or bast parenchyma
Phloem parenchyma is thin-walled living parenchyma without intercellular spaces and associated with phloem (See Figure 7 b.). They help in storage and slow lateral conduction of food.

d. Phloem fibres
Phloem fibres are thick-walled dead sclerenchyma in phloem. They help in mechanical support (See Figure 7 c.).

Figure 7 a. Phloem and sieve cell

2. Xylem

Xylem is a complex permanent tissue that helps in transport of water and minerals. It also provides mechanical strength to plants. It has four components – tracheids, vessels, xylem or wood parenchyma, and xylem or wood fibres (See Figure 8).

a. Tracheids
Tracheids are elongated thick-walled lignified dead cells with wide lumen and narrow end walls. Their shape is polygonal or tetrahedral. These are only conducting elements of xylem in gymnosperms and pteridophytes. In angiosperms, they are present along with other conducting elements - vessels. 

The walls of tracheids have annular (ring like), spiral (spiral or helix like), reticulate (network like), scalariform (ladder like), and pitted (uniformly thick except for small unthicken areas) thickenings for mechanical support. Annular is the most primitive while pitted is the most advanced thickening.

b. Vessels
Vessels are much elongated tubes with either of the end closed. They are formed by union of several short wide and thickened cells. Their end walls are transverse or oblique. Their walls are lignified. They also have annular, spiral, reticulate, scalariform, and pitted thickenings on the walls. Pitted thickening is more common than others. 

In T. S., they are circular in monocots whereas angular in dicots. They are absent in gymnosperms and pteridophytes.

c. Xylem parenchyma
It is thin-walled living parenchyma without intercellular spaces. It helps in storage of water and slow lateral conduction of sap.

d. Xylem fibres
Xylem fibres are thick-walled dead fibre type of sclerenchyma. They help in mechanical support.

Figure 8 a. T. S. of Xylem showing its elements

Figure 8 b. Thickenings in tracheids

Figure 8 c. Vessels with different end walls

C. Special type of Tissues

Some tissues help in secretion or excretion, called secretory or excretory tissues. They are embedded in parenchyma of cortex, phloem, xylem or pith. Special types of tissues are laticifers or laticiferous tissue and Glandular tissues. 

I. Laticifers

Thin walled, multinucleate, highly branched tube or duct that secretes latex is called laticifer. Laticifer helps in following functions:

• Storage of reserve foods like sugar, proteins, and oils;
• Formation and storage of excretory products like alkaloids, resins, tannins, and rubber etc.
• Conduction or translocation of products and regulate;
• Regulation of water balance in plants.

Laticifers are of two types – Latex cells and latex vessels.

1. Latex cells
Latex cells are branched, long individual cells which do not fuse to form a network (See Figure 9 a.). Examples are latex cells in Calotropis, Euphorbia, Nerium, Vinca, Cannabis, Urtica, Ficus, and Mulberry.

2. Latex vessels
Latex vessels are long branched ducts which fuse to form a network (See Figure 9 b.). Examples are latex vessels in Papaver, Argemone, Sunflower, Papaya, Banana, Cactus, robber plant, etc.

Figure 9 a. Latex cell in plants

Figure 9 b. Latex vessel in plants

II. Glands or Glandular tissue
Glands secrete oils, gums, mucilage, tannins, and resins. Glands are either external or internal. External glands are hydathode (See Figure 10 a.), nectar glands, glandular hairs, stinging hairs, digestive glands. Internal glands are oil glands (See Figure 10 b.), mucilage glands, and resin ducts, etc.

III. Special cavities or Canals
There may be three kinds of special cavities or canals – schizogenous, lysigenous, and schizo-lysigenous cavity - for the storage of their products (See Figure 11).

1. Schizogenous cavity
These cavities are formed by separation of cells to leave an empty space. Examples are resin ducts in stems of Pinus, Sunflower, Coriander, Fennels etc.

2. Lysigenous cavity
It is formed by break down of cells at particular spaces to store aromatic oils or water. Examples are oil cavities of citrus, cloves, and Eucalyptus, etc.

3. Schizo-lysigenous cavity
It is formed by separation of cells and disintegration of cells. Example is protoxylem cavity in maize stem.

Figure 11 Special cavities or canals surrounded by special tissues in plants