Concept of Plant Tissues

Q.N. 1. Define tissue. Give a brief account on the classification of plant tissues.

Tissue is defined as a group of cells having a common origin and co-operating with one-another to perform a similar function. Based on their capacity to divide, tissues can be classified as meristematic tissues or meristems and permanent tissues, in plants.

1. Meristems

Meristems are groups of young cells which retain the capacity of cell division. They occur in the growing regions of plants.

Meristems are categorised as promeristems, primary meristems, and secondary meristems on the basis of origin and development; as apical meristems, intercalary meristems, and lateral meristems based on position; protoderm, procambium, and ground meristems on the basis of functions.

2. Permanent tissues

Permanent tissues are the tissues that have their growth have stopped either completely or they do not divide till their dedifferentiation happens. Sometimes, they regain meristematic activity partially or wholly.

Permanent tissues can be simple permanent tissues and complex permanent tissues when they form homogenous mass of tissues with similar cells and heterogeneous mass of tissue with dissimilar cells, respectively.

Simple permanent tissues are epidermis, epiblema, simple parenchyma, chlorenchyma, aerenchyma, prosenchyma, idioblast, phloem parenchyma, xylem parenchyma as they perform special functions.

Complex permanent tissues include xylem and phloem. Phloem contains sieve elements, companion cells and/or albuminous cells, phloem parenchyma, and phloem fibres. Xylem consists of tracheids, vessels, xylem or wood parenchyma, and xylem or wood fibres.

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The term ‘meristem’ is derived from Greek words 'Meristos' which means 'Divisible'. It was coined by Nageli in 1858.
The term 'tissue' was coined by Nehemiah Grew, an English Anatomist and Physiologist in 1682. Nehemiah Grew is also known as the Father of Plant Anatomy, in 1682.

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Q.N. 2. Define meristems. Also, list the characteristics of meristems.

For Definition of meristems, see Q.N.1.

In principle, meristems have following features:

Immature cells with capability to divide and grow are present in meristem
The intercellular space between cells is absent.
Cells are oval, rounded, polygonal or rectangular.
The cell wall is thin, elastic, and made of cellulose.
There is no store reserved food material in cells.
The cells contain dense cytoplasm with large, distinct and prominent nuclei.
Vacuoles are either absent or very small.
Metabolic activities are very high.
Colourless proplastids are present.

Q.N. 3. Give a brief account on meristems based on their origin and development.

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

a. Promeristems

These are the youngest cells in growing regions 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.).

b. Primary meristems

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

c. 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.).

Q.N. 4. Give an account on meristems based on their position.

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

a. Apical meristems

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

b. 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.

c. Lateral meristems

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

Q.N. 5. Give an account on meristems based on their function.

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

a. Protoderm

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

b. Procambium

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

c. 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.

Q.N. 6. Define permanent tissues. Describe the simple permanent tissues with their functions.

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 fully or partially. The living permanent tissues have thin-walled or thick-walled cells whereas the dead permanent tissues have only thick-walled cells. Permanent tissues are of two types – simple permanent tissues and complex permanent tissues.

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.

a. Parenchyma

Parenchyma is a tissue made of thin-walled living, similar, oval, rounded or polygonal isodiametric cells with or without intercellular spaces. The 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 functions, they are classified as epidermis, epiblema, simple parenchyma, chlorenchyma, aerenchyma, prosenchyma, idioblast, phloem parenchyma, xylem parenchyma.

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 helps in photosynthesis.
Aerenchyma provides buoyancy and storage of metabolic gases.
Phloem and xylem parenchyma helps in slow lateral conduction of materials.
Prosenchyma helps in mechanical support.
Epiblema helps in absorption of sap.

b. Collenchyma

Collenchyma is a simple living permanent tissue with the cells having deposition of cellulose and pectin in specific areas of their wall. The cells are elongated, circular, oval or angular in the transverse section. Like parenchyma, they also possess large central vacuoles and a peripheral cytoplasm along with the 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 the cell wall, they are angular, lamellate, and lacunate collenchyma.

Functions of Collenchyma

Collenchyma provides mechanical strength to the young stem, leaves, and petioles of herbaceous plant.
It helps in cell elasticity and supports 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 amounts of food.
It allows growth and elongation of organs.
Cells of collenchyma undergo dedifferentiation and can form cork cambium or phellogen.

c. Sclerenchyma

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

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.

Q.N. 7. Describe the parenchyma with their functions.

For the introduction of parenchyma, see a. Parenchyma in Q.N.6.

Here is briefly described about different types of parenchyma:

i. Chlorenchyma

Parenchyma having chloroplast are called chlorenchyma. They are found in green stems, 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.).

ii. 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.).

iii. 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 of food.

For functions of parenchyma, see Q.N.6.

Q.N. 8. Describe the collenchyma with their functions.

For the introduction of Collenchyma, see b. Collenchyma in Q.N.6.

Here is briefly described about different types of collenchyma:

i. Angular collenchyma

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

ii. Lamellate collenchyma

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

iii. 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 the stem of Cucurbita and petiole of Salvia etc.

For functions of collenchyma, see Q.N.6.

Q.N.9. Describe the sclerenchyma with their functions.

For the introduction of sclerenchyma, see c. Sclerenchyma in Q.N.6.

Here is briefly described about different types of sclerenchyma:

i. 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.).

ii. 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 grittiness 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 the stem of hydrophytes.

For functions of sclerenchyma, see Q.N.6.

Q.N.10. What are complex permanent tissues? Describe the phloem tissues and their components.

Complex Permanent Tissues

Complex permanent tissues are those tissues made up of different groups of cells to perform a common function. So, they are heterogenous tissues. They are also called conducting tissues or vascular tissues, or physicomechanical tissues. They consist of phloem and xylem.

1. Phloem

Phloem is a complex permanent tissue that transports food in plants (See Figure 7 and 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 a 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 a peripheral layer of cytoplasm without any nucleus. The central part of the 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. Internally, a sieve tube or sieve cell has a 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 c.). 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 d.). 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 d.).

Q.N.11. What are complex permanent tissues? Describe xylem and their components.

For the introduction of complex permanent tissues, see Q.N.10.

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 and Figure 8 a.).

a. Tracheids

Tracheids are elongated thick-walled lignified dead cells with wide lumen and narrow end walls (See Figure 8 b.). 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 (Figure 8 c.). Annular is the most primitive while pitted is the most advanced thickening.

b. Vessels

Vessels are much elongated tubes with either of the ends closed. They are formed by union of several short, wide and thickened cells (Figure 8 d.). Their end walls are transverse or oblique. Their walls are lignified. They also have annular, spiral, reticulate, scalariform, and pitted thickenings on the walls (See Figure 8 e.). 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 a thin-walled living parenchyma without intercellular spaces (See Figure 8 f.). It helps in storage of water and slow lateral conduction of sap.

d. Xylem fibres

Xylem fibres are thick-walled dead fibre types of sclerenchyma (See Figure 8 g.). They help in mechanical support.

Q.N. 12. What are secretory or excretory tissues? Describe different types of special types of tissues and their associated cavities and canals. Also, mention their functions.

The tissues that are embedded in the parenchyma of cortex, phloem, xylem or pith and help in secretion or excretion of different plant chemicals are secretory or excretory tissues. They are also called special types of tissues. Those tissues include 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.

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 breakdown 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. An example is the protoxylem cavity in maize stem.

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List of figures

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Figure 1 a.

Figure 1 b.

Figure 1 c.

Figure 2