Tissues & The Tissue System

Table of Contents
1. What is Tissue?
2. Types of Plant Tissues
3. Meristematic Tissue
4. Permanent Tissue
5. Complex Tissues
6. Xylem
7. Phloem
8. The Tissue System
9. Summary
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Note: We are learning " Tissue and its Types"  first cause it is a part of NEET syllabus , though its not mentioned in latest 2025- 2026 Edition of NCERT.

What is Tissue?

Tissue is a group of cells that have a common origin, similar structure and work together to perform a specific function in the body of a plant or animal. In plants, tissues are organised into systems that carry out protection, support, storage, photosynthesis and conduction.

Types of Plant Tissues

• Meristematic tissue
• Permanent (non-meristematic) tissue

Meristematic Tissue

Meristematic tissue consists of cells that retain the ability to divide and give rise to other tissues. These cells are usually small, isodiametric, have thin cell walls, dense cytoplasm and a prominent nucleus. Meristems are responsible for growth in length and girth of plants.

• Characteristics of meristematic cells: small size, thin primary cell walls, dense cytoplasm, prominent nucleus, practically no vacuole, ability to divide repeatedly.
• Functions: produce new cells for growth, replace damaged tissues, and provide cells that differentiate into permanent tissues.

Types of meristems (based on position):

• Apical meristems - located at the tips of roots and shoots; responsible for primary (length) growth.
• Lateral meristems - found along the sides of stems and roots (e.g., vascular cambium and cork cambium); responsible for secondary (girth) growth.
• Intercalary meristems - present at the base of internodes or at leaf bases (common in grasses); allow elongation of internodes.

Permanent Tissue

Permanent tissues develop from meristematic tissue when cells lose the capacity to divide and become specialised for particular functions. Permanent tissues are of two major categories: simple tissues (formed of one type of cell) and complex tissues (formed of more than one type of cell).

• Simple tissues: parenchyma, collenchyma and sclerenchyma.
• Complex tissues: xylem and phloem (conducting tissues).

Types of Permanent Tissue

Types of Simple Tissues in Plants

1. Parenchyma

2. Collenchyma

3. Sclerenchyma

1. Parenchyma

• Forms the major component within organs.
• Cells are generally isodiametric.
• Shape of cells may be spherical, oval, round, polygonal, or elongated.
• Cell walls are thin and made up of cellulose.
• Cells may be closely packed or may have small intercellular spaces.

• Functions: Photosynthesis, Storage, Secretion

2. Collenchyma

• Occurs in layers below the epidermis in most dicotyledonous plants.
• Found either as a homogeneous layer or in patches.
• Cells are thickened at the corners due to deposition of cellulose, hemicellulose, and pectin.
• Cells may be oval, spherical, or polygonal.
• Often contain chloroplasts.
• When chloroplasts are present, cells assimilate food.
• Intercellular spaces are absent.
• Function: Provides mechanical support to growing parts such as young stem and petiole of leaf.

3. Sclerenchyma

• Consists of long, narrow cells with thick and lignified cell walls.
• Cell walls have few or numerous pits.
• Cells are usually dead and without protoplasts.
• Based on variation in form, structure, origin, and development, sclerenchyma is of two types: Fibres and Sclereids

Fibres

• Thick-walled, elongated, pointed cells.
• Generally occur in groups in various plant parts.

Sclereids

• Spherical, oval, or cylindrical, highly thickened dead cells.
• Have very narrow cavities (lumen).
• Commonly found in: Fruit walls of nuts, Pulp of fruits like guava, pear, sapota, Seed coats of legumes and Leaves of tea
• Function of sclerenchyma: Provides mechanical support to plant organs.

Complex Tissues

Definition

Complex tissues are made of more than one type of cells, which work together as a unit.

Types of Complex Tissues

1. Xylem

2. Phloem

Xylem

Functions

• Conducts water and minerals from roots to stem and leaves.
• Provides mechanical strength to plant parts.

Elements of Xylem

Xylem is composed of four kinds of elements:

1. Tracheids

2. Vessels

3. Xylem fibres

4. Xylem parenchyma

• Gymnosperms lack vessels in their xylem.

1. Tracheids

• Elongated or tube-like cells.
• Walls are thick and lignified with tapering ends.
• Cells are dead and without protoplasm.
• Inner cell wall layers show varied thickenings.
• Along with vessels, they are main water transporting elements in flowering plants.

2. Vessels

• Long cylindrical tube-like structures.
• Made of many cells called vessel members.
• Walls are lignified and contain a large central cavity.
• Cells are devoid of protoplasm.
• Vessel members are connected through perforations in common walls.
• Presence of vessels is a characteristic feature of angiosperms.

3. Xylem Fibres

• Have highly thickened walls.
• Central lumen is obliterated.
• May be septate or aseptate.

4. Xylem Parenchyma

• Cells are living and thin-walled.
• Cell walls are made of cellulose.
• Store food in the form of starch or fat.
• Also store substances like tannins.
• Radial conduction of water occurs through ray parenchymatous cells.

Primary Xylem

Two types:

• Protoxylem - first formed primary xylem

• Metaxylem - later formed primary xylem

Arrangement

• Stem:

  • Protoxylem towards centre (pith)

  • Metaxylem towards periphery

  • Arrangement called endarch

• Root:

  • Protoxylem towards periphery

  • Metaxylem towards centre

  • Arrangement called exarch

Phloem

Function

Transports food materials, usually from leaves to other parts of the plant.

Components of Phloem in Angiosperms

1. Sieve tube elements

2. Companion cells

3. Phloem parenchyma

4. Phloem fibres

• Gymnosperms have sieve cells and albuminous cells.

• They lack sieve tubes and companion cells.

Sieve Tube Elements

• Long, tube-like structures arranged longitudinally.

• End walls are perforated to form sieve plates.

• Mature sieve tube element:

  • Has peripheral cytoplasm

  • Has a large vacuole

  • Lacks a nucleus

• Functions are controlled by the nucleus of companion cells.

Companion Cells

• Specialised parenchymatous cells.

• Closely associated with sieve tube elements.

• Connected through pit fields in their common walls.

• Help in maintaining the pressure gradient in sieve tubes.

Phloem Parenchyma

• Made of elongated, tapering cylindrical cells.

• Have dense cytoplasm and nucleus.

• Cell wall is composed of cellulose and has pits.

• Plasmodesmatal connections exist between cells.

• Store food and substances like resins, latex, mucilage.

• Absent in most monocotyledons.

Phloem Fibres (Bast Fibres)

• Made of sclerenchymatous cells.

• Generally absent in primary phloem but present in secondary phloem.

• Elongated, unbranched, with pointed needle-like apices.

• Cell walls are quite thick.

• At maturity, fibres lose protoplasm and become dead.

• Fibres of jute, flax, and hemp are used commercially.

Primary Phloem

Protophloem:

• First formed primary phloem

• Contains narrow sieve tubes

Metaphloem:

• Later formed phloem

• Contains bigger sieve tubes

The Tissue System

In higher plants several tissues, having the same origin and working together to perform a particular function, form a tissue system. On the basis of position, division of labour and morphology, Sachs classified plant tissues into three major tissue systems: the epidermal tissue system, the ground (or fundamental) tissue system and the vascular (or conducting) tissue system. Each system is an association of tissues adapted to perform specific functions.

1. Epidermal Tissue System

(a) Structure of Epidermis

• The epidermis forms the outermost covering of the primary plant body (roots, stems and leaves).
• It is usually a single layer of compactly arranged cells forming a continuous layer without intercellular spaces.
• Epidermal cells are generally parenchymatous, living and may contain chloroplasts in some regions (e.g., guard cells).
• Cells are often elongated in a direction parallel to the surface and have a cuticle on the outer wall in aerial parts.
• The epidermis also contains specialised structures such as stomata, trichomes (hairs), and glands.

(b) Role of Cuticle

• The outer surface of the epidermis of aerial parts is commonly covered by a waxy layer called the cuticle.
• The cuticle reduces water loss by forming a hydrophobic barrier and protects against desiccation and pathogens.
• The cuticle is absent in roots, where water absorption is required.

(c) Stomata and their Function

• Stomata are small openings in the epidermis of leaves and young stems that permit transpiration and gaseous exchange.
• Each stoma is delimited by two bean-shaped guard cells that enclose the stomatal pore.
• Guard cells are the only epidermal cells containing chloroplasts and they change shape by altering their turgor; this regulates opening and closing of the stomatal pore.
• In grasses the guard cells are dumb-bell shaped, which allows rapid opening and closing.
• Guard cells exhibit differential wall thickening: the inner wall (towards the pore) is more thickened than the outer wall; this asymmetry helps the change in shape during opening/closing.
• Some epidermal cells near guard cells become specialised as subsidiary cells that assist in stomatal movement; the guard cells, subsidiary cells and the stomatal aperture together form the stomatal apparatus.
• Types of stomatal complexes (based on subsidiary cells) include anomocytic, anisocytic, paracytic and diacytic; different plant families show characteristic types.
• Stomatal movement is influenced by light, CO2 concentration, humidity, temperature and plant water status; opening generally occurs in light and closing in the dark or during water stress.

(d) Epidermal Hairs (Trichomes)

• Root hairs are unicellular, tubular outgrowths of epidermal cells of roots; they increase absorptive surface area and aid water and mineral uptake.
• Stem hairs (trichomes) are usually multicellular and show great diversity in form - simple, branched or glandular.
• Trichomes may be soft or stiff and some are secretory (glandular trichomes) producing aromatic oils or other substances.
• Functions of trichomes include reduction of water loss, protection from herbivores, reflection of excess light, secretion of substances and trapping of insects in insectivorous plants.

2. Ground Tissue System

All tissues, except for the epidermis and vascular bundles, make up the ground tissue. This ground tissue is made up of simple tissues such as: 

• Parenchyma
• Collenchyma
• Sclerenchyma

3. Vascular (Conducting) Tissue System

The vascular system comprises complex tissues specialised for the transport of water, minerals and organic nutrients throughout the plant. The two principal conducting tissues are xylem and phloem, which together form vascular bundles.

(a) Vascular bundles in dicotyledonous stems

• In many dicot stems, vascular bundles are conjoint (xylem and phloem present on the same radius) and open - a layer of cambium lies between xylem and phloem.
• The cambium is a lateral meristem that produces secondary xylem (wood) towards the inside and secondary phloem towards the outside, resulting in secondary growth (increase in girth).
• Open vascular bundles can form secondary tissues because of the presence of cambium.

(b) Vascular bundles in monocotyledons

• Monocot stems generally have conjoint, closed vascular bundles; cambium is absent from these bundles and therefore they do not produce secondary xylem and phloem.
• Vascular bundles in monocots are scattered in ground tissue rather than arranged in a ring (as in dicots).

(c) Radial vascular bundles

• In roots, vascular bundles are typically radial - xylem and phloem occur in alternate radii (xylem patches alternating with phloem patches).
• This radial arrangement facilitates longitudinal conduction and anchorage.

(d) Conjoint vascular bundles

• Conjoint vascular bundles have xylem and phloem on the same radius. This arrangement is common in stems and leaves.
• In collateral conjoint bundles, phloem is located on the outer side of xylem; in bicollateral bundles phloem is present on both sides of xylem.

Summary

Plant tissues are organised as meristematic and permanent types and grouped into three tissue systems - epidermal, ground and vascular. Meristems supply new cells for growth while permanent tissues are specialised for protection, support, storage, photosynthesis and transport. Understanding the structure and function of these tissues and their systems is essential to explain plant growth, adaptation and physiology.