Plant Anatomy

The study of the internal structure of plants through a microscope is called plant anatomy or phytotomy. N. Grew is known as the "father of plant anatomy". A plant is multicellular and is composed of many different cells. Cells of the same kind that have the same origin and function constitute a group called a tissue. In plants, tissue is the highest form of cellular organization, and tissues together constitute a tissue system. In plants, three types of tissue system are recognized: the dermal system, the ground system, and the vascular system. The dermal system gives protection, the ground system stores several chemicals, and the vascular system conducts water, minerals, and food.

Depending on the nature of the cells, plant tissues are classified into three groups: meristematic, permanent, and secretory.

Meristematic Tissue (Meristem)

A group of immature, non-specialized cells that undergo repeated division is called meristematic tissue. Meristem is therefore responsible for growth, and it can give rise to different tissues that go on to form permanent tissue.

General features of meristem cells:

1. The cells are immature and unspecialized.
2. They are in a state of continuous division.
3. They are thin walled.
4. They are compactly arranged without intercellular spaces.
5. They are densely packed with cytoplasm.

Types of meristem

1. On the basis of origin

• Promeristem: found in the embryo of the seed, so it is also called embryonic meristem. It is the earliest type of meristem, also named primordial meristem.
• Primary meristem: promeristem divides and differentiates into primary meristem, located at the apices of growing parts. This meristem brings about an increment in the length of plant parts, called primary growth.
• Secondary meristem: develops from the cells of primary meristem and brings about an increment in girth (diameter) of plant parts, a process called secondary growth. Cambium is an example of secondary meristem.

2. On the basis of location

Promeristem is found in seeds; the growing plant has only primary and secondary meristem. Depending on location, these are classified into three types:

• Apical meristem: a type of primary meristem found in the growing tips of roots, stems, leaves, and buds; it brings about primary growth in plant parts.
• Intercalary meristem: a type of primary meristem distributed at the nodes; it is mixed with other tissue and gives rise to branches, buds, and flowers.
• Lateral meristem: an example of secondary meristem, located at the lateral position of plant parts; it brings about an increment in girth of the plant. This meristem is generally found in dicot stems.

3. On the basis of function

• Protoderm: the meristem that forms the dermal (epidermal) system in plants.
• Ground meristem: the meristem that forms the ground system, where storage of food and other chemicals takes place.
• Procambium: the meristem that forms the vascular system in plants.

Shoot apex (shoot apical meristem, SAM)

The shoot apex has meristems that repeatedly divide and form different layers in plant parts. Two theories explain the differentiation of the SAM.

1. Tunica-Corpus theory

Proposed by Schmidt in 1924. According to this theory, the SAM has two distinct regions: the outer region is called the tunica and the inner region the corpus. The tunica is a few layers thick and composed of smaller cells; it is differentiated into the dermal system. Inner to the tunica is the many-layered corpus, composed of larger cells; the corpus cells later give rise to the ground system and vascular system.

2. Histogen theory

Proposed by Hanstein, who stated that there are three layers, collectively called histogens (tissue builders). According to this theory, the SAM has the following histogens:

• Dermatogen: the outermost layer of the SAM; its cells differentiate into the dermal layer.
• Periblem: inner to the dermatogen, the middle layer, which forms the cortex (ground tissue).
• Plerome: the central region of the SAM, which forms the vascular system.

Functions of meristem

• Results in growth and development.
• Primary meristem causes primary growth, an increment in the length of plant parts.
• Secondary meristem causes an increment in girth, called secondary growth.
• Helps in healing wounds.
• The SAM gives rise to structures such as leaves, buds, flowers, and branches.

Permanent Tissue

A group of living or dead cells that have a specific shape and function is called permanent tissue. It has lost the power of division, and is derived from the cells of meristematic tissue. It takes part in absorption of water and minerals, conduction, photosynthesis, and mechanical support.

Permanent tissue is of two types: simple permanent tissue and complex permanent tissue.

Simple permanent tissue

When the cells are of the same kind, the permanent tissue is called simple permanent tissue. It may or may not have intercellular spaces, and is the most common type of permanent tissue, with several roles. Based on the shape of the cells, simple permanent tissue is of three types:

1. Parenchyma.
2. Collenchyma.
3. Sclerenchyma.

a) Parenchyma

The term parenchyma was coined by N. Grew. It is the most common type of simple permanent tissue and is distributed in almost all parts of the plant. The cells of parenchyma are loosely arranged, with intercellular spaces; they are thin walled and living, with a distinct nucleus.

Types of parenchyma:

• Aerenchyma: parenchyma with wide intercellular spaces called air cavities, which provide buoyancy to the plant. It is found in free floating hydrophytes.
• Chlorenchyma: parenchyma with cells packed with chlorophyll; it takes part in photosynthesis and is distributed in the leaves and other green parts.
• Prosenchyma: parenchyma composed of elongated, compactly arranged cells; it provides mechanical support and rigidity to plant parts.
• Velamen: parenchyma available in the roots of epiphytes; it takes part in absorption of moisture and minerals from the atmosphere. The cells are living and thin walled.
• Palisade parenchyma: cells that are elongated and compactly arranged, with abundant chlorophyll.
• Spongy parenchyma: cells that are loosely arranged and distributed with a lesser amount of chlorophyll.

Functions of parenchyma:

1. Stores several chemicals, such as water and organic compounds.
2. Chlorenchyma takes part in photosynthesis.
3. Aerenchyma helps in exchange of gases.
4. Aerenchyma gives buoyancy to plant parts.
5. Prosenchyma gives mechanical support.

b) Collenchyma

The simple permanent tissue made of similar kinds of cells, with deposition of cellulose, hemicellulose, and pectin in the intercellular spaces, is called collenchyma.

Features of collenchyma:

1. The term collenchyma was coined by Schleiden.
2. It is a group of thick walled, living, and compactly arranged cells.
3. It provides rigidity with flexibility to plant parts.
4. The intercellular spaces among the cells are packed with cellulose, hemicellulose, and pectin.

Types of collenchyma (based on the pattern of deposition):

1. Angular collenchyma: cellulose, hemicellulose, and pectin are deposited at the corners where two cells meet.
2. Lacunar collenchyma: deposition of cellulose, hemicellulose, and pectin occurs in the intercellular spaces.
3. Tangential (lamellar) collenchyma: deposition occurs between the layers of cells, appearing plate-like, so it is also called plate collenchyma.

Functions of collenchyma:

1. Provides mechanical strength to plant parts.
2. Provides a degree of flexibility to plant parts.
3. Helps in photosynthesis to some degree, since the cells may contain chloroplasts.
4. Helps in storage of food.

c) Sclerenchyma

A group of simple permanent tissue made of thick walled, dead, non-nucleated cells. Each cell has abundant deposition of cellulose, hemicellulose, and lignin. As the cells are thick walled, sclerenchyma provides mechanical strength, and is found in the hard parts of plants.

Types of sclerenchyma (depending on the amount of deposition):

1. Fibre: composed of elongated, narrow, thick walled cells with a narrow lumen; it provides mechanical strength to plant parts, and is commercially important in the production of rope and textiles. The dead tissue found on the surface of coconut is an example of fibre.
2. Sclereid: dead cells with extreme deposition of cellulose, hemicellulose, and lignin. They have different shapes (spherical, oval, cylindrical, or dumb-bell shaped) and provide mechanical protection.

Types of sclereids:

1. Brachysclereids: also called stone cells; spherical, oval, or rounded, highly thickened dead cells with tubular pits.
2. Macrosclereids: rod-like or columnar shaped; common in the seed coats of many leguminous plants.
3. Osteosclereids: barrel-shaped cells resembling a bone; found in the leaves and seed coats of several monocotyledons.
4. Astrosclereids: star shaped; found in petioles.
5. Trichosclereids: hair-like, branched or unbranched; found in the intercellular spaces of leaves and stems of some aquatic plants.

Functions of sclerenchyma:

1. Provides mechanical strength to plant parts.
2. Provides protection to plant parts from extreme environmental conditions.
3. Fibre has commercial value in the production of ropes and textiles.
4. Sclereids provide stiffness to plant parts.

Complex permanent tissue

When a tissue is made of two or more types of cells, it is called complex permanent tissue. It includes xylem and phloem, which conduct water and food and are therefore also called conducting tissue. Xylem and phloem together form a bundle called a vascular bundle, so they are also called vascular tissue.

Xylem

The term xylem was introduced by Nageli. It is the chief conducting tissue of vascular plants, responsible for conduction of water and solutes; it also provides mechanical strength.

Components of xylem:

1. Tracheids: elongated, tube-like dead cells with tapering ends and no cytoplasm at maturity. They have hard, thick, and lignified walls, and an empty lumen. The walls usually have one or more rows of bordered pits. They are the only conducting elements in the wood of ferns and gymnosperms. They conduct water and minerals from the root through the empty lumen; water passes from one tracheid to another through pits, without obstruction by living tissue.
2. Vessels (trachea): elongated tubes consisting of a series of drum-shaped cells placed one above another, with the end walls perforated or dissolved. Each cell appears circular, oval, or polygonal, with a wide lumen; the cells become dead and lose their cytoplasm due to deposition of lignified secondary walls. Found in angiosperms. The long, empty tube provides an ideal system for transport of water in tall plants and also provides mechanical support.
3. Xylem (wood) parenchyma: made up of generally small, thin walled parenchyma cells associated with the conducting elements. It stores food, resin, and water/sap, and helps in lateral conduction of water and sap; it is the only living element of xylem.
4. Xylem (wood) fibres: sclerenchyma fibres with much thicker walls and overlapping ends, with a narrower lumen than vessels. They cannot conduct water, but being stronger, they provide mechanical strength to the vessels and tracheids. They are abundant in woody plants.

Functions of xylem:

1. Conducts water and mineral salts.
2. The components of xylem (tracheids, vessels, and fibres), having thick lignified walls, give mechanical strength to the plant body.

Types of xylem (depending on stage of development and structure):

1. Primary xylem, which further differentiates into:

• Protoxylem: the first-formed xylem elements.
• Metaxylem: the later-formed primary xylem.

Depending on the arrangement of protoxylem and metaxylem, primary xylem is said to be:

• Endarch: protoxylem lies toward the centre, metaxylem toward the periphery; occurs in leaf and stem.
• Exarch: protoxylem lies toward the periphery, metaxylem toward the centre; occurs in the root.
• Mesarch: protoxylem lies in the middle of the metaxylem, or vice versa; example, the leaves of some dicotyledons.
• Centrarch: protoxylem lies at the centre of the metaxylem; example, ferns.

2. Secondary xylem: formed during secondary growth, produced from the secondary meristem (vascular cambium) after redifferentiation.