What do you Mean by Cell Types?

Plants are made up of a variety of cell types that include both living and dead cells. These cells provide them mechanical and structural support along with the growth of plants. Plant tissue is a group or collection of cells, which are similar in structure and work together to accomplish a specific function. Plant cells are most similar to animal cells, both being eukaryotic cells.

Division of Plant Tissues

Plant tissues are categorized into two main types based upon the ability to divide. The two divisions are:

1. Meristematic tissue
2. Permanent tissues.

Meristematic tissue

• This type of tissue has the potential to divide very fast.
• The cells of meristematic tissue cells are small, cuboidal, living, and have a big nucleus.
• This tissue has no intracellular spaces.

The meristematic tissues are further divided into three main tissues depending on the location in the plant. These three tissues are apical, lateral, and intercalary meristems.

• Apical meristem: this type of meristem is present at the growing roots and shoot tip and is accountable for increasing the plant length.
• Lateral meristem: This type of meristem is present at the radial portion of the stem or root. Lateral meristem is accountable for plant thickness.
• Intercalary meristem: this type of meristem is present at the internodes or the base of the leaves and is accountable for the size of the internode.

Permanent tissue

These are the tissue that has lost its ability to divide but important for the plant as it provides strength and flexibility to the plant.

Permanent tissue has been divided further into two types:

• Simple Permanent tissue 
• Complex Permanent tissue

Simple permanent tissue is formed by collecting similar types of plant cells that perform the same function. It is further divided into three types:

• Sclerenchyma Cells
• Collenchyma Cells
• Parenchyma Cells

Sclerenchyma Cells: These are dead plant cells, elongated in shape, and have lignin present in their cell wall.  It is mainly present in the covering of seeds, nuts, and veins of leaves and stems vascular tissues. They have no intercellular spaces. The function of this tissue is to provide mechanical strength to the plant. The sclereids are the reduced form of sclerenchyma cells having high lignified walls. Sclereids are present in the seed coats of legumes, nuts, fruit walls, and grit of guava and pear.

Collenchyma Cells: These are living plant cells with minute intercellular gaps and cell walls made up of pectin and cellulose. The function of the collenchyma cells is to provide a structural framework and mechanical support to plants as the collenchyma cells have a thick cell wall with very few intracellular spaces. They are present mainly in the cells that have a primary cell wall and secondary cell wall.

Parenchyma Cells: These are living plant cells and have different shapes like oval, polygonal or elongated.  These cells have big central vacuoles and intercellular spaces between them. They are present in the cells that have a primary cell wall. There are mainly three types of parenchyma cells depending upon the function they perform.

• Parenchyma cells having chloroplasts are known as chlorenchyma which helps in absorbing light during photosynthesis.
• Parenchyma cells having big air voids are called aerenchyma, whose function is to maintain buoyancy in water plants.
• Few parenchyma cells act as storage chambers for starch in the case of vegetables and fruits.

Complex Permanent tissue is formed by collecting different types of plant cells that perform the same function. It is divided into two types.

• Xylem
• Phloem

Xylem

Xylem helps in the transportation of dissolved substances and water throughout the plant-like stem to the leaves from the plant's root. The xylem comprises four types of cells: vessel elements, tracheids, xylem fibers, and xylem parenchyma.

1. Tracheid's are dead, tube-like cells and have tapered ends. Tracheids are devoid of protoplasm and have thick lignified walls. These cells are involved in water and mineral transport.
2. Vessels are advanced xylem elements that mainly present in flowering plants. Similar to tracheids, vessels are dead cells and have a tube-like appearance. Lignin is present in their cell wall and has a central cavity. The function of the vessels is to provide rigidity and resistance to the cell wall. Moreover, vessels also prevent the cell wall from collapsing during excess transpiration.
3. Xylem fibers are dead cells having lignified walls and contain a large central lumen. They provide support to the plants and are involved in the transport of water.
4. Xylem parenchyma:  are the living parenchyma cells of the xylem that help transport water to a short distance only.

The xylem is divided into two types: protoxylem and metaxylem.

• Protoxylemis the first xylem formed in the plant.
• Metaxylemis the xylem formed after protoxylem.

Phloem

Phloem helps in the transportation of food made in the leaves to all different parts of the plant. The phloem tissue comprises phloem fibers, sieve elements, phloem parenchyma, and companion cells.

1. Sieve elements are the conducting elements of phloem and connect the cytoplasm of neighboring cells, transporting food material to different parts. Their end wall has numerous pores, and therefore they are named "sieve."  Sieve elements are divided into two types: sieve cells and sieve tubes. Sieve cells are primitive cells and present in pteridophytes and gymnosperms, whereas sieve tubes are new cells and are present in angiosperms.

2. Phloem parenchyma is a thin wall of plant cells and has a nucleus. Primary pit fields are present on the cell wall of these cells. The phloem parenchyma is further divided into axial parenchyma (parallel to elongation) and radial parenchyma (parallel to the axis of elongation).

3. Companion cells are the specialized parenchyma plant cells. These cells are connected to sieve tube elements by pit fields. The function of the companion cells is to maintain the pressure gradient in sieve tubes.

4. Phloem fibers are the only dead cells found in phloem when compared to the other four components. Phloem fibers are made up of sclerenchyma and are narrow, vertically elongated cells with a very thick but small lumen.

Thephloem is divided into two types: protophloem and metaphloem.

• Protophloem is the first formed phloem and has narrow sieve tubes.
• Met phloem is formed after primary phloem and has broad sieve tubes.

Tissue System

Thetissues in plants are further divided into three types depending upon the location and structure. These tissues are epidermal tissue, vascular tissue, and ground tissue.

1. Epidermal Tissue System: This tissue system is found at the outermost covering of all plant bodies. Its main function is to protect the plant against water loss, regulate gas exchange, and absorb water and nutrients. Epidermal tissue is further divided into epidermal cells, stomata, and epidermal appendages.

• Epidermal cells: This layer is the outermost layer of plants and presents the point of contact between the outer environment and the plant. The epidermis is the outer layer formed by the single layer of dermal tissues and covers the outer surface of the plant. Epidermal cells are diverse in structure and do not contain any intracellular spaces. The function of this dermal tissue is to provide protection to the plant, reduce water loss in the plant, water and mucilage storage. The suberin, cutin, and lignin are the complex polymers present in the epidermis of a plant whose function is to provide a protective barrier around the plant.
• Stomata: Theyare the specific structures present in the leaves of the plant. The function of these cell structures is to regulate the process of transpiration (loss of water) and gas exchange. The stomata are covered by two specific cells on both sides called guard cells that look like dump-bell. The function of the guard cells is to regulate the opening of stomata
• Epidermal appendages:  They are of two types: root hairs and trichomes. Root hair is present in the root, and its main function is to absorb water and minerals from the soil. Trichome prevents excess water loss due to transpiration.

2. The Ground Tissue System arises from the ground tissue meristem. It consists of simple tissues like parenchyma, collenchyma cell, sclerenchyma, and specific cells in the leaves known as mesophyll cells. The mesophyll cells contain chloroplast and chromoplast. The ground tissue system is heterogeneous as it has different kinds of cells to perform diverse kinds of functions. Its main function is to synthesize organic compounds and provide support to the plant. Sometimes, the ground tissue also stores food in the form of starch.

3. The Vascular Tissue System forms the third type of tissue system in plants and comprises two tissues: the phloem and xylem. Between the primary xylem and phloem, a specific meristematic tissue known as cambium in the case of dicot stems. The presence of cambium in vascular bundles gives them the ability to form secondary xylem and phloem tissues, known as open vascular bundles. On the contrary, vascular bundles in monocotyledons have no cambium, and hence they are known as closed vascular bundles.

Context and Application:

• Bachelor of Science in Biology:  The plant anatomy study allows students to conceptually integrate organismal structure and function and learn about plant cell types.
• Masters of Science in Biology: For post-graduate student more in-depth understanding of the plant cell types help them to conduct novel research in plant sciences.