The cells of plants are broadly divided into two types. The permanent tissues generally don’t divide further. The types of plant cells are
- Collenchyma, and
However, these cells are arranged into tissues as
- Simple tissues
- Complex tissues
Simple tissues making up the ground layers in plants. They are of three types as
- Collenchyma, and
Ground tissue includes all the tissues except the vascular tissue or part of the dermis (skin) of the plant.
Parenchyma is simple and consists of thin-walled cells which are non-specialized in structure. These are living cells, and their cell walls are made of cellulose.
The parenchyma cells form the major components of organs and are the most prevalent cells in plants. The shape of these cells has many variations. Though most of them are isodiametric some of them are also either oval, round, or even polygonal depending on their location and the function.
These cells are arranged in a closely packed manner or have small intercellular spaces.
They have a large central vacuole that stores water and ions. Some parenchymal cells are even specialized and have chloroplasts. These are called chlorenchyma cells.
Mesophyll cells in leaves are a type of parenchymatous cells. These mesophyll cells are of two types –
- Palisade parenchyma; This is present on the surface of leaves and is involved in photosynthesis.
- Spongy parenchyma; It is present beneath the palisade parenchyma and acts as a packing tissue.
- Parenchymal cells are meristematic in some cases and may show pluripotent ability. These cells divide into other types of cells and help in healing and repair.
- Specialized parenchyma cells called chlorenchyma contain chloroplasts and help in photosynthesis, and produce carbohydrates. These carbohydrates are either stored or broken down to provide energy.
- The large vacuoles in parenchymatous cells help in the storage of ions and soluble nutrients dissolved in water. These nutrients provide energy for the normal functioning of the plant.
- The turgor pressure in plants is maintained by the parenchymatous cells. The large vacuole stores water and determines the osmotic pressure.
- Parenchymatous cells also perform secretory functions.
Collenchyma cells may be oval, spherical, polygonal or elongated in shape. Their walls are non-uniformly thickened due to the deposition of cellulose, hemicellulose, and pectin (by secondary growth). These cells are usually thickened at the corners. They occur in layers under the epidermis in most dicotyledonous plants. These cells may occur in a homogeneous layer or in patches. They are often found under the epidermal layer or the outer layer of cells in young stems and leaf veins. Collenchymas cells often contain chloroplasts. These cells help to absorb food. Intercellular spaces are absent. Collenchyma cells can be classified into four main types: angular, tangential, annular, and lacunar.
Angular collenchyma- they are thickened only at the contact points between adjacent cells.
Tangential collenchyma– the cells are arranged into regularly ordered rows. The walls of the cells are thickened at the tangential face of the cell.
Annular collenchyma – the cells have uniformly thickened walls of the same width.
Lacunar collenchyma– these are the only type of collenchyma cells with intercellular spaces.
- Structural support is provided by collenchyma cells. They are elastic and hard.
- They protect the plant structures by forming an inner framework. Due to the thickened walls and deposition, they can provide additional strength.
- Plants have to withstand many external stresses and face environmental factors like heavy rains, strong winds, etc. These cells help them withstand the damage.
- Collenchyma cells also contain chloroplasts in many cases and take part in photosynthesis.
- collenchyma cells also take part in food assimilation.
- Collenchyma cells are present in growing parts of the plant, like shoots. They provide support and fill vacant spaces, which may later grow.
Sclerenchyma (mechanical tissue)
Sclerenchyma cells can be easily identified by observing the presence of their thick and lignified cell walls and high cellulose content. These are called secondary cell walls, and they are thickened due to the presence of lignin. Thus they do not allow the passage of water through these walls. These cell walls have few or numerous hollow pits through which substances can pass. Pits are the regions of a cell where there is no deposition of lignin. Sclerenchyma cells are dead and do not have protoplasm. Thus they do not perform photosynthesis, and neither do they need nutrients. The cells are rigid and nonstretchable and are usually found in nongrowing regions of plant bodies, cortex, hypodermis, vascular regions of stem, leaves, fruit wall, etc.
On the basis of variation in structure, origin, and development, sclerenchyma cells are classified into two types.
- Fibers -Fibres are thick-walled, greatly elongated cells. They are spindle-shaped. They have long, tapering ends which interlock. This interlocking provides greater support. They possess a narrow linen with rounded pits. Fiber is of great commercial importance and serves as sources of raw material for textiles and other woven products. Fibers can be divided into three groups –
Surface fibers: found in the wall of fruits and seed coats.
Xylary or Wood fibers: present in xylem
Extraxylary/Bast fibers: present in cortex, pericycle, and phloem.
- Sclereids-Sclereids are extremely variable in shape and are present in various tissues of the plant. The sclereids are spherical, oval, or cylindrical in shape with highly thickened walls. They have very narrow cavities (lumen). Sclereids are of four types-
Astrosclereids: star-shaped sclereids, present in dicot leaves and gymnosperms.
Brachysclereids: isodiametric in shape, also called stone cells. Present in the cortex, pith, etc. Grit cells are a collection of stone cells present in the pulp of fruits.
Osteosclereids: Bone like sclereids having lobed ends, present in the wall of fruits and seed coat.
Macrosclereids: rod-like in shape. It is present in bark and seed coat of leguminous plant.
- The mechanical tissue is thick and provides strength and support to the plant.
- Due to its lignified walls, it is able to protect plant parts from damage.
- Prevents water conduction in certain areas.
- Xylem fibers help in water conduction.
- Surface fibers of sclerenchyma help in the dispersal of seeds and fruits.
- They also protect against bacterial and other infections.
They are made up of a combination of many types of cells.
It is composed of sieve tube elements, companion cells, phloem parenchyma, and phloem fibers.
Sieve tube elements are long, tubular structures. Their end walls are perforated to form sieve plates. They have a peripheral cytoplasm and a large vacuole but no nucleus. Companion cells (parenchymatous)control functions of sieve tubes and maintain pressure. They are connected by pits present between their common walls. Phloem fibers (sclerenchymatous) are elongated, unbranched, and have pointed ends. The cell wall of fibers is quite thick. At maturity, these fibers do not contain protoplasm and are dead. Phloem parenchyma cells are long, tapering with dense cytoplasm and nucleus. It has a cellulosic cell wall with plasmodesmata connections.
- Transports food materials, from leaves to other parts.
- Storage of food material.
- It stores other substances like resins, latex, etc.
- It provides mechanical strength.
It is made up of tracheids, vessels, xylem fibers, and xylem parenchyma.
Tracheids are tube-like cells with lignified walls. They are dead cells and do not contain protoplasm. They have tapering ends. The vessel is a long tube-like structure made up of many cells
( vessel members). These cells have lignified walls surrounding a large central cavity with no cytoplasm. Vessel members are interconnected through perforations in their walls. Xylem fibers are dead and thick-walled, while xylem parenchyma cells are living and thin-walled.
- It is involved in the conduction of water and minerals from roots to the stem and leaves.
- It may also conduct nutrients in some cases.
- Stores food materials as starch.
- It also stores tannins.
- It provides mechanical strength to the plant parts.