Question

4) Briefly describe the three fundamental types of tissues that make up higher plant organs.  For each tissue type give an example of a specific cell type associated with it and its function.

Answer 1

The tissues of a plant are organized into three tissue systems:

1.The dermal tissue system

2.The ground tissue system

3. The vascular tissue system

1.Dermal tissue covers and protects the plant, and controls gas exchange and water absorption (in roots). Dermal tissue of the stems and leaves is covered by a waxy cuticle that prevents evaporative water loss.

Stomata are specialized pores that allow gas exchange through holes in the cuticle. Unlike the stem and leaves, the root epidermis is not covered by a waxy cuticle which would prevent absorption of water.

Root hairs, which are extensions of root epidermal cells, increase the surface area of the root, greatly contributing to the absorption of water and minerals.

Trichomes, or small hairlike outgrowths of epidermal tissue, may be present on the stem and leaves, and aid in defense against herbivores.

2.Ground tissue carries out different functions based on the cell type and location in the plant, and includes parenchyma (photosynthesis in the leaves, and storage in the roots), collenchyma (shoot support in areas of active growth), and schlerenchyma (shoot support in areas where growth has ceased) is the site of photosynthesis, provides a supporting matrix for the vascular tissue, provides structural support for the stem, and helps to store water and sugars.

3.Vascular tissue transports water, minerals, and sugars to different parts of the plant.

Vascular tissue is made of two specialized conducting tissues: xylem and phloem.

Xylem tissue transports water and nutrients from the roots to different parts of the plant, and also plays a role in structural support in the stem.

Phloem tissue transports organic compounds from the site of photosynthesis to other parts of the plant. The xylem and phloem always lie adjacent to each other in a vascular bundle.

Plant Cell types

Each plant tissue type is comprised of specialize cell types which carry out vastly different functions:

Vascular tissue cells:Tracheids, Vessel elements, Sieve tube cells,Companion cells.

Dermal tissue cells:Epidermal cells, Stomata or more accurately, guard cells, Trichomes.

Ground tissue cells:Parenchyma, Collenchyma, Sclerenchyma.

Cells in dermal tissue

The outer layer of tissue surrounding the entire plant is called the epidermis, usually comprised of a single layer of epidermal cells which provide protection and have other specialized adaptations in different plant organs.

In the root, the epidermis aids in absorption of water and minerals. Root hairs, which are extensions of root epidermal cells, increase the surface area of the root, greatly contributing to the absorption of water and minerals. Roots also contain specialized dermal cells called endodermis, which is found only in the roots and and serves as a checkpoint for materials entering the root’s vascular system from the environment. A waxy substance is present on the walls of the endodermal cells. This waxy region, known as the Casparian strip, forces water and solutes to cross the plasma membranes of endodermal cells instead of slipping between the cells.

In the stem and leaves, epidermal cells are coated in a waxy substance called a cuticle which prevents water loss through evaporation. The cuticle is not present on root epidermis and is the same as the Casparian strip, which is present in the roots. To permit gas exchange for photosynthesis and respiration, the epidermis of the leaf and stem also contains openings known as stomata. Two cells, known as guard cells, surround each leaf stoma, controlling its opening and closing and thus regulating the uptake of carbon dioxide and the release of oxygen and water vapor. Stems and leaves may also have trichomes, hair-like structures on the epidermal surface, that help to reduce transpiration (the loss of water by aboveground plant parts), increase solar reflectance, and store compounds that defend the leaves against predation by herbivores.

Cells in vascular tissue

Just like in animals, vascular tissue transports substances throughout the plant body. But instead of a circulatory system which circulates by a pump (the heart), vascular tissue in plants does not circulate substances in a loop, but instead transports from one extreme end of the plant to the other (eg, water from roots to shoots). Vascular tissue in plants is made of two specialized conducting tissues: xylem, which conducts water, and phloem, which conducts sugars and other organic compounds. A single vascular bundle always contains both xylem and phloem tissues. Unlike the animal circulatory system, where the vascular system is composed of tubes that are lined by a layer of cells, the vascular system in plants is made of cells – the substance (water or sugars) actually moves through individual cells to get from one end of the plant to the other.

Xylem tissue transports water and nutrients from the roots to different parts of the plant, and includes vessel elements and tracheids, both of which are tubular, elongated cells that conduct water. Tracheids are found in all types of vascular plants, but only angiosperms and a few other specific plants have vessel elements. Tracheids and vessel elements are arranged end-to-end, with perforations called pits between adjacent cells to allow free flow of water from one cell to the next. They have secondary cell walls hardened with lignin, and provide structural support to the plant. Tracheids and vessel elements are both dead at functional maturity, meaning that they are actually dead when they carry out their job of transporting water throughout the plant body.

Phloem tissue, which transports organic compounds from the site of photosynthesis to other parts of the plant, consists of sieve cells and companion cells. Sieve cells conduct sugars and other organic compounds, and are arranged end-to-end with pores called sieve plates between them to allow movement between cells. They are alive at functional maturity, but lack a nucleus, ribosomes, or other cellular structures. Sieve cells are thus supported by companion cells, which lie adjacent to the sieve cells and provide metabolic support and regulation.

The xylem and phloem are always next to each other. In stems, the xylem and the phloem form a structure called a vascular bundle; in roots, this is termed the vascular stele or vascular cylinder.

Cells in ground tissue

Ground tissue is all the other tissue in a plant that isn’t dermal tissue or vascular tissue. Ground tissue cells include parenchyma, (photosynthesis in the leaves, and storage in the roots), collenchyma (shoot support in areas of active growth), and sclerenchyma (shoot support in areas where growth has ceased).

Parenchyma are the most abundant and versatile cell type in plants. They have primary cell walls which are thin and flexible, and most lack a secondary cell wall. Parenchyma cells are totipotent, meaning they can divide and differentiate into all cell types of the plant, and are the cells responsible for rooting a cut stem. Most of the tissue in leaves is comprised of parenchyma cells, which are the sites of photosynthesis, and parenchyma cells in the leaves contain large quantities of chloroplasts for photosynthesis. In roots, parenchyma are sites of sugar or starch storage, and are called pith (in the root center) or cortex (in the root periphery). Parenchyma can also be associated with phloem cells in vascular tissue as parenchyma rays.

Collenchyma, like parenchyma, lack secondary cell walls but have thicker primary cells walls than parenchyma. They are long and thin cells that retain the ability to stretch and elongate; this feature helps them provide structural support in growing regions of the shoot system. They are highly abundant in elongating stems.

Sclerenchyma cells have secondary cell walls composed of lignin, a tough substance that is the primary component of wood. Sclerenchyma cells therefore cannot stretch, and they provide important structural support in mature stems after growth has ceased. Interestingly, sclerenchyma cells are dead at functional maturity.