Plant Tissue Systems

Like other organisms, plant cells are grouped together into various tissues. These tissues can be simple, consisting of a single cell type, or complex, consisting of more than one cell type. Above and beyond tissues, plants also have a higher level of structure called plant tissue systems. There are three types of tissue systems: dermal tissue, vascular tissue, and ground tissue systems.

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Plant Tissue Systems

Leaf structure diagram
The medium scale structure of a leaf featuring the major tissues; the upper and lower epithelia (and associated cuticles), the palisade and spongy mesophyll and the guard cells of the stoma. Vascular tissue (veins), made up of xylem, phloem and sheath cells, and example trichromes are also shown. The green spots within cells represent chloroplasts and indicate which tissues undergo photosynthesis. By Zephyris (Own work) [ CC BY-SA 3.0 or GFDL], via Wikimedia Commons

Dermal Tissue

The dermal tissue system consists of the epidermis and the periderm. The epidermis is generally a single layer of closely packed cells. It both covers and protects the plant. It can be thought of as the plant's "skin." Depending on the part of the plant that it covers, the dermal tissue system can be specialized to a certain extent. For instance, the epidermis of a plant's leaves secretes a coating called the cuticle that helps the plant retain water. The epidermis in plant leaves and stems also contain pores called stomata. Guard cells in the epidermis regulate gas exchange between the plant and the environment by controlling the size of the stomata openings.

The periderm, also called bark, replaces the epidermis in plants that undergo secondary growth. The periderm is multilayered as opposed to the single layered epidermis. It consists of cork cells (phellem), phelloderm, and phellogen (cork cambium). Cork cells are nonliving cells that cover the outside of stems and roots to protect and provide insulation for the plant. The periderm protects the plant from pathogens, injury, prevents excessive water loss, and insulates the plant.

Ground Tissue

The ground tissue system synthesizes organic compounds, supports the plant and provides storage for the plant. It is mostly made up of plant cells called parenchyma cells but can also include some collenchyma and sclerenchyma cells as well. Parenchyma cells synthesize and store organic products in a plant. Most of the plant's metabolism takes place in these cells. Parenchyma cells in leaves control photosynthesis. Collenchyma cells have a support function in plants, particularly in young plants. These cells help to support plants while not restraining growth due to their lack of secondary cell walls and the absence of a hardening agent in their primary cell walls. Sclerenchyma cells also have a support function in plants, but unlike collenchyma cells, they have a hardening agent and are much more rigid.

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Vascular Tissue System

Diagram of Xylem and Phloem in a stem.
Diagram of Xylem and Phloem in a stem. 1. Xylem 2. Phloem 3. Cambium 4. Pith 5. Companion Cells. By Michael Salaverry ( barakplasma ) (Own work) [ CC BY-SA 3.0], via Wikimedia Commons

Xylem and phloem throughout the plant make up the vascular tissue system. They allow water and other nutrients to be transported throughout the plant. Xylem is consists of two types of cells known as tracheids and vessel elements. Tracheids and vessel elements form tube-shaped structures that provide pathways for water and minerals to travel from the roots to the leaves. While tracheids are found in all vascular plants, vessels are found only in angiosperms.

Phloem is composed mostly of cells called sieve-tube cells and companion cells. These cells assist in the transport of sugar and nutrients produced during photosynthesis from the leaves to other parts of the plant. While tracheid cells are nonliving, sieve-tube and companion cells of the phloem are living. Companion cells possess a nucleus and actively transport sugar into and out of sieve-tubes.

Plant Tissue Systems: Plant Growth

Areas within a plant that are capable of growth via mitosis are called meristems. Plants undergo two types of growth, primary and/or secondary growth. In primary growth, plant stems and roots elongate by cell enlargement as opposed to new cell production. Primary growth occurs in areas called apical meristems. This type of growth allows plants to increase in length and to extend roots deeper into the soil. All plants undergo primary growth. Plants that undergo secondary growth, such as trees, have lateral meristems that produce new cells. These new cells increase the thickness of stems and roots. Lateral meristems consist of the vascular cambium and the cork cambium. It is the vascular cambium that is responsible for producing xylem and phloem cells. The cork cambium is formed in mature plants and yields bark.

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Bailey, Regina. "Plant Tissue Systems." ThoughtCo, Oct. 27, 2017, thoughtco.com/plant-tissue-systems-373615. Bailey, Regina. (2017, October 27). Plant Tissue Systems. Retrieved from https://www.thoughtco.com/plant-tissue-systems-373615 Bailey, Regina. "Plant Tissue Systems." ThoughtCo. https://www.thoughtco.com/plant-tissue-systems-373615 (accessed November 23, 2017).