Tissues

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Tissues

Plants are a composite of cells organized into tissues. Every cell within these tissues has a unique size and shape and is surrounded by a wall composed of a complex carbohydrate called cellulose. Plant cells are attached to each other by a gluelike substance, pectin, that cements them together.

All plant tissues originate in meristems, which are unique tissues of the plant body. They are the sites of new cell production and of the genetic events necessary for cellular specialization. Meristems can be categorized by their locations. Apical meristems are composed of groups of dividing cells at the tips of shoots (branches) and roots. When meristematic cells produced by apical meristems begin elongating, they are classified as primary meristems. There are three types of primary meristems: protoderm, ground meristem, and procambium. As primary meristem cells stop dividing and begin differentiating, their classification changes to primary tissues. There are four primary tissues: the epidermis derived from protoderm, the ground tissues (parenchyma, sclerenchyma, and collenchyma) derived from ground meristem, and two types of vascular tissue—xylem and phloem—derived from procambium. Primary tissues have specific positions in the plant body and specific functions.

As the organs of the plant body age, the stems and roots often grow wider. Growth in width is called lateral growth and is initiated by secondary meristems. There are two types of secondary meristems: the vascular cambium and the cork cambium. The vascular cambium is located near the outside of stems and roots. Its function is to produce new cells that become part of the secondary xylem and secondary phloem tissues. The cork cambium is located near the outer edge of older stems and roots, and it produces the periderm, which is part of the bark.

The Concept of the Tissue System

In 1875, German scientist Julius von Sachs introduced a scheme that is still used today: the tissue systems. A tissue system is composed of tissues with a common position and function. The plant body has three tissue systems: dermal, ground, and vascular.

The dermal tissue system forms the outer, protective covering of the plant body. In young plants, the dermal tissue system consists of epidermis tissue. In some plants and plant organs, the epidermis remains intact

PLANT TISSUE SYSTEMS
Tissue System	Tissue	Meristem Origin	Primary or Secondary?	Function
Dermal	Epidermis	Protoderm	Primary	Protect against pathogen entry and inhibit water loss
	Periderm	Cork cambium	Secondary	Protect against pathogen entry and inhibit water loss
Ground*	Parenchyma	Ground meristem†	Both	Storage of food products and water, photosynthesis, and other basic processes
	Collenchyma	Ground meristem	Both	Structural support of leaves and young stems
	Sclerenchyma	Ground meristem	Both	Structural support of plant organs
Vascular	Xylem	Procambium and vascular cambium	Both	Movement of water and dissolved materials throughout the plant
	Phloem	Procambium and vascular cambium	Both	Movement of sugars throughout the plant
* The gound tissues are located mostly in the cortex and pith of stems, roots, and fruits and in the mesophyll of leaves.
† These tissues can also appear as component parts of other complex tissues, e.g., parenchyma cells are an important part of the xylem.

throughout the life of the plant. However, in some aging plants the epidermis is torn and an underlying secondary meristem, the cork cambium, produces a secondary tissue, the periderm. The periderm, which consists of stacks of cells with waxy cell walls, then takes over the protective function. Although both the epidermis and the periderm occupy the same outer position and perform the same protective functions, they are derived from different meristems.

The ground tissue system consists of tissues that are produced by the ground meristem, one of the primary meristems. It is responsible for functions such as photosynthesis and storage and is comprised of the following tissues: parenchyma, collenchyma, and sclerenchyma.

The vascular tissue system is composed of the water- and mineral-conducting tissue, xylem, and the food-conducting tissue, phloem. Primary vascular tissue is produced by the primary meristem, procambium, and secondary vascular tissue is produced by the vascular cambium.

The Ground Tissue System

The ground meristem produces the primary ground tissue of stems, leaves, roots, flowers, and fruits. The three ground tissues are parenchyma, collenchyma, and sclerenchyma. In stems and roots, ground tissues are located in the cortex and pith. The cortex lies between the epidermis and vascular tissue; the pith, when it is present, is located in the center of the organ. In leaves, the ground tissue is called mesophyll and is located between the upper and lower epidermis.

Parenchyma Tissue.

Typically the cortex of roots, the pith of stems, the mesophyll of leaves, and the edible parts of fruits are composed entirely of parenchyma tissue. Although parenchyma cells making up this tissue vary in size, shape, wall structure, and function, generally they are living at maturity, are polyhedral in shape, have thin cell walls, and perform many of the basic physiological functions of the plant. Parenchyma tissue in the cortex and pith regions of roots and stems are often specialized for storage of carbohydrates, proteins, fats, and oils. Water is stored in the mesophyll of succulent plants. Cells in the mesophyll of leaves contain chloroplasts and are specialized for photosynthesis. There are two types of leaf mesophyll: palisade mesophyll is composed of columnar-shaped cells near the upper surface of the leaf, and branched spongy mesophyll is next to the lower surface.

Collenchyma Tissue.

Collenchyma is a supporting tissue found in the leaves and young stems. This tissue provides strong but flexible support. Collenchyma cells are living at maturity and have plastic walls that can expand with the growing organ. Collenchyma cells are elongated and have a thick cell wall containing large amounts of pectins and water. The unevenly thickened cell wall is a definitive feature of collenchyma cells. Strands of collenchyma tissue often occur just beneath the epidermis in stems and petioles and between vascular bundles and the epidermis in leaves.

Sclerenchyma Tissue.

Sclerenchyma is also supporting tissue, but its cells are generally dead at maturity. Sclerenchyma cells have thick cell walls that are usually lignified. Lignin is a complex polymer that is impervious to water. Supporting sclerenchyma tissue is common in plant organs that have completed elongation growth. Lignification of the cell walls of sclerenchyma cells not only makes them harder and stronger, it also makes them resistant to decay. There are two major categories of sclerenchyma cells: sclereids and fibers. Generally sclereids are short and irregular in shape, while fibers are long and narrow. Sclereids often have massively thick lignified cell walls. Columnar-shaped sclereids form the outer seed coat tissue of bean seeds; clusters of sclereids give pear fruits their gritty texture; branched sclereids are sometimes present in the mesophyll of leaves; and strands of elongate fibers are important structural components in the cortex, associated with vascular tissue, or extending from vascular bundles to the epidermis in leaves. Extremely long fibers from flax and ramie plants, and other plant species, are commercially important sources of fibers used in the manufacture of ropes and fabrics.

The Dermal Tissue System

The dermal tissue system consists of the epidermis and the periderm. The epidermis is a primary tissue derived from the protoderm, and the periderm is a secondary tissue derived from the cork cambium.

The Epidermis.

The epidermis is a complex tissue and is the outermost covering of the primary plant body. It provides a protective barrier between the internal tissues of the plant and the outside. Although the epidermis is generally one cell layer thick, it is made up of several different cell types. From the surface, epidermal cells appear slightly elongated in stems and leaf petioles and irregular in outline like jigsaw puzzle pieces. Epidermal cells generally lack chloroplasts and their outer surfaces are covered by a waxy layer, the cuticle . The cuticle reduces water loss from the plant surface. Another common epidermal cell type, the guard cell, contains chloroplasts. Guard cells are always present in pairs surrounding a pore. A pair of guard cells and its pore is called a stoma (plural, stomata ). The exchange of gases essential for photosynthesis, such as the uptake of carbon dioxide, and the loss of water vapor through transpiration occur through stomata. Stomata are common in photosynthetic leaves and other photosynthetic organs, such as herbaceous stems and aerial roots. In many plants, stomata are surrounded by cells that differ from ordinary epidermal cells; these are called subsidiary cells. Another common type of epidermal structure is the trichome. Trichomes may be single-celled outgrowths, such as root hairs, or they may be simple or complex multicellular structures.

The Periderm.

The periderm forms the outer covering over most older stems and roots. It may have a few to several cell layers. During the first year of secondary growth, the cork cambium produces tabular-shaped cork cells to the outside, and the epidermis is crushed and destroyed. Additional cork cambia may arise deeper within the plant body in subsequent years. In some plants, cylinders or arcs of cork cambia activate, resulting in multiple layers of periderm tissue. Suberized (waxy) cork cells form an impervious outer plant tissue.

The Vascular Tissue System

Xylem and phloem are complex tissues comprising the vascular tissue system. Vascular tissue is present in all stems, roots, leaves, flower parts, seeds, and fruits.

Xylem Tissue.

Xylem is a complex tissue specialized to transport water and dissolved minerals. In young organs, xylem is formed from a primary meristem, the procambium, and is a primary tissue. In older stems and roots it continues to form as a secondary tissue from a secondary meristem, the vascular cambium.

The two cell types specifically charged with water transport are vessel members and tracheids. These cell types are fundamentally similar in that both are dead at maturity and tend to have thickened cell walls with openings called pits. A pit is a thin region in the wall that extends between two adjacent cells. Pits allow water to pass freely, but air bubbles are trapped. Differences are apparent in the end walls of these cells. The end walls of tracheids are tapered and cells connected end-to-end communicate through pits. The end walls of vessel members are completely open. The open area is called a perforation plate. When two vessel members are connected endto-end they form an open tube called a vessel, which is typically composed of several vessel members connected end-to-end. The end walls of vessel members at opposite ends of a vessel have non-perforated end walls, so a vessel is like a length of tube with closed ends. The non-perforated end walls do have pits, allowing water and minerals to easily pass from one vessel to another, but restricting air bubbles to a single vessel. Because air bubbles block the movement of water, any that form inside tracheids and vessel members create conducting problems. Isolating air bubbles and restricting their movement improves conducting efficiency of tracheids and vessels.

Xylem is composed of several different cell types. In addition to vessel members and tracheids, parenchyma cells and fibers are commonly found in xylem tissue. Parenchyma cells and fibers contribute to the transport and support function of this tissue.

Phloem Tissue.

The complex phloem tissue transports sugars from leaves to the other plant organs. It is a primary tissue in young organs, and in older stems and roots it is a secondary tissue formed from the vascular cambium.

Within phloem tissue, cells that collect and transport sugars are called sieve-tube members (STMs). STMs are living cells joined end-to-end to form a tubelike structure called a sieve tube. Each STM has a unique attachment to another important phloem cell type called a companion cell. Companions cells help regulate cellular functions inside STMs and assist in loading and unloading sugars into STMs. Other cells in the phloem include parenchyma cells and fibers. Fibers act to strengthen and support the weight of the phloem and surrounding tissues, and phloem parenchyma cells store water and help with loading and unloading sugars.

see also Anatomy of Plants; Cells, Specialized Types; Meristems; Sachs, Julius von; Vascular Tissues.

Thomas L. Rost

Deborah K. Canington