Plant Tissues and Tissue Systems

A tissue, as we learned in Chapter 1, is an organized group of cells that have features in common and that work together as a structural and functional unit. Parenchyma cells make up parenchyma tissue, a simple tissue—that is, a tissue composed of only one type of cell. Sclerenchyma and collenchyma are other simple tissues, composed, respectively, of sclerenchyma and collenchyma cells.

Different cell types combine to form complex tissues. Xylem and phloem are complex tissues, each composed of more than one type of cell. As a result of its cellular complexity, xylem can perform a variety of functions, including transport, support, and storage. The xylem of angiosperms contains vessel elements and tracheids for conduction, thick-walled fibers for support, and parenchyma cells that store nutrients. The phloem of angiosperms includes sieve tube elements, companion cells, fibers, sclereids, and parenchyma cells.

Tissues, in turn, are grouped into tissue systems that extend throughout the body of the plant, from organ to organ, in a concentric arrangement. Vascular plants have three tissue systems: vascular, dermal, and ground (Figure 35.12).

As sieve tube elements mature, plasmodesmata in their end walls enlarge to form pores, enhancing the connection between neighboring cells. The result is end walls that look like sieves, called sieve plates (see Figure 35.11). As the holes in the sieve plates expand, the membrane that encloses the central vacuole, called the tonoplast, disappears. The nucleus and some cytoplasmic components also break down, and thus do not clog the pores of the sieve.

At functional maturity, a sieve tube element is filled with sieve tube sap, consisting of water, dissolved sugars, and other solutes. This solution moves from cell to cell along the sieve tube. The moving sap solution is distinct from the layer of cytoplasm at the periphery of a sieve tube element, next to the cell wall. This stationary layer of cytoplasm contains the organelles remaining in the sieve tube element.

Each sieve tube element has one or more companion cells (see Figure 35.11), produced as a daughter cell along with the sieve tube element when a parent cell divides. Numerous plasmodesmata link a companion cell with its sieve tube element. Companion cells retain all their organelles and, through the activities of their nuclei, they may be thought of as the "life-support systems" of the sieve tube elements.

All of these types of plant cells play important roles. Next we'll see how they are organized into tissues and tissue systems.

The dermal tissue system is the outer covering of the plant.

The vascular tissue system conducts water and solutes throughout the plant.

The dermal tissue system is the outer covering of the plant.

The vascular tissue system conducts water and solutes throughout the plant.

The ground tissue system carries out photosynthesis, stores photosynthetic products, and helps support the plant.

Dermal Ground

Dermal Ground Vascular

35.12 Three Tissue Systems Extend throughout the Plant Body

The arrangement shown here is typical of eudicots.

The ground tissue system carries out photosynthesis, stores photosynthetic products, and helps support the plant.

^Dermal n^ Ground Vascular

Dermal Ground Vascular

35.12 Three Tissue Systems Extend throughout the Plant Body

The arrangement shown here is typical of eudicots.

The vascular tissue system, which includes the xylem and phloem, is the plant's plumbing or transport system. All the living cells of the plant body require a source of energy and chemical building blocks. The phloem transports carbohydrates from sites of production (called sources, primarily leaves) to sites of utilization or storage (called sinks, such as growing tissue, storage tubers, and developing flowers). The xylem distributes water and mineral ions taken up by the roots to all the cells of the stem and leaves.

The dermal tissue system is the outer covering of the plant. All parts of the young plant body are covered by an epidermis, which may be a single layer of cells or several layers. The epidermis contains epidermal cells and may also include specialized cell types, such as the guard cells that form stomata (pores) in leaves. The shoot epidermis secretes a layer of wax-covered cutin, the cuticle, that helps retard water loss from stems and leaves. The stems and roots of woody plants have a protective covering called the periderm, which will be discussed later in this chapter.

The ground tissue system makes up the rest of the plant. It consists primarily of parenchyma tissue, often supplemented by collenchyma or sclerenchyma. Ground tissue functions primarily in storage, support, photosynthesis, and the production of defensive and attractive substances.

In the discussions that follow, we'll examine how the tissue systems are organized in the different organs of a flowering plant. Let's begin by seeing how this organization develops as the plant grows.

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Responses

  • SEBASTIAN
    Which human body system resemble the xylem and phloem of a plant?
    8 years ago
  • awet
    What tissue distributes water and dissolves ions to all parts of plants living cells th?
    8 years ago
  • burtuka
    What is concentric arrangement of tissues in plants?
    8 years ago
  • nadine reinhardt
    What plant tissue does not play an important role in supporting a plant?
    8 years ago

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