Info

Tissue system

Type of cells

Location

Function in roots

Function in stems

Function in leaves

Dermal tissue system

flat, living parenchyma (epidermal cells) in nonwoody parts; flat, dead parenchyma (cork cells) in woody parts

outermost layer(s) of cells

absorption, protection

gas exchange, protection

gas exchange, protection

Ground tissue system

mostly parenchyma, usually with some collenchyma and fewer sclerenchyma

between dermal and vascular in nonwoody plant parts

support, storage

support, storage

photosynthesis

Vascular tissue system

elongated sclerenchyma cells, with some parenchyma

tubes throughout plant

transport, support

transport, support

transport, support

Ground Tissue System

Dermal tissue surrounds the ground tissue system, which consists of all three types of plant cells. Ground tissue functions in storage, metabolism, and support. Parenchyma cells are the most common type of cell found in ground tissue. Nonwoody roots, stems, and leaves are made up primarily of ground tissue. Cactus stems have large amounts of parenchyma cells for storing water in dry environments. Plants that grow in waterlogged soil often have parenchyma with large air spaces that allow air to reach the roots. Nonwoody plants that must be flexible to withstand wind have large amounts of collenchyma cells. Sclerenchyma cells are found where hardness is an advantage, such as in the seed coats of hard seeds and in the spines of cactuses.

Vascular Tissue System

Ground tissue surrounds the vascular tissue system, which functions in transport and support. Recall that the term vascular tissue refers to both xylem and phloem. Xylem tissue conducts water and mineral nutrients primarily from roots upward in the plant. Xylem tissue also provides structural support for the plant. Phloem tissue conducts organic compounds and some mineral nutrients throughout the plant. Unlike xylem, phloem is alive at maturity.

In angiosperms, xylem has two major components—tracheids and vessel elements. Both are dead cells at maturity. Look at Figure 29-2a. A tracheid (TRAY-kee-id) is a long, thick-walled sclerenchyma cell with tapering ends. Water moves from one tra-cheid to another through pits, which are thin, porous areas of the cell wall. A vessel element, shown in Figure 29-2b, is a scle-renchyma cell that has either large holes in the top and bottom walls or no end walls at all. Vessel elements are stacked to form long tubes called vessels. Water moves more easily in vessels than in tracheids. The xylem of most seedless vascular plants and most gymnosperms contains only tracheids, which are considered a primitive type of xylem cell. The vessel elements in angiosperms probably evolved from tracheids. Xylem also contains parenchyma cells.

The conducting parenchyma cell of angiosperm phloem is called a sieve tube member. Look at Figure 29-2c. Sieve tube members are stacked to form long sieve tubes. Compounds move from cell to cell through end walls called sieve plates. Each sieve tube member lies next to a companion cell, a specialized parenchyma cell that assists in transport. Phloem also usually contains scle-renchyma cells called fibers. Commercially important hemp, flax, and jute fibers are phloem fibers.

Vascular tissue systems are adapted to different environmental conditions. For example, xylem forms the wood of trees, providing the plants with strength while conducting water and mineral nutrients. In aquatic plants, such as duckweed, xylem is not necessary for support or water transport and may be nearly absent from the mature plant.

figure 29-2

(a) Tracheids are long and thin, and they contain pits in their cell walls. (b) Vessel elements are shorter and wider than tracheids. Both tracheids and vessel elements transport water. (c) Sieve tube members are long and tubular, and they contain pores in their cell walls. Sugar is transported through sieve tube members.

Word Roots and Origins meristem from the Greek meristos, meaning "divided"

TABLE 29-2 Types of Meristems

Type

Location

Growth function

Apical meristem

tips of stems and roots

increase length at tips

Intercalary meristem

between the tip and base of stems and leaves

increase length between nodes

Lateral meristem

sides of stems and roots

increase diameter

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