A hierarchy of meristems generates a plants body

There are two types of meristems:

► Apical meristems give rise to the primary plant body, which is the entire body of many plants.

► Lateral meristems give rise to the secondary plant body. The stems and roots of some plants (most obviously trees) form wood and become thick; it is the lateral meristems that give rise to the tissues responsible for this thickening.

apical meristems. Apical meristems are located at the tips of roots and stems and in buds. They extend the plant body by producing the cells that subsequently expand and differentiate to form all plant organs (Figure 35.13).

► Shoot apical meristems supply the cells that extend stems and branches, allowing more leaves to form and photo-synthesize.

► Root apical meristems supply the cells that extend roots, enabling the plant to "forage" for water and minerals.

Lateral buds

Lateral buds

Meristems Plants Lateral Apical

-Cork cambium

Vascular cambium

35.13 Apical and Lateral Meristems Apical meristems produce the primary plant body;lateral meristems produce the secondary plant body.

35.13 Apical and Lateral Meristems Apical meristems produce the primary plant body;lateral meristems produce the secondary plant body.

Leaf primordia

The apical bud contains a shoot apical meristem.

Lateral bud primordia

In woody plants the vascular cambium and cork cambium thicken the stem and root.

-Cork cambium

Leaf primordia

The apical bud contains a shoot apical meristem.

Lateral bud primordia

In woody plants the vascular cambium and cork cambium thicken the stem and root.

Lateral Bud Primordium

Vascular cambium

Root apical meristem Root hairs

Root apical meristem

Apical Meristem Root And Shoot

Root apical meristem Root hairs

Root apical meristem

Both root and shoot apical meristems give rise to a set of cylindrical primary meristems that produce the primary tissues of the plant body. From the outside to the inside of the root or shoot, which are both cylindrical organs, the primary meristems are the protoderm, the ground meristem, and the pro-cambium. These in turn give rise to the three tissue systems:

Apical meristems —*- Primary meristems —»- Tissue systems

Root or jt shoot apical > meristem

Protoderm-»- Dermal tissue system

Ground meristem —»- Ground tissue system

Procambium-»- Vascular tissue system

Apical meristems are responsible for primary growth, which leads to lengthening of the plant body and organ formation. All plant organs arise ultimately from cell divisions in the apical meristems, followed by cell expansion and differentiation. Primary growth gives rise to the entire body of many plants.

Because meristems can continue to produce new organs throughout the lifetime of the plant, the plant body is much more variable in form than the animal body, whose organs are produced only once.

lateral meristems. Some roots and stems develop a secondary body, the tissues of which we commonly refer to as wood and bark. These complex tissues are derived from two lateral meristems: the vascular cambium and the cork cambium (Figure 35.14).

The vascular cambium is a cylindrical tissue consisting predominantly of vertically elongated cells that divide frequently. Toward the inside of the stem or root, the dividing cells form new xylem, the secondary xylem, and toward the outside they form new phloem, the secondary phloem.

As a tree trunk grows in diameter, the outermost layers of the stem crack and fall off. Without the activity of the cork cambium, this sloughing off of tissues, including the epidermis, would expose the tree to potential damage, including excessive water loss or invasion by microorganisms. The cork cambium produces new protective cells, primarily in the outward direction. The walls of these cork cells become impregnated with suberin. The mass of waterproofed cells produced by the cork cambium is called the periderm.

Growth in the diameter of stems and roots, produced by the vascular and cork cambia, is called secondary growth. It is the source of wood and bark. Wood is secondary xylem. Bark is everything external to the vascular cambium (peri-derm plus secondary phloem).

Each year, deciduous trees lose their leaves, leaving bare branches and twigs in winter. These twigs illustrate both pri-

This year's -< growth

Primary phloem yXx ^^ Vascular / f^C cambium

Last year's growth

This year's -< growth

Last year's growth

Growth from two years ago

Woody Growth

Epidermis Cortex

Primary xylem

Vascular cambium

35.14 A Woody Twig Apical meristems produce primary growth. Lateral meristems produce secondary growth.

Epidermis Cortex

Growth from two years ago

35.14 A Woody Twig Apical meristems produce primary growth. Lateral meristems produce secondary growth.

tribute to a root cap, which protects the delicate growing region of the root as it pushes through the soil. The cells of the root cap are often damaged or scraped away and must therefore be replaced constantly. The root cap is also the structure that detects the pull of gravity and thus controls the downward growth of roots.

Part of the root apical meristem nearest the tip of the root forms a quiescent center, in which cell divisions are rare. The quiescent center can become more active when needed—following injury, for example.

The daughter cells that are produced at the basal end of the apical meristem (away from the root cap) elongate and lengthen the root. Following elongation, these cells differentiate, giving rise to the various tissues of the mature root. The growing region above the apical meristem comprises the three cylindrical primary meristems: the protoderm, the ground meristem, and the procambium (Figure 35.15). These primary meristems give rise to the three tissue systems of the root.

The apical and primary meristems constitute the zone of cell division, the source of all the cells of the root's primary tissues. Just above this zone is the zone of cell elongation, where the newly formed cells are elongating and thus causing the root to reach farther into the soil. Above this is the zone of mat-

Primary xylem

Periderm

Cork cambium I

Cortex

Primary phloem

Secondary phloem

Vascular cambium mary and secondary growth (Figure 35.14). The apical meristems of the twigs and their branches are enclosed in buds protected by bud scales. When the buds begin to grow in the spring, the scales fall away, leaving scars that show us where the bud was and identifying each year's growth. The dormant twig shown in Figure 35.14 is the product of primary and secondary growth. Only the buds consist entirely of primary tissues.

In some plants, meristems may remain active for years— even centuries. The bristlecone pine mentioned at the beginning of this chapter provides a dramatic example. Such plants grow in size, or at least in diameter, throughout their lives. In the sections that follow, we'll examine how the various meristems give rise to the plant body.

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Responses

  • Layton
    Why do meristems of plants remain active?
    7 years ago
  • tuukka
    What can lateral stems give rise to?
    7 years ago
  • kimberly
    Is protoderm primary or lateral meristem?
    7 years ago
  • mackenzie
    Which type of primary meristem gives rise to the vascular tissue system?
    6 years ago
  • EVERARD
    Are all lateral meristems cylindrical meristem?
    3 years ago
  • leeann
    Which are cylinderical meristem?
    3 years ago

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