Many stems and roots undergo secondary growth

Some stems and roots remain slender and show little or no secondary growth. However, in many eudicots, secondary growth thickens stems and roots considerably. This process gives rise to wood and bark, and it makes the support of tall trees possible.

Secondary growth results from the activity of the two lateral meristems: vascular cambium and cork cambium (see Figure 35.13). Vascular cambium consists of cells that divide to produce secondary xylem and phloem cells, while cork cambium produces mainly waxy-walled cork cells.

Initially, the vascular cambium is a single layer of cells lying between the primary xylem and the primary phloem (see Figure 35.18a). The root or stem increases in diameter when the cells of the vascular cambium divide, producing secondary xylem cells toward the inside of the root or stem and producing secondary phloem cells toward the outside (Figure 35.19). In the stems of woody plants, cells in the pith rays between the vascular bundles also divide, forming a continuous cylinder of vascular cambium running the length of the stem. This cylinder, in turn, gives rise to complete cylinders of secondary xylem (wood) and secondary phloem, which contributes to the bark.

As the vascular cambium produces secondary xylem and phloem, its principal cell products are vessel elements, supportive fibers, and parenchyma cells in the xylem and sieve tube elements, companion cells, fibers, and parenchyma cells in the phloem. The parenchyma cells in the xylem and phloem store carbohydrate reserves in the stem and root.

Living tissues such as this storage parenchyma must be connected to the sieve tubes of the phloem, or they will starve to death. These connections are provided by vascular rays, which are composed of cells derived from the vascular cambium. These rays, laid down progressively as the cambium divides, are rows of living parenchyma cells that run perpendicular to the xylem vessels and phloem sieve tubes (Figure 35.20). As the root or stem continues to increase in diameter, new vascular rays are initiated so that this storage and transport tissue continues to meet the needs of both the bark and the living cells in the xylem.

The vascular cambium itself increases in circumference with the growth of the root or stem. To do this, some of its cells divide in a plane at right angles to the plane that gives rise to secondary xylem and phloem. The products of each of these divisions lie within the vascular cambium itself and increase its circumference.

Only eudicots and other non-monocot angiosperms have a vascular cambium and a cork cambium and thus undergo secondary growth. The few monocots that form thickened stems—palm trees, for example—do so without using vascular cambium or cork cambium. Palm trees have a very wide apical meristem that produces a wide stem, and dead leaf bases also add to the diameter of the stem. Basically, monocots grow in the same way as do other angiosperms that lack secondary growth.

Wood and bark, consisting of secondary phloem, are unique to plants showing secondary growth. These tissues have their own patterns of organization and development.

Woody stem

Primary Growth Woody Stems

The vascular cambium thickens the stem by producing secondary xylem and secondary phloem.

-Pith Primary xylem Secondary xylem Vascular cambium Secondary phloem Primary phloem

Woody stem

The vascular cambium thickens the stem by producing secondary xylem and secondary phloem.

35.19 Vascular Cambium Thickens Stems and Roots

Stems and roots grow thicker because a thin layer of cells, the vascular cambium, remains meristematic.

-Pith Primary xylem Secondary xylem Vascular cambium Secondary phloem Primary phloem

When a vascular cambium cell divides, it produces either a new xylem cell toward the inside of the stem or root, or a new phloem cell toward the outside.

Outer margin of primary xylem

When a vascular cambium cell divides, it produces either a new xylem cell toward the inside of the stem or root, or a new phloem cell toward the outside.

Outer margin of primary xylem

Xylem Cell

Next new secondary phloem cell

Outward growth secondary Next new xylem cell secondary xylem cell

Vascular cambium cell New secondary Next new xylem cell secondary xylem cell

Next new secondary phloem cell

Outward growth

Vessel ray

Vascular element

Vessel ray

Vascular element

Ray Cells Wood

35.20 Vascular Rays and Vessel Elements In this sample of wood from the tulip poplar, the orientation of vascular rays is perpendicular to that of the vessel elements. The vascular rays transport sieve tube sap horizontally from the phloem to storage parenchyma cells.

35.20 Vascular Rays and Vessel Elements In this sample of wood from the tulip poplar, the orientation of vascular rays is perpendicular to that of the vessel elements. The vascular rays transport sieve tube sap horizontally from the phloem to storage parenchyma cells.

wood. Cross sections of most tree trunks (mature stems) in temperate-zone forests show annual rings (Figure 35.21), which result from seasonal environmental conditions. In spring, when water is relatively plentiful, the tracheids or vessel elements produced by the vascular cambium tend to be large in diameter and thin-walled. Such wood is well adapted for transporting water and minerals. As water becomes less available during the summer, narrower cells with thicker walls are produced, making this summer wood darker and perhaps more dense than the wood formed in spring. Thus each growing season is usually recorded in a tree trunk by a clearly visible annual ring. Trees in the moist Tropics do not undergo seasonal growth, so they do not lay down such obvious regular rings. Variations in temperature or water supply can lead to the formation of more than one "annual" ring in a single year.

The difference between old and new regions of wood also contributes to its appearance. As a tree grows in diameter, the xylem toward the center becomes clogged with water-insoluble substances and ceases to conduct water and minerals; this heartwood appears darker in color. The portion of the xylem that is actively conducting water and minerals throughout the tree is called sapwood and is lighter in color and more porous than heartwood.

The knots that we find attractive in knotty pine but regard as a defect in structural timbers are cross sections of branches.

As a trunk grows, the bases of branches become buried in the trunk's new wood and appear as knots when the trunk is cut lengthwise.

BARK. As secondary growth of stems or roots continues, the expanding vascular tissue stretches and breaks the epidermis and cortex, which ultimately flake away. Tissue derived from the secondary phloem then becomes the outermost part of the stem. Before the dermal tissues are broken away, cells lying near the surface of the secondary phloem begin to divide and produce layers of cork, a tissue composed of cells with thick walls, waterproofed with suberin. The cork soon becomes the outermost tissue of the stem or root (see Figure 35.14). The dividing cells, derived from the secondary phloem, form a cork cambium. Sometimes the cork cambium produces cells to the inside as well as to the outside; these cells constitute what is known as the phelloderm.

Cork, cork cambium, and phelloderm make up the peri-derm of the secondary plant body. As the vascular cambium continues to produce secondary vascular tissue, the corky layers are in turn lost, but the continuous formation of new cork cambia in the underlying phloem gives rise to new corky layers.

When periderm forms on stems and roots, the underlying tissues still need to release carbon dioxide and take up oxygen. Lenticels are spongy regions in the periderm of stems and roots that allow such gas exchange (Figure 35.22).

Annual ring

Pith

Annual ring

Pith

Secondary Growth Root

Secondary phloem

Secondary xylem

35.21 Annual Rings Rings of secondary xylem are the most noticeable feature of this cross section from a 3-year-old basswood stem.

Secondary phloem

Secondary xylem

35.21 Annual Rings Rings of secondary xylem are the most noticeable feature of this cross section from a 3-year-old basswood stem.

Lenticel

Lenticel

Twig Lenticel

35.22 Lenticels Allow Gas Exchange through the Periderm The region of periderm that appears broken open is a lenticel in a year-old elder twig; note the spongy tissue that constitutes the lenticel.

35.22 Lenticels Allow Gas Exchange through the Periderm The region of periderm that appears broken open is a lenticel in a year-old elder twig; note the spongy tissue that constitutes the lenticel.

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Responses

  • Gabriel
    Why do stems need to undergo secondary growth?
    8 years ago
  • anna
    Do leaves undergo secondary growth?
    8 years ago
  • erkki
    Why monocot does not undergo secondary growth?
    8 years ago
  • LARGO
    Why do roots undergo secondary grouth?
    8 years ago
  • sophia kruger
    What divides the primary xylem?
    8 years ago
  • Jose
    Can roots undergo secondary growth?
    8 years ago
  • Margaret
    Is xylem or pholem more noticable in a cross section?
    8 years ago
  • negisti
    How gas exchange through lenticel?
    8 years ago
  • vitale
    Is tue xylem a membrain?
    7 years ago
  • BERND
    Why monocots can't undergo secondary growth?
    6 years ago
  • Leah
    Which group of angiosperms that lack of secondary growth?
    3 years ago
  • melanie
    Which additional tissues develop when the cross section stem undergoes secondary growth?
    2 years ago

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