Apical Meristem

figure 29-9

Apical meristems, responsible for the primary elongation of the plant body, are found in the shoot system and root system. Each leaf is attached to the stem at a node. The space between nodes, called the internode, is much larger in the older part of the stem.

Epidermis

Phloem

Vascular bundle

Xylem

Ground tissue

(a) MONOCOT STEM

(a) MONOCOT STEM

Epidermis

Cortex

Phloem

Vascular bundle

Xylem

Pith

(b) DICOT STEM

figure 29-10

Epidermis

Phloem

Vascular bundle

Xylem

Ground tissue

Epidermis

Cortex

Phloem

Vascular bundle

Xylem

Pith

(b) DICOT STEM

figure 29-10

Primary growth in stems produces dermal, ground, and vascular tissues. (a) The cross section of a herbaceous stem of corn, a monocot, shows the vascular bundles scattered throughout the ground tissue. (b) In the sunflower, a dicot, the vascular tissues appear as a single ring of bundles between the cortex and the pith. Notice that the cortex lies just inside the epidermis, as it does in the root.

In some plants that live in cold winter environments, the terminal bud opens and the bud scales fall off when growth resumes in the spring. The bud scales leave scars on the stem surface. Trees and shrubs often are identified in winter by stem characteristics.

Root tips usually have a protective layer, the root cap. The stem apical meristem is protected by bud scales only when the stem is not growing. A surface bud forms very close to the stem tip with one or more buds at each node. In contrast, lateral roots originate farther back from the root tip and form deep inside the root at no particular location along the root axis.

Primary Growth in Stems

As in roots, apical meristems of stems give rise to the dermal, ground, and vascular tissues. Locate each of these tissues in Figure 29-10. The dermal tissue is represented by the epidermis, or the outer layer of the stem. Its main functions are to protect the plant and to reduce the loss of water to the atmosphere while still allowing gas exchange through stomata.

In gymnosperm and dicot stems, ground tissue usually forms a cortex and a pith. The cortex frequently contains flexible collenchyma cells. The pith is located in the center of the stem. The ground tissue of monocot stems is usually not clearly separated into pith and cortex.

Vascular tissue formed near the apical meristem occurs in bundles—long strands that are embedded in the cortex. Look at the vascular bundles in Figure 29-10. Each bundle contains xylem tissue and phloem tissue. Xylem is usually located toward the inside of the stem, while phloem is usually located toward the outside.

Compare the arrangement of vascular bundles in monocots and dicots. Monocot stem vascular bundles are usually scattered throughout the ground tissue. Stem vascular bundles of dicots and gymnosperms usually occur in a single ring. Most monocots have little or no secondary growth, and they retain the primary growth pattern their entire lives. However, in many dicots and gymnosperms, the primary tissues are eventually replaced by secondary tissues.

Secondary Growth in Stems

Stems increase in thickness due to the division of cells in the vascular cambium. The vascular cambium in dicot and gymnosperm stems first arises between the xylem and phloem in a vascular bundle. Eventually, the vascular cambium forms a boundary. The vascular cambium produces secondary xylem to the inside and secondary phloem to the outside. It usually produces much more secondary xylem than it does secondary phloem. Secondary xylem is called wood. Occasionally, the vascular cambium produces new cambium cells, which increase the stem's diameter.

Older portions of the xylem eventually stop transporting water. They often become darker than the newer xylem due to the accumulation of resins and other organic compounds produced by the few live cells remaining in the xylem. This darker wood in the center of a tree is called heartwood, as shown in Figure 29-11a. The functional, often lighter-colored wood nearer the outside of the trunk is sapwood. In a large-diameter tree, the heartwood keeps getting wider while the sapwood remains about the same thickness.

The phloem produced near the outside of the stem is part of the bark. Bark is the protective outside covering of woody plants. It consists of cork, cork cambium, and phloem. The cork cambium produces cork near the outside. However, cork cells are dead at maturity and cannot elongate, so the cork ruptures as the stem continues to expand in diameter. This results in the bark pattern of some trees, such as oaks and maples, appearing rough or irregular in texture.

During spring, if water is plentiful, the vascular cambium can form new xylem with cells that are wide and thin walled. This wood is called springwood, as shown in Figure 29-11b. In summer, when water is limited, the vascular cambium produces summerwood, which has smaller cells with thicker walls. In a stem cross section, the abrupt change between small summerwood cells and the following year's large springwood cells produces an annual ring. The circles that look like rings on a target in Figure 29-11a are the annual rings of the stem. Because one ring is usually formed each year, you can estimate the age of the stem by counting its annual rings. Annual rings also form in dicot and gymnosperm roots, but they are often less pronounced because the root environment is more uniform. Annual rings often do not occur in tropical trees because of their uniform year-round environment.

Bark

Sapwood Heartwood

(a) HEARTWOOD AND SAPWOOD

Springwood

Summerwood figure 29-11

(a) Wood consists of secondary xylem. The dark-colored wood is called heartwood, and the light-colored wood is called sapwood. Bark is the outside protective covering of woody plants.

(b) You can see the annual rings produced by alternating springwood cells and summerwood cells. The abundant water of the spring season helps expand the cell walls of springwood cells.

Annual ring

Bark

Sapwood Heartwood

Springwood

Summerwood

Annual ring

(b) SPRINGWOOD AND SUMMERWOOD

(a) HEARTWOOD AND SAPWOOD

(b) SPRINGWOOD AND SUMMERWOOD

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