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23–3 Stems
Photo Credit: Getty Images
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2. Stem Structure and Function
Stems have three important functions:
they produce leaves, branches and flowers
they hold leaves up to the sunlight
they transport substances between roots and leaves
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3. Stem Structure and Function
Stems make up an essential part of the water and mineral transport systems of the plant.
Xylem and phloem form continuous tubes from the roots through the stems to the leaves.
This allows water and nutrients to be carried throughout the plant.
In many plants, stems also function as storage systems and aid in the process of photosynthesis.
Stems are surrounded by epidermal cells that have thick cell walls and a waxy protective coating.
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4. Stem Structure and Function
Leaves attach to the stem at structures called nodes.
The regions of stem between the nodes are internodes.
Small buds are found where leaves attach to nodes.
Bud
Node
Internode
Node
Stems produce leaves and branches and hold leaves up to the sunlight, where they carry out photosynthesis. Leaves are attached to a stem at structures called nodes. These nodes are separated by regions of the stem called internodes.
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5. Stem Structure and Function
Buds contain undeveloped tissue that can produce new stems and leaves.
In larger plants, stems develop woody tissue that helps support leaves and flowers.
Bud
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6. Monocot and Dicot Stems
The arrangement of tissues in a stem differs among seed plants.
In monocots, vascular bundles are scattered throughout the stem.
In dicots and most gymnosperms, vascular bundles are arranged in a ringlike pattern.
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7. Monocot and Dicot Stems
Monocot Stems
Monocot stems have a distinct epidermis, which encloses vascular bundles.
Each vascular bundle contains xylem and phloem tissue.
Phloem faces the outside of the stem, and xylem faces the center.
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8. Monocot and Dicot Stems
Epidermis
Vascular bundles
Vascular bundles are scattered throughout the ground tissue.
Ground tissue consists mainly of parenchyma cells.
Ground tissue
In a monocot, vascular bundles are scattered throughout the stem. Photo Credit: ©Ed Reschke/Peter Arnold, Inc.
Monocot
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9. Monocot and Dicot Stems
Dicot stems have vascular bundles arranged in a ringlike pattern.
The parenchyma cells inside the vascular tissue are known as pith.
The parenchyma cells outside of the vascular tissue form the cortex of the stem.
Vascular bundles
Epidermis
In a dicot, vascular bundles are arranged in a ring. Photo Credit: ©Ed Reschke/Peter Arnold, Inc.
Cortex
Pith
Dicot
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10. Primary Growth of Stems
All seed plants undergo primary growth, which is an increase in length at the ends of the plant.
New cells are produced at the tips of roots and shoots throughout the plant’s life.
Primary growth of stems is produced by cell divisions in the apical meristem. It takes place in all seed plants.
Primary growth
Apical meristem
Primary growth
All seed plants undergo primary growth, which is an increase in length. Every year, apical meristems, shown in red, divide to produce new growth. The primary growth for one season consists of a stem and several leaves.
Leaf scar
Year 3
Year 2
Year 1
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11. Secondary Growth of Stems
The method of growth in which stems increase in width is called secondary growth.
In conifers and dicots, secondary growth takes place in the lateral meristematic tissues called the vascular cambium and cork cambium.
Vascular cambium produces vascular tissues and increases the thickness of stems over time.
Cork cambium produces the outer covering of stems.
The addition of new tissue in these cambium layers increases the thickness of the stem.
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12. Secondary Growth of Stems
Formation of the Vascular Cambium
Once secondary growth begins, the vascular cambium appears as a thin layer between the xylem and phloem of each vascular bundle.
Vascular cambium
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13. Secondary Growth of Stems
Vascular cambium divides producing xylem toward the center of the stem and phloem toward the outside.
These different tissues form the bark and wood of a mature stem.
Secondary phloem
Secondary xylem
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14. Secondary Growth of Stems
Formation of Wood
Wood is actually layers of xylem. These cells build up year after year.
Bark
Wood
 In a mature tree that has undergone several years of secondary growth, the vascular cambium lies between layers of xylem to the inside, and layers of phloem to the outside. The youngest xylem, called sapwood, transports water and minerals.
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15. Secondary Growth of Stems
As woody stems grow thicker, older xylem cells near the center of the stem no longer conduct water.
This is called heartwood.
Heartwood usually darkens with age because it accumulates impurities that cannot be removed and supports the tree.
Xylem: Heartwood
 In a mature tree that has undergone several years of secondary growth, the vascular cambium lies between layers of xylem to the inside, and layers of phloem to the outside. The youngest xylem, called sapwood, transports water and minerals.
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16. Secondary Growth of Stems
Xylem: Sapwood
Heartwood is surrounded by sapwood.
Sapwood is active in water and mineral transport and therefore lighter in color.
 In a mature tree that has undergone several years of secondary growth, the vascular cambium lies between layers of xylem to the inside, and layers of phloem to the outside. The youngest xylem, called sapwood, transports water and minerals.
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17. Secondary Growth of Stems
In most of the temperate zone, tree growth is seasonal. Growth begins in the spring and the vascular cambium begins to grow rapidly, producing large light-colored xylem cells with thin cell walls. The result is a light-colored layer of wood called early wood. As the growing season continues, the cells become smaller with thicker cell walls, forming a layer of darker wood called late wood.
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18. Secondary Growth of Stems
This alternation of dark and light wood produces what we normally call tree rings. Each ring represents a year of growth. The size of the rings may even provide information about weather conditions, such as wet or dry years. Thick rings indicate that weather conditions were favorable for the tree growth and visa versa.
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19. Secondary Growth of Stems
Vascular cambium
Formation of Bark
On most trees, bark includes all of the tissues outside the vascular cambium — phloem, the cork cambium and cork.
The vascular cambium produces new xylem and phloem, which increase the width of the stem.
Bark
 In a mature tree that has undergone several years of secondary growth, the vascular cambium lies between layers of xylem to the inside, and layers of phloem to the outside. The youngest xylem, called sapwood, transports water and minerals.
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20. Secondary Growth of Stems
The phloem transports sugars produced by photosynthesis.
As the vascular cambium increases in diameter, it forces the phloem tissue outward.
This causes the oldest tissues to split and fragment as they are stretched by the expanding stem.
Phloem
 In a mature tree that has undergone several years of secondary growth, the vascular cambium lies between layers of xylem to the inside, and layers of phloem to the outside.
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21. Secondary Growth of Stems
The cork cambium produces a thick protective layer of cork which surrounds the cortex.
Cork cambium
 In a mature tree that has undergone several years of secondary growth, the vascular cambium lies between layers of xylem to the inside, and layers of phloem to the outside. The youngest xylem, called sapwood, transports water and minerals.
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22. Secondary Growth of Stems
The cork contains cells that have thick cell walls and usually contain fats, oils, or waxes.
These waterproof substances help to prevent loss of water.
The outermost cork cells are usually old, dead, nonfunctioning phloem that protects the tree.
Cork
 In a mature tree that has undergone several years of secondary growth, the vascular cambium lies between layers of xylem to the inside, and layers of phloem to the outside. The youngest xylem, called sapwood, transports water and minerals.
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