Plant Stems: Structure, Function, and Growth Processes - Prof. Robert E. Reeder, Study notes of Biology

Download Plant Stems: Structure, Function, and Growth Processes - Prof. Robert E. Reeder and more Study notes Biology in PDF only on Docsity! BIOL 1120 REEDER STEMS AND PLANT TRANSPORT I. Stems A. Stem Functions 1. Support leaves and reproductive structures (flowers and fruits) 2. Provides internal transport: water and dissolved minerals from the roots to the leaves; conduct sugar produced in leaves to roots and other parts; lenticels will function for gas exchange on woody stems 3. Produces new living tissue throughout a plant's life: produces buds that develop into stems with new leaves and/or reproductive structures B. External Structure of a Woody Twig 1. All stems have buds: undeveloped embryonic shoots containing dormant apical meristems 2. Terminal bud: embryonic shoot located at the stem's tip a. When dormant (unopened and not actively growing), it is covered and protected by outer bud scales (modified leaves) b. When it resumes growth, the bud scales fall off leaving bud-scale scars 1) These scars may persist for a number of years until they are finally obscured by the branch's secondary growth c. The distance between any two sets of bud-scale scars marks the growth in length that twig made in a season; therefore, the age of a small branch may be determined by counting the scars from the terminal bud down the branch. 3. Lateral buds: located in the leaf axils (upper angle between a leaf and the stem to which it is attached) 4. Both terminal and lateral buds can form stems that bear leaves and/or flowers when they grow a. Normally, most of a plant's axillary buds are repressed by hormones from the primary stem's apical meristem (called apical dominance of the terminal bud: initially has produced the new stem tissues and leaf primordia of the main axis of the main shoot). b. Most buds develop into leafy twigs and may be termed leaf buds c. Others, called mixed buds, contain the rudiments of leafy twigs and embryonic flowers (apple and lilac) d. Flower buds hold embryonic flowers only, as in the peach, cherry, red maple and elm. e. Flower and mixed buds may be terminal or axillary in position 5. A leaf scar shows where each leaf was attached previous to abscission a. A layer of corky material seals the scar b. Small pattern of corky dots may be seen as the bundle scars remaining from strands of conducting vascular tissue that linked the stem with those of the leaf. c. Lateral buds are found above the leaf scars 6. Lenticels can be found as tiny marks, or specks on the bark of a woody twig: loosely arranged cells that allow for gas exchange C. Stem Primary and Secondary Growth 1. Apical meristems are responsible for primary growth (plant length) and are located at the tips of roots and stems a. As length growth occurs, apical meristem leave behind leaf primordia and lateral buds b. The new tissues formed from apical meristems are called primary tissues 2. Lateral meristems are located along the stem and root sides and are responsible for increases in width a. The new tissues formed from lateral meristems are called secondary tissues * 3. All plants have primary growth, but vary with secondary growth * a. Herbaceous plants typically have primary growth only * b. Woody plants have both primary and secondary growth 1) Increases in length occurs at the tips of its stems and roots, while its older stems and roots further back from the tips increase in width 2) Secondary growth adds wood and bark (causing the stem to thicken), while at the same time, primary growth (stem length increases) continues D. Distinguishing Herbaceous Dicot and Monocot Stems 1. Herbaceous dicot, such as the sunflower a. Outer covering of epidermis (with cuticle) b. Several layer of cells, called the cortex, may contain parenchyma, collenchyma, and sclerenchyma 1) Functions may include photosynthesis, storage, and support c. the vascular tissues are arranged as vascular bundles in a circle in cross section bio1120_stems&plant_transport.doc 4/2/09 1) Lengthwise, the vascular bundles extend as long strands throughout the stem's length and are continuous with the vascular tissues of both leaves and roots 2) Each vascular bundle: a) Xylem is located toward a stem's interior, and phloem is toward the exterior b) Sandwiched between the two vascular tissues is a single cell layer called the vascular cambium (lateral meristem responsible for secondary growth) c) Fibers and fiber clusters are found (more extensively in phloem) to help strengthen the stem d) No clear separation of cortex and pith between the vascular bundles 2. Herbaceous monocots, such as corn a. Outer covering of epidermis (with cuticle) b. Vascular tissues run in strands lengthwise * c. In cross section, the vascular bundles are scattered throughout the stem 1) Xylem arranged toward bundles inside and phloem toward the outside 2) Each bundle is enclosed in a sheath of sclerenchyma d. No distinct areas of cortex or pith * e. Lateral meristems (vascular cambium or cork cambium) do not occur 1) No secondary growth: wood or bark 2) Palm trees (a monocot) attain considerable size due to modified primary growth 3) Stems of bamboo and palm are extremely hard due to excessive sclerenchyma E. Secondary Growth of Woody Dicots: Angiosperms (apple, hickory, maple) and Gymnosperms (pine, spruce) 1. Occurs as a result of two lateral meristems: vascular cambium and cork cambium 2. Vascular cambium: a. Produces secondary tissues: secondary xylem (wood) to replace primary xylem, and secondary phloem (inner bark) to replace primary phloem (ONLY a single layer of functional secondary phloem is maintained per year). * 1) Primary vascular tissues must be replaced in order for the plant to have an extended life span 3. Cork cambium: a. Produces cork cells and cork parenchyma 1) These tissues are collectively called the periderm (outer bark) 2) Functions to replace the epidermis 4. Secondary growth: a. Although vascular cambium is not initially a solid cylinder of cells, it becomes continuous when secondary tissue production begins 1) Certain parenchyma cells in each pith ray retain division capability and form a complete ring of vascular cambium b. As vascular cambium cells divide radially (inward or outward), one of the resulting cells remains meristematic as part of the vascular cambium, while the other cells continue dividing several times until they stop, mature, and form secondary tissue. c. Vascular cambium produces tissue in two directions: 1) Inside the ring forms secondary xylem, or wood 2) Outside the ring forms secondary phloem, or inner bark d. As the stem increases in circumference, vascular cambium cells also produce daughter cells that remain meristematic: maintains the continuous ring of vascular cambium e. Vascular cambium produces new wood on the inside of the ring, allowing the plant to grow in diameter 1) Vascular cambium continues to divide and add more cells to the ring, which continues to surround the most recently formed ring of secondary xylem completely as successive seasons pass. f. Cells produced outside the vascular cambium becomes secondary phloem, which carries on food transport in place of the primary phloem which may be destroyed (stretched and crushed by expansion of the inner part of the plant). g. As more secondary xylem forms, the existing secondary phloem is destroyed in its turn, and more secondary phloem is added to the outside of the vascular cambium; functional secondary phloem remains about the same thickness. h. The phloem that dies is incorporated into the bark, so that the bulk of the stem comes to be made of xylem. i. As the secondary xylem and phloem are added by vascular cambium, increasing the stem's diameter, the cortex and epidermis originally lying outside the vascular tissues are also stretched and destroyed; fortunately, a new outer protective layer of secondary tissue will form (periderm). bio1120_stems&plant_transport.doc 4/2/09