Dynamic Plant Focus on Stems Carnegiea gigantea - Saguaro cactus External Form of A Woody Twig Axil - Angle between petiole and stem - Axillary Bud located in axil. Will become branches or flowers in flowering plants Bud scales protect buds. Terminal Bud at twig tip - Growth makes twig longer. - Number of groups of bud scale scars tells age of twig. Stipules - Paired, often leaflike appendages at base of a leaf External Form of A Woody Twig Deciduous trees and shrubs (lose all leaves annually) - After leaves fall, have dormant axillary buds with leaf scars below Growth Apical meristem increases length Ground meristem makes cortex & pith Procambium produces primary Xylem & Phloem Vascular cambium produces secondary Xylem & Phloem Cork cambium/phellogen produces bark to reduce water loss & protects stem (in woody plants only). Bundle scars mark food and water conducting tissue within leaf scars. 1
Origin and Development of Stems Narrow band of cells between the primary xylem and primary phloem may become vascular cambium. Cells produced by the vascular cambium become components of secondary xylem toward center and secondary phloem toward surface. Internal Vascular System Similar to interconnected pipes in a house. Tissue Patterns in Stems Cotyledons - Seed leaves attached to embryonic stems Function: Store food needed by young seedling Tissue Patterns in Stems Herbaceous Dicot Stems Have discrete vascular bundles arranged in a cylinder. Vascular cambium arises between primary xylem and primary phloem. - Adds secondary xylem and secondary phloem Dicotyledons (Dicots) - Flowering plants that develop from seeds having two cotyledons Monocotyledons (Monocots) - Flowering plants that develop from seeds with a single cotyledon Dicot stem 2
Tissue Patterns in Stems Wood Black locust tree (longitudinal section) Tissue Patterns in Stems - Monocots Have neither a vascular cambium nor a cork cambium. Produce no secondary vascular tissues or cork Primary xylem and phloem in discrete vascular bundles scattered throughout the stem Vascular bundles oriented with xylem closer to center of stem and phloem closer to surface. Parenchyma (ground tissue) surrounds vascular bundles. Cross section of monoco stem Tissue Patterns in Stems - Monocots In a typical monocot vascular bundle: Two large vessels with several small vessels First formed xylem cells stretch and collapse. Leave irregularly shaped air space Phloem consists of sieve tubes and companion cells. Vascular bundle surrounded by sheath of sclerenchyma cells. Monocot vascular bundle Transpiration and cohesion tension theory Method by which water moves from the roots to the shoot system through the xylem. Loss of water out of stomata by evaporation. Hydrogen bonds link water molecules together. Water moves up and the xylem in a long chain. Water molecules pull each other up one molecule at a time from previous location below. Column of water is under tension. - Cohesion 3
How Materials move in the Xylem Water Stress Occurs when a break in the water chain contained in the xylem vessel elements breaks. May occur: When transpiration rates increase During very hot, dry weather Note- Extreme wilting can kill the plant. Transport of Sugars Sugars flow from: Source (leaf) Sink Sink = any structure that uses up sugars or stores them e.g. fruits, roots, stems. Pressure-flow theory relies on differential hydrostatic pressure to move fluid through the phloem cells. Specialized Stems Rhizomes - Horizontal stems that grow below-ground and have long to short internodes Irises, some grasses, ferns Runners - Horizontal stems that grow above ground and have long internodes. Strawberry Stolons - Produced beneath the surface of the ground and tend to grow in different directions. Potato 4
Specialized Stems Tubers - Swollen, fleshy, underground stem Store food Potatoes - Eyes of potato are nodes Bulbs - Large buds surrounded by numerous fleshy leaves, with a small stem at lower end Store food Onions, lilies, hyacinths, tulips Specialized Stems Corms - Resemble bulbs, but composed almost entirely of stem tissue, with papery leaves Store food Crocus and gladiolus Cladophylls - Flattened, leaf-life stems Prickly pear cactus Formation of Vascular Tissues in Woody Plants Secondary meristem Increase Girth 1) Vascular cambium: generates vascular tissues. 2) Cork cambium: generates protective outer covering, including bark. Elements: Fusiform initials > Generates secondary Ph & Xy Ray initials > Vascular rays (for lateral transport) 5
Dendrochronology Putting trees to work to see into the past. Tree time keeping. > Early Wood (less dense rainy season) > Late Wood (more dense arid season) 6