Chapter 6 Stems - PDF

Because learning changes everything. ® Chapter 6 Stems FIFTEENTH EDITION James E. Bidlack, Shelly H. Jansky © 2021 McGraw Hill. All rights reserved. Authorized only for instructor use in...

Because learning changes everything. ® Chapter 6 Stems FIFTEENTH EDITION James E. Bidlack, Shelly H. Jansky © 2021 McGraw Hill. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written consent of McGraw Hill.  Outline Introduction to Stems External Form of a Woody Twig Origin and Development of Stems Tissue Patterns in Stems Steles Herbaceous Dicotyledonous Stems Woody Dicotyledonous Stems Specialized Stems Rhizomes Runners and Stolons Tubers Bulbs Corms Cladophylls Other Specialized Stems Wood and Its Uses © McGraw Hill © Doug Sherman/Geofile 2  Introduction to Stems Most wood in household items comes from stems. Grafting has been used by humans for centuries. This involves artificially uniting stems or parts of stems of different but related varieties of plants. Some plants have indeterminate growth, meaning they can grow indefinitely. © McGraw Hill 3  External Form of A Woody Twig Cells produced by stem meristem become shoot system with branches and leaves. Woody twig consists of an axis with attached leaves. Node - Area of stem where leaves are attached Alternate or spiral Opposite - Attached in pairs Whorled - In groups of three or more Internode - Stem region between nodes Leaf has a flattened blade and is usually attached to the twig by petiole. © McGraw Hill 4  Anatomy 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 © McGraw Hill 5  Deciduous Tree and Shrubs Deciduous trees and shrubs (lose all leaves annually) - After leaves fall, have dormant axillary buds with leaf scars below Bundle scars mark food and water conducting tissue within leaf scars. © McGraw Hill 6  Origin and Development of Stems Apical meristem at stem tip Contributes to increase in stem length Dormant before growing season begins Protected by bud scales and by leaf primordia Leaf primordia - Tiny embryonic leaves that develop into mature leaves Longitudinal section through stem tip © McGraw Hill © G. S. Ellmore 7  Primary Meristems Apical meristem cells form 3 primary meristems. Protoderm - Gives rise to epidermis Procambium - Produces primary xylem and phloem Ground Meristem - Produces pith and cortex, both composed of parenchyma cells Longitudinal section through stem tip © McGraw Hill © G. S. Ellmore 8  Leaf Primordia and Bud Primordia Leaf primordia and bud primordia develop into mature leaves and buds. Traces branch off from cylinder of xylem and phloem, and enter leaf or bud. Trace = strand of xylem and phloem © McGraw Hill 9  Vascular Cambium 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. © McGraw Hill 10  Cork Cambium In many plants cork cambium (phellogen) produces cork cells with suberin and phelloderm cells. Function to reduce water loss and to protect stem against injury Lenticels - Parenchyma cells in cork for exchange of gases © McGraw Hill 11  Tissue Patterns in Stems Steles Stele - Central cylinder of primary xylem, primary phloem, and pith (if present) Protostele - Solid core, phloem surrounds xylem In primitive seed plants, whisk ferns, club mosses and ferns Siphonosteles - Tubular with pith in center Common in ferns Eusteles - Discrete vascular bundles In flowering plants and conifers © McGraw Hill 12  Seed Leaves Cotyledons - Seed leaves attached to embryonic stems Store food needed by young seedling Dicotyledons (Dicots) - Flowering plants that develop from seeds having two cotyledons Monocotyledons (Monocots) - Flowering plants that develop from seeds with a single cotyledon © McGraw Hill 13  Herbaceous Dicotyledonous Stems Annuals - Plants that die after going from seed to maturity within one growing season Usually green, herbaceous plants Most monocots are annuals, but many dicots are also annuals. Tissues largely primary © McGraw Hill 14  Tissue Patterns in Herbaceous Dicotyledonous Stems Herbaceous dicots have discrete vascular bundles arranged in a cylinder. Vascular cambium arises between primary xylem and primary phloem. Adds secondary xylem and secondary phloem © McGraw Hill (a-b) © James E. Bidlack 15  Woody Dicotyledonous Stems Wood = Secondary xylem Differences in wood: Vascular cambium and cork cambium active all year: Ungrained, uniform wood produced Some tropical trees © McGraw Hill © Kingsley Stern 16  Seasonal Production of Wood In spring: Relatively large vessel elements of secondary xylem produced - Spring wood. After spring wood: Fewer, smaller vessel elements in proportion to tracheids and fibers - Summer wood. Spring and summer wood alternate as light and dark rings. Environment impacts the size of the rings Xylem growth in one year = annual ring © McGraw Hill © Clouds Hill Imaging Ltd./Science Source 17  Conifers Vessels and fibers absent Tracheids in spring larger than later in season © McGraw Hill 18  Annual Rings and Vascular Rays One year’s growth of xylem = annual ring Vascular cambium produces more secondary xylem than phloem. Bulk of tree trunk consists of annual rings of wood. Indicates age of a tree Indicates climate during tree’s lifetime Vascular Rays consist of parenchyma cells that function in lateral conduction of nutrients and water. Xylem Ray - Part of ray within xylem Phloem Ray - Part of ray through phloem © McGraw Hill 19  Cross Section of a Young Stem Cross section of young stem with secondary growth © McGraw Hill © Kingsley Stern 20  A Block of a Woody Dicotyledonous Stem 3-D view of dicot wood © McGraw Hill 21  Heartwood and Sapwood Resins, gums, and tannins accumulate, and darken wood, forming heartwood. Heartwood - Older, darker wood in center Sapwood - Lighter, still- functioning xylem closest to cambium © McGraw Hill © James E. Bidlack 22  Removal of Wood Heartwood supports the tree but cannot conduct materials. A tree may still function after heartwood is removed. It is possible to remove portions of sapwood without destroying the tree. Coastal redwoods of California © McGraw Hill © Kingsley Stern 23  Softwood and Hardwood Softwood - Wood of conifers No fibers or vessel elements Hardwood - Wood of dicot trees Resin canals - Tubelike canals scattered throughout xylem and other tissues Resin canals in pine © McGraw Hill © James E. Bidlack 24  Bark and Laticifers Bark - Tissues outside the vascular cambium, including secondary phloem Mature bark may consist of alternating layers of crushed phloem and cork. Laticifers - Ducts found mostly in phloem that have latex-secreting cells Cross section of young stem with Used to make rubber, secondary growth chicle (chewing gum), morphine © McGraw Hill © Kingsley Stern 25  Monocotyledonous Stems Monocots stems 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 Cross section of monocot stem © McGraw Hill © James E. Bidlack 26  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 Monocot vascular bundle surrounded by sheath of sclerenchyma cells. © McGraw Hill © Kingsley Stern 27  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 © McGraw Hill 28  Tubers and Bulbs 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 © McGraw Hill 29  Corms and Cladophylls Corms - Resemble bulbs, but composed almost entirely of stem tissue, with papery leaves Store food Crocus and gladiolus Cladophylls - Flattened, leaf-life stems Greenbriars, some orchids, prickly pear cactus © McGraw Hill (Bottom Right):© Kingsley Stern 30  Wood and Its Uses In a living tree, 50% of the wood weight comes from water content. Dry part of wood composed of about 60-75% cellulose and about 15-25% lignin. Density - Weight per unit volume Durability - Ability to withstand decay Tannins and oils repel decay organisms. © McGraw Hill 31  Types of Sawing Radially cut (quartersawed) boards show annual rings in side view. Tangentially cut (plain- sawed or slab cut) boards are cut perpendicular to rays. Show annual rings as irregular bands of light and dark streaks © McGraw Hill 32  Knots Knots - Bases of lost branches covered by new annual rings produced by the cambium In greater concentrations in older parts of a log Lower most branches often die from insufficient light © McGraw Hill 33  Wood Products About half of U.S. and Canadian wood production is used as lumber, primarily for construction. Sawdust and waste used for particle board and pulp. Veneer - Thin sheet of desirable wood glued to cheaper lumber Pulp is second most widespread use of wood. Paper, synthetic fibers, plastics, linoleum In developing countries, about half of cut timber is used for fuel. Less than 10% in US and Canada. © McGraw Hill 34  End of Main Content Because learning changes everything. ® www.mheducation.com © 2021 McGraw Hill. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written consent of McGraw Hill.