Stems

Chapter 6: Stems

Introduction to Stems

  • Most wood in household items comes from stems.

  • Grafting: A horticultural technique involving the artificial uniting of stems or parts of stems of different but related varieties of plants. This method has been used by humans for centuries.

  • Indeterminate Growth: Some plants exhibit a growth pattern where they can grow indefinitely, continuing to produce new tissues and structures throughout their lives.

External Form of A Woody Twig

  • Composition of a Woody Twig:

    • Cells produced by the stem meristem develop into the shoot system which includes branches and leaves.

    • A woody twig consists of a central axis with attached leaves.

  • Key Terms:

    • Node: The specific area of the stem where leaves are attached.

    • Leaf Arrangements:

    • Alternate or Spiral: Leaves attached one at a time at each node.

    • Opposite: Leaves attached in pairs at each node.

    • Whorled: Leaves attached in groups of three or more at a node.

    • Internode: The segment of the stem between two nodes.

    • Leaf Structure: Typically has a flattened blade attached to the twig via a structure called a petiole.

Anatomy of A Woody Twig

  • Axil: The angle formed between the petiole and the stem.

  • Axillary Bud: Found in the axil, capable of developing into branches or flowers in flowering plants.

  • Bud Scales: Protective structures that cover the buds.

  • Terminal Bud: Located at the tip of the twig, responsible for growing the twig longer.

  • Bud Scale Scars: The number of scars on the twig indicates its age.

  • Stipules: Paired structures that can be found at the base of petioles.

Deciduous Trees and Shrubs

  • Deciduous Trees and Shrubs: These plants lose all their leaves annually. After leaf fall, they have dormant axillary buds protected by leaf scars situated below.

  • Bundle Scars: Marks within leaf scars that indicate the presence of food and water conducting tissues.

Origin and Development of Stems

  • Apical Meristem: The growth region located at the tip of the stem, contributing to an increase in stem length. It remains dormant before the growing season starts, protected by bud scales and leaf primordia.

  • Leaf Primordia: Tiny, embryonic leaves developing into mature leaves.

Primary Meristems

  • Apical meristem cells give rise to three primary meristems:

    • Protoderm: The layer that leads to the formation of the epidermis.

    • Procambium: Develops into primary xylem and phloem.

    • Ground Meristem: Produces the pith and cortex, both primarily composed of parenchyma cells.

Leaf Primordia and Bud Primordia

  • Leaf and bud primordia mature into leaves and buds, respectively.

  • Trace: A strand composed of xylem and phloem that branches off from the vascular cylinder into the leaf or bud.

Vascular Cambium

  • A narrow band of cells located between the primary xylem and primary phloem that can become a vascular cambium, responsible for producing secondary xylem toward the center and secondary phloem toward the surface.

Cork Cambium

  • Found in many plants, the cork cambium (also known as phellogen) is responsible for producing cork cells, which contain suberin, and phelloderm cells. This structure functions to reduce water loss and protect the stem from injury.

  • Lenticels: Specialized structures composed of parenchyma cells in cork, providing gas exchange.

Tissue Patterns in Stems Steles

  • Stele: The central cylinder that comprises primary xylem, primary phloem, and pith (if present).

    • Types of Steles:

    • Protostele: A solid core with phloem surrounding the xylem, typically seen in primitive seed plants, whisk ferns, club mosses, and ferns.

    • Siphonosteles: Tubular structure with pith in the center, common in ferns.

    • Eusteles: Discrete vascular bundles found in flowering plants and conifers.

Seed Leaves

  • Cotyledons: Seed leaves attached to embryonic stems that store food necessary for the young seedling.

    • Dicotyledons (Dicots): Flowering plants developing from seeds containing two cotyledons.

    • Monocotyledons (Monocots): Flowering plants developing from seeds with a single cotyledon.

Herbaceous Dicotyledonous Stems

  • Annuals: Plants that complete their life cycle from seed to maturity within one growing season, typically characterized by green, herbaceous structure.

    • Most monocots are annuals, with many dicots also exhibiting this growth pattern.

    • The tissues of annuals are largely primary.

Tissue Patterns in Herbaceous Dicotyledonous Stems

  • Herbaceous dicots contain discrete vascular bundles arranged in a cylindrical formation.

  • Vascular Cambium: Arises between primary xylem and primary phloem, facilitating the addition of secondary xylem and secondary phloem.

Woody Dicotyledonous Stems

  • Wood: Composed primarily of secondary xylem.

    • The differences in wood include the activity levels of the vascular cambium and cork cambium:

    • Continuous activity throughout the year produces ungrained, uniform wood found in some tropical trees.

Seasonal Production of Wood

  • Seasonal variations produce distinct wood types:

    • Spring Wood: Characterized by relatively large vessel elements of secondary xylem.

    • Summer Wood: Consists of fewer, smaller vessel elements in proportion to tracheids and fibers.

  • Spring and summer wood alternate, resulting in light and dark annual rings, the size of which can be influenced by environmental conditions.

  • Annual Ring: The total growth of xylem in one year forms the annual ring,

Conifers

  • Conifers: Lack vessels and fibers in their wood structure. Tracheids tend to be larger in spring than later in the growing season.

Annual Rings and Vascular Rays

  • One year’s growth of xylem results in an annual ring, with the vascular cambium generating more secondary xylem than secondary phloem.

  • The majority of the tree trunk consists of these annual rings which indicate:

    • The age of the tree.

    • Climatic conditions experienced during the tree's life.

  • Vascular Rays: Composed of parenchyma cells functioning in the lateral conduction of nutrients and water:

    • Xylem Ray: A part of the ray located within the xylem.

    • Phloem Ray: A part of the ray found within the phloem.

Cross Section of a Young Stem

  • Illustrates the structure of a young stem exhibiting secondary growth.

A Block of a Woody Dicotyledonous Stem

  • Represents a 3-D view of dicot wood, revealing internal structures and characteristics.

Heartwood and Sapwood

  • Heartwood: Older, darker wood found at the center of a tree, containing accumulated resins, gums, and tannins that darken the wood.

  • Sapwood: The lighter, still-functioning xylem surrounding the cambium.

Removal of Wood

  • Heartwood serves as structural support but cannot conduct materials essential for tree survival.

  • A tree can potentially continue functioning even after its heartwood has been removed.

  • Portions of sapwood can also be removed without compromising the entire tree structure, as seen in some coastal redwoods of California.

Softwood and Hardwood

  • Softwood: Refers to wood derived from conifers, lacking fibers and vessel elements.

  • Hardwood: Wood sourced from dicot trees, comprising resin canals that are tubelike canals scattered within xylem and other tissues, such as those found in pine trees.

Bark and Laticifers

  • Bark: All tissues external to the vascular cambium, including secondary phloem, potentially consisting of alternate layers of crushed phloem and cork.

  • Laticifers: Specialized ducts primarily residing in phloem containing latex-secreting cells, which are utilized for various products, including rubber, chicle (used for chewing gum), and morphine.

Monocotyledonous Stems

  • Monocot stems lack both vascular cambium and cork cambium, resulting in the absence of secondary vascular tissues or cork.

  • The primary xylem and phloem develop in scattered, discrete vascular bundles throughout the stem.

Typical Monocot Vascular Bundle

  • Consists of two large vessels complemented by several smaller vessels.

  • Early formed xylem cells exhibit stretching and collapsing, leading to irregular airspace shapes in the tissue.

  • Phloem includes sieve tubes and companion cells, all encased by a sclerenchyma cell sheath.

Specialized Stems

  • Rhizomes: Horizontal stems that grow underground with long to short internodes, seen in plants like irises, some grasses, ferns.

  • Runners: Horizontal stems growing above ground with longer internodes, exemplified by strawberries.

  • Stolons: Horizontal stems produced beneath the ground that tend to grow in various directions, as seen in potatoes.

Tubers and Bulbs

  • Tubers: Swollen, fleshy underground stems that serve as food storage, such as potatoes (with 'eyes' acting as nodes).

  • Bulbs: Large, fleshy buds surrounded by numerous fleshy leaves, containing a small stem at the base crucial for food storage, examples include onions, lilies, hyacinths, and tulips.

Corms and Cladophylls

  • Corms: Similar to bulbs but primarily consist of stem tissue with papery leaves, found in plants like crocus and gladiolus.

  • Cladophylls: Leaf-like stems that are flattened, exemplified by greenbriars, some orchids, and prickly pear cactus.

Wood and Its Uses

  • In living trees, approximately 50% of the wood's weight is from water content.

  • The dry part of the wood typically comprises about 60-75% cellulose and about 15-25% lignin.

  • Density: Measured as weight per unit volume.

  • Durability: Wood's ability to withstand decay, influenced by the presence of tannins and oils which repel decay organisms.

Types of Sawing

  • Radially Cut (Quarter-Sawed): Boards that exhibit annual rings in profile view.

  • Tangentially Cut (Plain-Sawed or Slab Cut): Boards are cut perpendicular to rays, displaying annual rings as irregular bands of light and dark streaks.

Knots

  • Knots: Formed from the bases of lost branches, encapsulated by new annual rings generated by the cambium.

  • Found in greater concentrations in the older parts of a log, while the lower branches may die off due to inadequate light exposure.

Wood Products

  • Approximately half of the wood production in the U.S. and Canada is utilized for lumber primarily in construction industries.

  • Sawdust and leftover materials are repurposed for particle board and pulp products.

  • Veneer: A thin layer of desirable wood adhered to less expensive lumber.

  • The production of pulp is the second most common use of wood, contributing to the creation of paper products, synthetic fibers, plastics, and linoleum.

  • In developing countries, nearly half of harvested timber is used for fuel, whereas this percentage is less than 10% in the U.S. and Canada.