Plant Stems - Anatomy and Growth

Introduction to Stems

• Most wood in household items originates from stems
• Humans have used grafting for centuries, which involves uniting stems or parts of stems of different but related plant varieties artificially
• Some plants exhibit indeterminate growth, capable of growing indefinitely

External Form of a Woody Twig

• Stem meristem cells produce the shoot system, including branches and leaves
• A woody twig comprises an axis with leaves attached
• Nodes are stem areas where leaves attach, arranged alternately, spirally, oppositely in pairs, or whorled in groups of three or more
• Internodes are stem regions located between nodes
• Leaves feature a flattened blade typically attached to the twig through a petiole

Anatomy of A Woody Twig

• Axils are located as the angle between petiole and stem
• Axillary buds are found in axils and develop into branches or flowers, protected by bud scales
• Terminal buds, located at the twig tip, facilitate twig elongation
• The quantity of bud scale scar groups indicates a twig's age
• Stipules are paired, often leaflike appendages at a leaf's base

Deciduous Trees and Shrubs

• Deciduous trees and shrubs shed leaves annually
• Dormant axillary buds with leaf scars develop following leaf fall
• Bundle scars mark food and water-conducting tissue within leaf scars

Origin and Development of Stems

• Apical meristem is located at the stem tip; it contributes to stem length increase
• Apical meristem remains dormant before each growing season
• Bud scales, along with leaf primordia, protect apical meristem
• Leaf primordia are tiny embryonic leaves which develop into mature leaves

Primary Meristems

• Apical meristem cells give rise to three primary meristems
• Protoderm gives rise to the plant epidermis
• Procambium produces primary xylem and phloem
• Ground meristem produces pith and cortex, both composed of parenchyma cells

Leaf Primordia and Bud Primordia

• Leaf and bud primordia develop into mature leaves and buds
• Traces, which are strands of xylem and phloem, branch off forming xylem/pholem cylinders and enter leaf or bud

Vascular Cambium

• Vascular cambium is a narrow band of cells between primary xylem and primary phloem
• Cells vascular cambium produces become secondary xylem components toward the center and produce secondary pholem toward the surface

Cork Cambium

• Cork cambium (phellogen) produces cork cells with suberin and phelloderm cells in many plants
• The function of cells is to reduce water loss and protect against injury
• Lenticels are parenchyma cells in cork to facilitate gas exchange

Tissue Patterns in Stems: Steles

• Stele refers to the central cylinder of primary xylem, primary phloem, and pith, if present
• Protostele is the solid core for xylem surrounded by phloem, found in primitive seed plants, whisk ferns, club mosses, and ferns
• Siphonosteles refers to a tubular structure with pith in the center, common in ferns.
• Eusteles are discrete vascular bundles found in flowering plants and conifers

Seed Leaves

• Cotyledons are seed leaves attached to embryonic stems that store food needed by young seedlings
• Flowering plants developing from seeds with two cotyledons are dicotyledons (dicots)
• Flowering plants developing from seeds with a single cotyledon are monocotyledons (monocots)

Herbaceous Dicotyledonous Stems

• Annuals are plants that die after progressing from seed to maturity within a single growing season
• Herbaceous dicotyledonous stems are usually green and herbaceous
• Mostly monocots are annuals, but many dicots are also annuals
• Tissues are largely primary

Tissue Patterns in Herbaceous Dicotyledonous Stems

• Herbaceous dicots have discrete vascular bundles arranged in a cylinder
• Arises between primary xylem and primary phloem is the vascular cambium
• Secondary xylem and secondary pholem are added

Woody Dicotyledonous Stems

• Wood is secondary xylem
• Vascular cambium and cork cambium remain active year-round resulting in ungrained, uniform wood in some tropical trees

Seasonal Production of Wood

• Spring wood comprises relatively large vessel elements of secondary xylem produced in spring
• Smaller vessel elements occur in proportion to tracheids and fibers post-spring wood which is summer wood
• These alternate as light and dark rings
• Environment affects the size of rings
• One year of xylem growth equates to one annual ring

Conifers

• Vessels and fibers are absent in conifers
• Tracheids are larger in spring than later in the season

Annual Rings and Vascular Rays

• One year's xylem growth equals one annual ring
• Vascular cambium leads to more secondary xylem production than pholem
• Majority of the tree trunk is annual rings of wood
• Annual rings indicate tree age and climate during lifetime
• Vascular rays consist of parenchyma cells which function in lateral water and nutrient conduction
• Xylem ray - Part of ray within xylem
• Phloem ray - Part of ray through the phloem

Heartwood and Sapwood

• Resins, gums, and tannins accumulate to darken wood and form heartwood
• Heartwood is in the center and darker than sapwood
• Sapwood is lighter, still-functioning xylem closest to cambium

Removal of Wood

• Heartwood supports the tree but cannot conduct materials
• A tree may still function even after heartwood removal
• Portions of sapwood can be removed without destroying the tree
• Coastal redwoods of California are a species that is resistant to losing portions of sapwood

Softwood and Hardwood

• Softwood refers to wood of conifers with no fibers or vessel elements
• Hardwood refers to wood of dicot trees
• Resin canals are tubelike canals scattered throughout xylem and other tissues

Bark and Laticifers

• Bark is tissues outside the vascular cambium, including secondary phloem
• Mature bark consists of alternating layers of crushed pholem/cork
• Laticifers are ducts mostly in pholem which contain latex-secreting cells
• Laticifers utilized for rubber, chewing gum, and morphine

Monocotyledonous Stems

• Monocots stems lack vascular or cork cambium
• Secondary vascular tissues or cork is absent
• Primary xylem and pholem in discrete bundles scatter throughout the stem

Typical Monocot Vascular Bundle

• There are two large vessels with several small vessels
• First xylem formed cells stretch then collapse which creates irregularly shapes air space
• Pholem consist of sieve tubes along with companion cells
• Bundle is surrounded by sheath of sclerenchyma cells

Specialized Stems

• Rhizomes are horizontal stems that grow below ground and have long/short internodes; Irises, grasses, and ferns
• Runners are horizontal stems growing above ground with long internodes; Strawberry
• Stolons are produced beneath the surface and tend to grow in different directions; Potato
• Tubers are swollen, fleshy underground stems used for food storage, such as potato eyes, which are nodes
• Bulbs are large buds surrounded by numerous fleshy leaves w/a small stem @ lower end; Store food; Onions, lilies, hyacinths, and tulips
• Corms resemble bulbs but are almost entirely stem tissue w/papery leaves; Food store; Crocus, gladiolus
• Cladophylls are flattened, leaf-like stems; Greenbriars, orchids, prickly pears

Wood and its Uses

• Living tree wood is 50% water content
• Dry wood: 60-75% cellulose, 15-25% lignin
• Density is the weight per unit volume
• Durability is ability to withstand decay
• Tannins and oils repel decay organisms

Types of Sawing

• Radially cut (quartersawed) boards present annual rings in side view
• Tangentially cut (plain-sawed or slab cut) boards are cut perpendicular to rays; Show annual rings as light/dark streaks

Knots

• Knots are bases of lost branches covered in new annual rings formed by cambium concentrated in older log parts
• Lower branches often die from lack of light

Wood Products

• Half of US/Canadian wood production for lumber, primarily for construction
• Waste sawdust used for particle board and pulp
• Veneer is a thin sheet of desirable wood glued to cheaper lumber
• Second-most widespread wood use is pulp made into paper, synthetic fibers, plastics, and linoleum
• In developing countries, half uses lumber for fuel compared to less than 10% of fuel production in US and Canada