Botany: Origin and Development of Stems

Origin and Development of Stems

• A narrow band of cells between the primary xylem and phloem may become vascular cambium
• Cells produced by the vascular cambium become components of secondary xylem towards the center and secondary phloem towards the surface.
• In many plants, cork cambium produces cork cells with suberin and phelloderm cells.
• Parenchyma cells in cork facilitate gas exchange.

Tissue Patterns in Stems

• Stele is the central cylinder of primary xylem, primary phloem, and pith (if present).
• Protostele has a solid core with phloem surrounding the xylem.
• Siphonosteles are tubular with pith in the center.
• Eusteles have discrete vascular bundles.
• Cotyledons, seed leaves attached to embryonic stems, store food for the young seedling.
• Dicotyledons have two cotyledons, while monocotyledons have a single cotyledon.
• Most monocots are annuals, but many dicots are also annuals.

Herbaceous Dicotyledonous Stems

• Herbaceous dicots have discrete vascular bundles arranged in a cylinder.
• Vascular cambium arises between primary xylem and phloem.

Woody Dicotyledonous Stems

• Wood is secondary xylem.
• Vascular cambium and cork cambium are active year-round, producing ungrained, uniform wood.
• Differences in wood production occur seasonally:
  • Spring wood has relatively large vessel elements.
  • Summer wood has fewer, smaller vessel elements.
• Conifers have larger tracheids in the spring.
• One year's growth of xylem forms an annual ring.
• Indicates age of a tree and climate during its lifetime.
• Vascular rays consist of parenchyma cells for lateral conduction of nutrients and water.
• Tyloses are protrusions of adjacent parenchyma cells into conducting cells of xylem, preventing water conduction and darkening wood.
• Heartwood is older, darker wood in the center.
• Sapwood is lighter and still-functioning xylem closest to the cambium.
• Softwood is wood of conifers, lacking fibers and vessel elements..
• Hardwood is wood of dicot trees.
• Resin canals are tubelike canals containing resin secreted by specialized cells, found in conifers and some tropical flowering plants.
• Bark is tissues outside the vascular cambium, including secondary phloem.
• Laticifers are ducts found mostly in phloem with latex-secreting cells.

Monocotyledonous Stems

• Monocot stems lack a vascular cambium and a cork cambium.
• Primary xylem and phloem in discrete vascular bundles are scattered throughout the stem.
• Vascular bundles are oriented with xylem closer to the center and phloem closer to the surface.
• Parenchyma tissue surrounds the vascular bundles.
• Vascular bundles in monocots have:
  • Two large vessels with several small vessels.
  • First formed xylem cells stretch and collapse, leaving irregularly shaped air spaces.
  • Phloem consists of sieve tubes and companion cells.
  • A sheath of sclerenchyma cells surrounds the vascular bundle.

Specialized Stems

• Rhizomes are horizontal stems that grow belowground with long to short internodes.
• Runners are horizontal stems that grow aboveground with long internodes.
• Stolons are produced beneath the surface of the ground and grow differently.
• Tubers are swollen, fleshy, underground stems that store food.
• Bulbs are large buds surrounded by fleshy leaves with a small stem at the base, storing food.
• Corms resemble bulbs, but are mostly stem tissue with papery leaves that store food.
• Cladophylls are flattened, leaf-like stems.

Wood and Its Uses

• In a living tree, 50% of the wood weight comes from water content.
• Dry wood is composed of 60-75% cellulose and 15-25% lignin.
• Wood products include lumber, particle board, pulp, veneer, and sawdust.
• Knots are bases of lost branches covered by new annual rings.
• Radially cut boards show annual rings in side view.
• Tangentially cut boards are cut perpendicular to rays.