Plant Stems: Anatomy and Tissue Types

• The text is a lab manual supplement about plant stems.
• Details the anatomy of vegetative parts, specifically stems, as well as primary and secondary tissues.

Objectives

• Identify stem structures.
• Distinguish between primary and secondary tissues.
• Identify vascular cambium and cork cambium structures and functions.
• Identify woody and herbaceous dicot stem cross-sections.
• Identify monocot stem cross-sections.
• Know the composition of wood and its annual rings.

Lab Focus

• Studies are done on the theoretical part of stems.
• Includes learning diagnostic features of monocot and dicot stems.

Woody and Leafy Stem Structures

• Woody twigs consist of an axis with attached leaves.
• Leaves can be alternately or oppositely arranged.
• Leaves are attached at a node.
• The internode is the stem region between nodes.
• Leaves have a flattened blade attached to the twig by a petiole.
• An axillary bud is located in the axil (between petiole and stem).
• A terminal bud is often found at the twig tip.
• Stipules are paired appendages at the leaf base, often present during the leaf's life.
• Deciduous trees and shrubs have dormant axillary buds, and leaf scars remain after leaves fall.
• Lenticels are airy cell aggregations on stems, roots, and other vascular plant parts.
• Lenticels function as pores, allowing direct gas exchange between internal tissues and the atmosphere.
• Lenticel shape is used for tree ID.

Primary vs. Secondary Tissue

• Apical meristem is dormant before the growing season and is protected by bud scales and primordia.
• When a bud begins to expand, apical meristem cells undergo mitosis, and three primary meristems develop.
• Protoderm gives rise to the epidermis.
• Procambium produces primary xylem and primary phloem cells.
• Ground meristem produces tissues of parenchyma cells, forming pith and cortex.

Functions and Structures

• A narrow band of cells between primary xylem and phloem may become vascular cambium.
• Vascular cambium cells become secondary xylem and phloem components.
• Cork cambium, a second cambium found in many plants produces cork cells and phelloderm cells.

Dicot Stem and Cross Section ID

• Herbaceous dicotyledonous stems: typically green, herbaceous annuals, but some dicots are perennials.
• Discrete vascular bundles of xylem and phloem are present.
• Procambium only produces primary xylem and phloem initially.
• Vascular cambium arises later, adding secondary phloem and xylem. - The annual growth of xylem is called an annual ring.

Woody Dicotyledonous Stems

• Vascular cambium produces large vessel elements of secondary xylem (spring wood).
• Xylem produced later has smaller or fewer elements (summer wood).
• Vascular cambium produces more secondary xylem than phloem, forming the bulk of a tree trunk with annual wood rings.
• Ring examination determines tree age and indicates climatic conditions.
• Vascular rays consist of parenchyma cells that function in lateral conduction of nutrients and water; occurs though xylem and phloem rays.

Monocot Stem and Cross Section ID

• Most monocot stems lack vascular cambium and cork cambium, so they don't produce secondary vascular tissues or cork.
• Xylem and phloem exist in discrete vascular bundles.
• Secondary meristem produces parenchyma cells to the outside and secondary vascular bundles to the inside.
• Monocot stems have many vascular bundles in a complex ground tissue pattern.
• Dicot stems have one ring of vascular bundles located in the periphery of ground tissues.

Wood Composition and Annual Rings

• Heartwood is the older, darker wood at the center.
• Sapwood is lighter, still-functioning xylem closest to the cambium and it forms at the same rate as heartwood.
• Softwood xylem consists primarily of tracheids without fibers or vessel elements; found in cone-bearing trees.
• Bark refers to all tissues outside the cambium, including the phloem.
• Mature bark may consist of alternating layers of crushed phloem and cork.

Secondary Stem Growth

• Secondary growth begins after the herbaceous dicot stem stage: Epidermis is replaced by cork, and vascular cambium produces secondary xylem and phloem.
• In a three-year-old stem where cork cambium produces new cork, primary phloem and the cortex eventually disappear.
• Secondary phloem produces the vascular cambium and remains active that year.
• Secondary xylem also produced by vascular cambium, builds up to become the annual growth rings.

Explore plant stem anatomy, focusing on primary and secondary tissues. Learn to identify structures such as vascular cambium and cork cambium, including their function. Distinguish between woody and herbaceous dicot stems and monocot stem cross-sections.