Today: Introduction to Plant Anatomy

Plant Morphology Reflects the demands of two very different environments:

Soil and AirIntro to Plant Anatomy

The Root SystemFunctions:???

Root SystemsMonocots: typically have fibrous root systems (mats of thin roots below the soil surface) Dicots: typically have a taproot system (one large vertical root with smaller lateral roots)

Root ModificationsBoth Monocots and Dicots use root hairs at the root tips.

Why??

Both may have adventitious roots arising from stems or leavesRoot Modifications

Many Modified Shoots Look Like Roots!RootsStemLeavesAxillary budsSwollen Ends of Rhizomes!

Plant Organs are Composed of 3 Tissues:1. Dermal Tissue 2. Vascular Tissue3. Ground Tissue

1. Dermal Tissue (Epidermis)Single layer of tightly-packed cellsCovers and protects young plant partsMay have other specialized functions:Example: Root hairs or the Cuticle secreted by leaves and stems

2. Vascular TissueTransports materials between roots and shootsComposed of xylem and phloem

2. Vascular TissueWood is composed primarily of vessels and tracheids

Plant Cell Walls: A Quick ReviewNote the thick, secondary wall!

2. Vascular Tissue: XylemTracheids have secondary walls hardened with lignin, allowing them to function in supportWater is transported through pits in the secondary cell wall.

Vessel elements are wider, shorter, and have thinner walls. They align end to end to form pipes or xylem vessels.

2. Vascular Tissue: PhloemSugars and other organic molecules and ions are transported through chains of specialized cells- the sieve tube members.

Sieve-tube members are alive, but have no nucleus or ribosomes!Each sieve-tube has a nonconducting companion cell connected by plasmodesmata2. Vascular Tissue: Phloem

3. Ground TissueGround tissue is tissue that is neither dermal or vascular!Functions include photosynthesis, storage and support.In dicot stems (above), ground tissue is divided into pith (C) inside the vascular tissue and cortex (D), outside the vascular tissue

Cell TypesAll plant tissues are composed of three basic cell types:

1. Parenchyma Cells2. Collenchyma Cells3. Sclerenchyma Cells

Cell Types: Parenchyma CellsThin, flexible primary wallsTypically no secondary wallLarge central vacuole

Typically generalists! But sieve-tube members (phloem) are also parenchyma cells

Developmentally important, and used in repair and replacement! Can generate an entire plant from a parenchyma cell!

Cell Types: Parenchyma Cells

Collenchyma CellsThicker (but uneven) primary wallsGrouped in strands or cylinders to support young plant shootsProvide support without restraining growth (no lignin!)

Sclerenchyma CellsThick secondary walls (with lignin) Many are dead at functional maturity Example: vessel elements and tracheidsFibers and sclereids are sclerenchyma cells specialized entirely for support.

Shoot AnatomyShoots are composed of stems and leaves, and may be vegetative or reproductive

Stems are alternating nodes (where leaves are attached) and internodes (stem segments between nodes)

At the intersection of each leaf and the stem is an axillary bud with the potential to form a vegetative branch.

Most axillary buds of a young shoot are dormant.Shoot Anatomy

Growth of a young shoot is typically concentrated at the terminal bud (apex)Presence of the terminal bud helps inhibit growth of axillary buds (apical dominance)Shoot Anatomy

LeavesMost leaves are composed of a flattened blade and a stalk, the petiole

So whats this? One petiole. No axillary buds

Many leaves are specialized for functions besides photosynthesis!Leaves

Plant Anatomy: A Quick Review

Tissue Systems in a Leaf

In dicots, two distinct regions of mesophyll:

1. Palisade Parenchyma- columnar cells

2. Spongy Parenchyma- multiple air spaces (esp. near stomata)

The Leaf Ground TissueLots of surface area! Hmm.

Plant GrowthMost plants have indeterminate growthMay be annual, biennial, or perennial

CREDIT: "Falling an old-growth redwood, 1985." Photo by Karen Tillson for "Timber and Forests: Post war to Present," a California Local Legacies project.

Indeterminate growth is possible because of meristems, regions of perpetually embryonic tissuesPlant GrowthGrowth in RootsGrowth in Shoots

Elongation of roots and shoots from the apical meristems is called primary growthProgressive thickening of roots and shoots as a product of lateral meristems is called secondary growthBoth occur simultaneously in woody plants!Plant Growth

Revisiting RootsWhat did you notice when you drew the root tip??

Primary Growth in Roots

A Typical Dicot Root

A Typical Monocot Root

Lateral roots form from the outermost layer of the stele, the pericyclePrimary Growth in Roots

Secondary Growth of RootsTwo lateral meristems create secondary growth:

1. The Vascular Cambium- produces secondary xlyem (wood) and phloem2. The Cork Cambium- produces a tough thick covering, the periderm, that replaces the epidermis (and is impermeable to water!!)

Primary Growth in ShootsApical MeristemLeaf PrimordiaAxillary bud meristems

Primary Growth in Shoots Apical meristem gives rise to protoderm, procambium and ground meristem

Primary Tissues In StemsVascular tissue is packed in vascular bundles

Primary Tissues In StemsVascular tissue is packed in vascular bundles

Secondary Growth in ShootsVascular cambium= cylinder of meristematic cells

Originates from parenchyma cells that regain the capacity to divide

Secondary Growth in Shoots

Secondary Growth in ShootsEach time a cambium cell divides (C), one daughter cells continues as an initial, the other becomes the derivative (D)

Secondary Growth in ShootsAs the diameter increases, a second lateral meristem, the cork cambium, develops from parenchyma cells in the cortex

Dead cork cells (from the cork cambium) provide a barrier to water loss, physical damage, and pathogensSecondary Growth in Shoots

Secondary Growth in ShootsOriginal cork cambium is a fixed size!New cork cambium must continually form deeper in the cortex, and eventually from the secondary phloem.(So only the youngest secondary phloem functions in sugar transport!)

BarkBark = all tissues external to the vascular cambium (secondary phloem, cork cambium, and cork)

BarkThe periderm (cork + cork cambium) may split open.The openings (lenticels) allow for gas exchange for cellular respirationThis species of Jatropha (Family Euphorbiaceae) of western Mexico has conspicuous horizontal lenticels on its smooth, peelable red-brown bark. Photo: Ontario Ministry of Agriculture and Food

Growth RingsIn temperate regions, secondary growth is interrupted each winter (vascular cambium becomes dormant)Early wood (spring) has larger tracheids and vessels with thinner wallsLate wood (summer) has thicker cell walls

Heartwood- older, no longer functions in water transport (support), cell cavities typically clogged with resinsSapwood- secondary xylem still functional

Generating a Stem Diagram:Primary xylem, secondary xylem, primary phloem, secondary phloem, pith, periderm, cortex, vascular cambium, cork cambium, cork, heartwood, sapwood

*Root hair are extensions of individual epidermal cells!*