Chapter 6

Lecture Outline

Stems

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Outline

External Form of a Woody Twig

Origin and Development of Stems

Tissue Patterns in Stems

• Herbaceous Dicotyledonous Stems

• Woody Dicotyledonous Stems

• Monocotyledonous Stems

Specialized Stems

Wood and Its Uses

External Form of A Woody Twig Cells produced by stem meristem become

shoot system with branches and leaves.

Woody twig consists of an axis with attached leaves.• Node - Area of stem where leaves are attached

– Alternate or spiral

– Opposite - Attached in pairs

– Whorled - In groups of three or more

• Internode - Stem region between nodes

• Leaf has a flattened blade and is usually attached to the twig by petiole.

External Form of A Woody Twig Axil - Angle between

petiole and stem• Axillary Bud located in axil.

– Will become branches or flowers in flowering plants

– Bud scales protect buds.

Terminal Bud at twig tip• Growth makes twig longer.• Number of groups of bud

scale scars tells age of twig. Stipules - Paired, often

leaflike appendages at base of a leaf

External Form of A Woody Twig Deciduous trees and

shrubs (lose all leaves annually) - After leaves fall, have dormant axillary buds with leaf scars below• Bundle scars mark food

and water conducting tissue within leaf scars.

Origin and Development of Stems Apical meristem at

stem tip

• Contributes to increase in stem length

• Dormant before growing season begins

• Protected by bud scales and by leaf primordia– Leaf primordia - Tiny

embryonic leaves that develop into mature leaves

Longitudinal section through stem tip

Origin and Development of Stems Apical meristem cells

form 3 primary meristems.

• Protoderm - Gives rise to epidermis

• Procambium - Produces primary xylem and phloem

• Ground Meristem - Produces pith and cortex, both composed of parenchyma cells

Longitudinal section through stem tip

Origin and Development of Stems Leaf primordia and bud

primordia develop into mature leaves and buds.

• Traces branch off from cylinder of xylem and phloem, and enter leaf or bud.– Trace = strand of xylem

and phloem

– Each trace leaves a gap filled with parenchyma in the cylinder of vascular tissue, forming leaf gap or bud gap.

Origin and Development of Stems Narrow band of cells between the primary

xylem and primary phloem may become vascular cambium.

• Cells produced by the vascular cambium become components of secondary xylem toward center and secondary phloem toward surface.

Origin and Development of Stems In many plants cork cambium (phellogen)

produces cork cells with suberin and phelloderm cells.

• Function to reduce water loss and to protect stem against injury

• Lenticels - Parenchyma cells in cork for exchange of gases

Tissue Patterns in StemsSteles

Stele - Central cylinder of primary xylem, primary phloem, and pith (if present)

• Protostele - Solid core, phloem surrounds xylem– In primitive seed plants, whisk ferns, club mosses and

ferns

• Siphonosteles - Tubular with pith in center– Common in ferns

• Eusteles - Discrete vascular bundles– In flowering plants and conifers

Tissue Patterns in Stems

Cotyledons - Seed leaves attached to embryonic stems

• Store food needed by young seedling

Dicotyledons (Dicots) - Flowering plants that develop from seeds having two cotyledons

Monocotyledons (Monocots) - Flowering plants that develop from seeds with a single cotyledon

Tissue Patterns in StemsHerbaceous Dicotyledonous Stems

Annuals - Plants that die after going from seed to maturity within one growing season• Usually green, herbaceous plants

• Most monocots are annuals, but many dicots are also annuals.

• Tissues largely primary

Tissue Patterns in StemsHerbaceous Dicotyledonous Stems

Herbaceous dicots have discrete vascular bundles arranged in a cylinder.

Vascular cambium arises between primary xylem and primary phloem.• Adds secondary xylem and secondary phloem

Dicot stem

Tissue Patterns in StemsWoody Dicotyledonous Stems

Wood = Secondary xylem Differences in wood:

• Vascular cambium and cork cambium active all year:– Ungrained, uniform wood produced– Some tropical trees

• If wood produced seasonally:– In spring: Relatively large vessel elements of secondary

xylem produced - Spring wood.– After spring wood: Fewer, smaller vessel elements in

proportion to tracheids and fibers - Summer wood.– In conifers, vessels and fibers absent

o Tracheids in spring larger than later in season

Tissue Patterns in StemsWoody Dicotyledonous Stems

One year’s growth of xylem = annual ring • Vascular cambium produces more secondary

xylem than phloem.– Bulk of tree trunk consists of annual rings of wood.

• Indicates age of a tree

• Indicates climate during tree’s lifetime

Vascular Rays consist of parenchyma cells that function in lateral conduction of nutrients and water.• Xylem Ray - Part of ray within xylem

• Phloem Ray - Part of ray through phloem

Tissue Patterns in StemsWoody Dicotyledonous Stems

Cross section of young stem

with secondary growth

Tissue Patterns in StemsWoody Dicotyledonous Stems

3-D view of dicot wood

Tissue Patterns in StemsWoody Dicotyledonous Stems

Tyloses - Protrusions of adjacent parenchyma cells into conducting cells of xylem• Prevent conduction of

water

• Resins, gums, and tannins accumulate, and darken wood, forming heartwood.– Heartwood - Older, darker

wood in center – Sapwood - Lighter, still-

functioning xylem closest to cambium

Tissue Patterns in Stems Woody Dicotyledonous Stems

Softwood - Wood of conifers• No fibers or vessel elements

Hardwood - Wood of dicot trees

Resin canals - Tubelike canals scattered throughout xylem and other tissues• Lined with specialized

cells that secrete resin• Common in conifers• In some tropical

flowering plants – Frankincense and myrrh

Resin canals in pine

Tissue Patterns in Stems Woody Dicotyledonous Stems

Bark - Tissues outside the vascular cambium, including secondary phloem• Mature bark may consist of alternating layers of

crushed phloem and cork.

Cross section of

young stem with secondary

growth

Laticifers - Ducts found mostly in phloem that have latex-secreting cells• Rubber, chicle (chewing gum), morphine

Tissue Patterns in Stems Monocotyledonous Stems

Monocots stems have neither a vascular cambium nor a cork cambium.• Produce no secondary vascular tissues or cork• Primary xylem and phloem in discrete vascular

bundles scattered throughout the stem– Vascular bundles

oriented with xylem closer to center of stem and phloem closer to surface.

– Parenchyma (ground tissue) surrounds vascular bundles.

Cross section

of monocot

stem

Tissue Patterns in Stems Monocotyledonous Stems

In a typical monocot vascular bundle:• Two large vessels with several small vessels• First formed xylem cells

stretch and collapse.– Leave irregularly shaped

air space

• Phloem consists of sieve tubes and companion cells.

• Vascular bundle surrounded by sheath of sclerenchyma cells. Monocot vascular bundle

Specialized Stems Rhizomes - Horizontal stems

that grow below-ground and have long to short internodes• Irises, some grasses, ferns

Runners - Horizontal stems that grow above ground and have long internodes.• Strawberry

Stolons - Produced beneath the surface of the ground and tend to grow in different directions.• Potato

Specialized Stems Tubers - Swollen, fleshy,

underground stem• Store food

• Potatoes - Eyes of potato are nodes

Bulbs - Large buds surrounded by numerous fleshy leaves, with a small stem at lower end• Store food

• Onions, lilies, hyacinths, tulips

Specialized Stems Corms - Resemble bulbs,

but composed almost entirely of stem tissue, with papery leaves• Store food• Crocus and gladiolus

Cladophylls - Flattened, leaf-life stems• Greenbriars, some

orchids, prickly pear cactus Prickly pear cactus

Wood and Its Uses In a living tree, 50% of the wood weight

comes from water content.

• Dry part of wood composed of about 60-75% cellulose and about 15-25% lignin.

Density - Weight per unit volume

Durability - Ability to withstand decay

• Tannins and oils repel decay organisms.

Wood and Its UsesTypes of Sawing

Radially cut (quartersawed) boards show annual rings in side view.

Tangentially cut (plain-sawed or slab cut) boards are cut perpendicular to rays.

• Show annual rings as irregular bands of light and dark streaks

Wood and Its Uses

Knots - Bases of lost branches covered by new annual rings produced by the cambium

Wood Products• About half of U.S. and Canadian wood production is

used as lumber, primarily for construction.– Sawdust and waste used for particle board and pulp.– Veneer - Thin sheet of desirable wood glued to cheaper

lumber

• Pulp is second most widespread use of wood.– Paper, synthetic fibers, plastics, linoleum

• In developing countries, about half of cut timber is used for fuel.– Less than 10% in US and Canada.

Review

External Form of a Woody Twig

Origin and Development of Stems

Tissue Patterns in Stems

• Herbaceous Dicotyledonous Stems

• Woody Dicotyledonous Stems

• Monocotyledonous Stems

Specialized Stems

Wood and Its Uses