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Plant Form

and Function

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Confusing vocabulary: Dicot = Eudicot. Your text book uses the

term Eudicot. They mean exactly the same thing.

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Dermal Tissue

• Forms the epidermis

• One cell layer thick in most plants

• Forms the outer protective covering of the plant

• Covered with a fatty cutin layer (cuticle).

• Contains special cells, including guard cells, trichomes and root hairs.

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• Guard cells

• Paired sausage-shaped cells

• Flank a stoma – epidermal opening

• Passageway for oxygen, carbon dioxide, and water vapor

• Stomatal patterning genes reveal a coordinated network of cell–cell communication that informs cells of their positions relative to other cells and determines cell fate.

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• Trichomes

• Cellular or multicellular hairlike outgrowths of the epidermis

• Keep leaf surfaces cool and reduce evaporation by covering stomatal openings

• Some are glandular, secreting substances that deter herbivory

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

a.3.5 mm

3.5 mmb. c.

Neighboring cell

1.

Inhibition

Activation

Trichome will

not develop

Trichome

develops

Trichome

Trichome cell

Trichome-

promoting

proteins,

including

GL3

Trichome-

inhibiting

proteins

2. Trichome initiation ON

3. Trichome initiation OFF

a-b: Courtesy of Allan Lloyd

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• Roots hairs

• Tubular extensions of individual epidermal cells

• Greatly increase the root’s surface area and efficiency of absorption

• Should not be confused with lateral roots

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a. Parenchyma b. Collenchyma c. Sclerenchyma

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Meristems

• Clumps of small cells with dense cytoplasm and large nuclei .

• Act as stem cells do in animals. Plant biologists use term meristem cell rather than stem cell to avoid confusion.

• One cell divides producing a differentiating cell and another that remains meristematic.

• Extension of shoot and root produced by apical meristems.

• Lateral meristems produce an increase in shoot and root diameter.

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Apical Meristems

• New cells are produced that enable a plant to grow in length (stem up and root down); a process called primary growth.

• Tissues produced by primary growth are called primary tissues.

• Apical meristems are composed of delicate cells that need protection (easily damaged).

• Root cap protects root apical meristem.

• Leaf primordia protect shootapical meristem.

Primary growth in roots

• Root has four regions of growth and differentiation:

• Root Cap

• Zone of Cell Division (meristem)

• Zone of Elongation

• Zone of Maturation

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Root cap

• Root cap protects the delicate meristem cells as it grows through the abrasive soil.

• Golgi bodies in the outer cap cells secrete a lubricant that eases the root through the soil.

• Root cap cells are constantly replacedas they are damaged.

• Columella cells in the center of the root cap function as gravity detectors. They contain amyloplasts (plastids with starch grains) that collect on the side facing down. Roots will bend in that direction. These also occur in stems.

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Zone of Cell Division• Derived from rapid divisions of the root apical meristem.

Daughter cells of the apical meristem.

• Cells in the center don’t divide as often as the outer areas. This is called the quiescent center.

• Apical meristem daughter cells soon subdivide into the three primary tissues:

• Protoderm

• Procambrium

• Ground meristem

Zone of Elongation

• Roots lengthen because cells become several times longer than wide.

• Width also increases slightly.

Zone of Maturation

• Previously elongated cells become differentiated into specific types of cells.

• Root surface cells become epidermal cells; cuticle, root hairs.

• Parenchyma cells produced by cortex (ground meristem just inside past the epidermis).

Monocot vs. Eudicot roots• Eudicots:

• Primary xylem are a solid core in the center and star-shaped. Arms of the star point to the pericycle layer.

• Monocots:• Primary xylem are in bundles arranged in a ring, which

surround the pith (center parenchyma cells).

Modified Roots

• Prop roots: Keep the plant upright.

• Aerial roots: Obtain water from the air.

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Modified Roots

• Pneumatophores: Facilitate oxygen uptake (snorkel).

• Water storage roots: Weigh 50 kg or more.

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Buttress roots

Modified Roots

• Parasitic roots: Penetrate host plants.

Modified Roots

• Food Storage Roots

Stems• Like roots, stems contain the three types of plant

tissue: dermal, ground and vascular.

• Also undergo growth from cell division in apical and lateral meristems.

• Major distinguishing feature between monocot and eudicot stems is the organization of the vascular tissue system.

• Monocot vascular bundles are usually scatteredthroughout ground tissue system.

• Eudicot vascular tissue is arranged in a ring with internal ground tissue (pith) and external ground tissue (cortex).

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49Monocot

Eudicot (Dicot)

Secondary growth in woody plants

• Increase in thickness of stems and roots is called secondary growth.

• Caused by dividing of lateral meristem cells.

• These lateral meristems are arranged in two cylinders known as the vascular cambium and the cork cambium.

Lateral meristems

• Outward growth from lateral meristem cells. This is called secondary growth.

• Found in woody plants.

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Vascular Cambium

• Growth occurs on both sides of the vascular cambium.

• Secondary xylem on the interior side and secondary phloem on the exterior side.

• Secondary xylem makes up the wood of the tree.

• Secondary phloem is part of the bark of the tree.

• One year’s growth of secondary xylem makes a tree ring.

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Tree Rings

• Annual growth of secondary xylem produce tree rings.

• Spring growth is greater than late summer, thus the ring has two parts: an early, lighter, wider section and a later, thinner, darker section.

Cork Cambium

• Cork is produced by the cork cambium.

• These external tissues slough off and replaced each year.

• Cork is the outer layer of bark composed of mature dead cork cells. These cells have thick waxy walls.

• Bark is composed of: secondary phloem, cork cambium and cork.

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Secondary Xylem

• Heartwood: darker center wood. Composed of older layers of secondary xylem. No longer transports water. Clogged with resin which helps resist rotting.

• Sapwood: lighter outer wood. Composed of younger layers of secondary xylem. Conducts sap and water.

• Wood is composed of: secondary xylem.

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Internal Woody Stem Structure

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Bark and Wood• Bark = cork, cork cambium and secondary phloem.

• Wood = secondary xylem which is made of heartwood and sapwood.

Tree Rings

• Annual growth of secondary xylem produce tree rings.

• Spring growth is greater than late summer, thus the ring has two parts: an early, lighter, wider section and a later, thinner, darker section.

Dendrochronology

• Using tree rings to determine absolute age is called dendrochronology. Helps confirm results of other dating methods (like carbon 14 decay).

• In the desert Southwest, tree ring chronologies go back as far as 8,700 years BP.

• Best sequences are regional and species specific.

Plant zones and nursery tags

• Selecting the right plant for your yard.

• Know your hardiness zone.

Hardiness Zone Map

• Standard by which growers determine which plants are most likely to thrive at a particular location.

• The map is based on the average minimal winter temperature.

• Divided into 10-degree F zones. Sometimes there are 5-degree subcategories.

• Zone 1 is the coldest. Zone 13 is the warmest (in the US/US territories).

• http://planthardiness.ars.usda.gov

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Nursery plant tags

• Tags covers basic information about the plant.

• Common name and scientific name.

• Hardiness zone: perennials only (those that return each year). No zone is listed for annuals (those that only last one season).

• Sun exposure.

• Water requirements.

• Size and growth information.

• Bloom time (if flowering).

• Other relevant information.