Chapter 36: Plant Form
Plant Body Organization * A vascular plant consists of * Root system * Anchors the plant * Used to absorb water and ions * Shoot system * Consists of supporting stems, photosynthetic leaves, and reproductive flowers * Repetitive units consist of internode, node, leaf, and axillary bud

* 3 basic tissue types * Dermal – outer protective cover * Ground – function in storage, photosynthesis, and secretion * Vascular – conducts fluids and dissolved substances * Tissues consist of one or more cell types * Tissue systems – each of these tissue types extends through root and shoot systems

Meristems * Clumps of small cells with dense cytoplasm and large nuclei * Act as stem cells do in animals * One cell divides producing a differentiating cell and another that remains meristematic * Plant biologists use term meristem cell rather than stem cell to avoid confusion * Extension of shoot and root produced by apical meristems * Lateral meristems produce an increase in shoot and root diameter

Apical meristem * Located at tips of stems and roots * Give rise to primary tissues which are collectively called the primary plant body * Apical meristems composed of delicate cells that need protection * Root cap protects root apical meristem * Leaf primordia protect shoot apical meristem

* Apical meristem gives rise to the three tissue systems by first initiating primary meristems * 3 primary meristems * Protoderm – forms epidermis * Procambrium – produces primary vascular tissue * Ground meristem – differentiates into ground tissue

Lateral meristems * Found in plants that exhibit secondary growth * Give rise to secondary tissues which are collectively called the secondary plant body * Woody plants have two types * Cork cambium produces outer bark * Vascular cambium produces secondary vascular tissue * Secondary xylem is the main component of wood

Plant Tissues * Three main types of tissue * Dermal * On external surfaces that serves a protective function * Ground * Forms several different internal tissue types and can participate in photosynthesis, serve a storage function, or provide structural support * Vascular * Conducts water and nutrients

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 constituting the cuticle * Contains special cells, including guard cells, trichomes, and root hairs * 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

Ground Tissue * 3 cell types * Parenchyma * Function in storage, photosynthesis, and secretion * Collenchyma * Provide support and protection * Sclerenchyma * Provide support and protection

Parenchyma cells * Most common type of plant cell * Living protoplasts – may live many years * Function in storage, photosynthesis, and secretion * Less specialized than other plant cells

Collenchyma cells * Provide support for plant organs * Allow bending without breaking * Living protoplasts – may live many years

Sclerenchyma cells * Tough thick walls * Usually lack living protoplasts at maturity * Secondary cell walls often contain lignin * Two general types – both strengthen tissues * Fibers – long, slender cells that are usually grouped in strands * Sclereids – variable shape, often branched, may occur singly or in groups

Vascular Tissue
Xylem
* Principal water-conducting tissue * Vessels: Continuous tubes of dead cylindrical cells arranged end-to-end * Tracheids: Dead cells that taper at the end and overlap one another * Vessel members tend to be shorter and wider than tracheids

Phloem * Principal food-conducting tissue in vascular plants * Contains two types of elongated cells * Sieve cells (seedless vascular plants and gymnosperms) and sieve tube members (angiosperms) * Living cells that contain clusters of pores called sieve areas or sieve plates * Sieve-tube members are more specialized (more efficient) * Associated with companion cells

Roots * Simpler pattern of organization and development than stems * Four regions are commonly recognized: * Root cap * Zone of cell division * Zone of elongation * Zone of maturation

400 µm

Root cap
Zone of cell division
Zone of elongation Zone of maturation dermal tissue ground tissue vascular tissue
Root in cross section
Endodermis
Root hair
Epidermis
Vascular tissue Ground meristem Procambium
Protoderm
Quiescent center Apical meristem Columella cells Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. © Biodisc/ Visuals Unlimited
Ground
tissue

STAGES IN THE DIFFERENTIATION OF PLANT TISSUES

Zygote
Embryo
Cork cambium
Leaf primordia
Bud primordia
Shoot elongation
Outer bark
Phloem
Xylem
Inner bark
Bark
Bark
Leaves
Lateral shoots
Cork cambium
Pericycle
Phloem
Xylem
Lateral roots
Root elongation
Outer bark
Inner bark
Vascular cambium
Vascular cambium
Wood
Wood

Shoot apical meristem
Root apical meristem
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Modified Roots * Most plants produce either/or: * Taproot system – single large root with small branch roots * Fibrous root system – many small roots of similar diameter * Some plants, however, produce modified roots with specific functions * Adventitious roots arise from any place other than the plant’s root

* Prop roots: Keep the plant upright * Aerial roots: Obtain water from the air * Pneumatophores: Facilitate oxygen uptake * Contractile roots: Pull plant deeper into soil * Parasitic roots: Penetrate host plants * Food storage roots: Store carbohydrates * Water storage roots: Weigh 50 kg or more * Buttress roots: Provide considerable stability
Prop roots
Aerial roots
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a.
© E.R. Degginger/Photo Researchers, Inc.
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b.
© Peter Frischmuth/Peter Arnold Inc. Pneumatophores
Water storage roots
c.
© Walter H. Hodge/Peter Arnold Inc.
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d.
© Gerald & Buff Corsi/Visuals Unlimited
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e.
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Stems * Like roots, stems contain the three types of plant tissue * Also undergo growth from cell division in apical and lateral stems * Shoot apical meristem initiates stem tissue and intermittently produces primordia * Develop into leaves, other shoots, and even flowers

Leaves may be arranged in one of three ways
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Whorled:
Sweet woodruff
Opposite:
Periwinkle
Alternate:
Ivy

* Node – point of attachment of leaf to stem * Internode – area of stem between two nodes * Blade – flattened part of leaf * Petiole – stalk of leaf * Axil – angle between petiole/blade and stem * Axillary bud – develops into branches with leaves or may form flowers * Terminal bud – extends the shoot system during the growing season

Bundle scar
Terminal bud
Axillary bud
b.
a.
Leaf scar
Terminal
bud scale scars Inter node
Node
Blade
Petiole
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* Major distinguishing feature between monocot and eudicot stems is the organization of the vascular tissue system * Monocot vascular bundles are usually scattered throughout ground tissue system * Eudicot vascular tissue is arranged in a ring with internal ground tissue (pith) and external ground tissue (cortex)
Monocot
Eudicot
Pith
Xylem
Phloem
Cortex
a.
1176 µm

Collenchyma
(layers below epidermis)
Epidermis
(outer layer)
Vascular bundle

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© Ed Reschke
b.
Xylem
Ground tissue
Phloem
2353 µm

Vascular bundle

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© Ed Reschke

* Vascular tissue arrangement is directly related to the stem’s ability for secondary growth * In eudicots, a vascular cambium develops between the primary xylem and phloem * Connects the ring of primary vascular bundles * In monocots, there is no vascular cambium – no secondary growth

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a.
b.
Primary xylem
Secondary xylem
Primary phloem
Secondary phloem
Epidermis
Primary xylem
Primary phloem
Vascular cambium
(lateral meristem)

c.
Primary xylem
Secondary xylem
Primary phloem
Secondary phloem
Vascular cambium
(lateral meristem)
Annual
growth layers

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* Rings in the stump of a tree reveal annual patterns of vascular cambium growth * Cell size depends on growth conditions * In woody eudicots and gymnosperms, the cork cambium arises in the outer cortex * Produces boxlike cork cells on outside and parenchyma-like phelloderm cells on inside * Collectively called the periderm

INTERNAL STEM STRUCTURE

Outer bark
Cork cambium
Phloem
Vascular cambium
Xylem
Heartwood
Sapwood
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* Periderm – cork cambium, cork, and phelloderm * Forms outer bark

* Lenticels – Cork cambium produces unsuberized cells that permit gas exchange to continue

* Bulbs – swollen underground stems, consisting of fleshy leaves * Corms – superficially resemble bulbs, but have no fleshy leaves * Rhizomes – horizontal underground stems, with adventitious roots

* Runners and stolons – horizontal stems with long internodes that grow along the surface of the ground

Stolon

Tuber (swollen tip of stolon)
d.

* Tubers – swollen tips of rhizomes that contain carbohydrates * Tendrils – twine around supports and aid in climbing * Cladophylls – flattened photosynthetic stems resembling leaves
Tendril

e.

f.
Cladophyll
Leaves

Leaves * Initiated as primordia by the apical meristems * Principal site of photosynthesis * Expand by cell enlargement and cell division * Determinate in structure – growth stops at maturity * Different patterns adaptive in different environments

* Leaves may have stipules * Outgrowths at base of petiole * May be leaf-forming or modified as spines

* Veins * Vascular bundles in leaves * Main veins are parallel in most monocot leaves * Veins of eudicots form an often intricate network
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b. a: © Kjell Sandved/Butterfl y Alphabet; b: © Pat Anderson/Visuals Unlimited

* Simple leaves contain undivided blades * May have teeth, indentations, or lobes * Compound leaves have blades that are divided into leaflets

* The leaf’s surface is covered by transparent epidermal cells * Most have no chloroplasts * Epidermis has a waxy cuticle * Different types of glands and trichomes may be present * Lower epidermis contains numerous stomata flanked by guard cells

* Most eudicot leaves have 2 types of mesophyll * Palisade mesophyll – usually two rows of tightly packed chlorenchyma cells * Spongy mesophyll – loosely arranged cells with many air spaces in between * Function in gas exchange and water vapor exit * Monocot leaves – mesophyll is usually not differentiated into palisade/spongy layers

Upper epidermis Cuticle
Palisade
mesophyll
Spongy
mesophyll
Vein
Xylem
Phloem
Guardcell
Stoma
Lower epidermis Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Modified Leaves * Floral leaves (bracts) – surround true flowers and behave as showy petals * Spines – reduce water loss and may deter predators * Reproductive leaves – plantlets capable of growing independently into full-sized plant * Window leaves – succulent, cone-shaped leaves that allow photosynthesis underground * Shade leaves – larger in surface area but with less mesophyll than sun-lit leaves * Insectivorous leaves – trap insects * Pitcher plants have cone-shaped leaves that accumulate rainwater * Sundews have glands that secrete sticky mucilage * Venus flytrap have hinged leaves that snap shut