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Secondary + ArborescentGrowth

Ch 26

Annuals, biennials, and perennials

• Plants often classified according to seasonal growth cycles– Annuals

• Weeds wildflowers garden flowers vegetables• Weeds, wildflowers, garden flowers, vegetables• The entire cycle from seed to vegetative plant to

flowering plant and to seed again within a single growing season.

– Biennials• Two seasons are needed for the period from seed

germination to seed formation.• 1st season is root production, short stem, and

tt f lrosette of leaves• 2nd season is flowering, fruiting, seed formation,

and death, completing the cycle.

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– Perennials • Vegetative structures live year after year. • Herb perennials pass unfavorable seasons as

dormant roots, rhizomes, bulbs, or tubers.

• Woody perennials may delay flowering until adulthood is reached.

– Example: Aesculus hippocastanum

Precocious flowering: flowering before leaves emerge.

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The Vascular Cambium

• Meristematic cells of VC are highly vacuolated.– Vertically oriented fusiform initials (longer than

wide)wide)– Horizontally oriented ray initials (elongated or

squarish)

Malus domestica Robinia pseudoacacia

• Secondary xylem and phloem are produced through periclinal divisions of the fusiform and ray initials.

• IOW the cell plate that forms is parallel to• IOW the cell plate that forms is parallel to the surface of the root or stem.

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Periclinal divisions Anticlinal divisions

• Xylem & Phloem produced by fusiform initials known as– Axial system

Ray initials produce horizontally oriented• Ray initials produce horizontally oriented ray cells – Form vascular rays or radial system

Vascular cambium:Two cells- Fusiform and ray initials

Periclinal division Initial vascular ray formation: Inner becomesxylem; outer becomes phloem.

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• In temperate regions the vascular cambium is dormant during winter and reactivated in the spring.

• Formation of secondary xylem & phloem– Results in cylinder of secondary vascular

tissues, with rays extending radially through the cylinder.y

Early development: 3 meristems

Completion of primary growth

Origin of vascular cambium

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Formation of some secondary xylem & phloem

End of 1st yr’sgrowth: showing periderm

Small amount of secondary growth

End of 1st yr’s growth: Secondary growth begins in 1st year.

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One-year-old stem ofTilia americana.

Two-year-old stem ofTilia americana.

Three-year-old stem of Tilia americana.

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Periderm is the dermal tissue of secondary plant body

• Periderm replaces the epidermis as protective covering.

• Periderm consists of three parts:C k bi ( h ll ) i t th t– Cork cambium (phellogen), meristem that produces periderm

– Cork (phellem), protective tissue formed to outside by the cork cambium

– Phelloderm, living parenchyma tissue formed to inside by meristem

Newly formed periderm

Periderm more advanced

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Initiation of a lenticel

In stems and roots containing periderms:Gas exchange accomplished by lenticels

Well-developed lenticel:Airy aggregation of cells within the structural surfaces of stems, roots

Melia azedarach – Chinaberry Tree

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Lenticel of Dutchman’s pipe Aristolochia:Unlike Sambucus; phelloderm consists of several layers of cells.

• Terms ‘periderm’, ‘cork’, and ‘bark’ often confused.

• Cork is one of three parts of the peridermS d ti th t l id i i– Secondary tissue that replaces epidermis in woody roots and stems

• Bark refers to all tissues outside vascular cambium

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Several periderm layers

External features of woody stems

Wood: secondary xylem

• Classified as hardwood or softwood.• Hardwoods are typically angiosperms

(magnoliid and eudicot).S ft d t i ll if• Softwoods are typically conifers.

• Terms do not accurately express the relative density or ‘hardness’ of the wood.

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Betula papyrifera

Carya ovata

Platanusoccidentalis

Quercus velutina

• Conifer wood lacks vessels• Have relatively small amounts of axial

parenchyma (wood)L t i t h id tit t d i t– Long, tapering tracheids constitute dominant cell type.

– Only parenchymal cells are associated with resin ducts.

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• Angiosperm wood contains vessels– Vessel elements, tracheids, several types of

fibers, and parenchymal cells– Presence of vessel elements distinguishesPresence of vessel elements distinguishes

angiosperm wood from conifer wood.

Quercus rubra Liriodendron tulipifera

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Growth Rings

• Growth rings– ‘growth’ in increments– If a growth layer represents one season’s

growthgrowth • Annual rings form

• Abrupt changes in environmental factors may lead to > 1 ring per year– False annual rings

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4240 B.C. to 4210 B.C.Begins about 6260 ya.

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• Sapwood – Part of wood in living tree that contains living

cells & reserve materials– Conducts water

• Heartwood – Believed to result from process that enables

the plant to remove from regions of growth secondary metabolites that may be toxic or inhibitory to growing cells.

– Does not conduct

Sapwood:Heartwood ratio

• Thick sapwood species– Acer– Betula

Fraxinus– Fraxinus• Thin sapwood species

– Robinia– Catalpa – Taxus

• No clear distinction between sapwood & heartwood– Populus

Salix– Salix – Abies

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Summary • Secondary growth causes an increase in girth of

stems and root.• Plants often classified by seasonal growth cycles.• VC – fusiform and ray initials

C k bi d i i• Cork cambium produces protective covering on secondary plant body.

• Bark – all tissues outside the VC• Wood is secondary xylem• Growth rings result from the periodic activity of

VC.