Plant Structure, Growth, & Development

Campbell and ReeceChapter 35

Organization of Plants

• Plants like most multicellular organisms have organs made of tissues that are made of different cell types

3 Basic Plant Organs

1. Stems2. Roots3. Leaves

Roots

• Organ that 1. anchors a

vascular plant in the soil

2. absorbs water & minerals

3. stores carbohydrates

Taproot System

• In most eudicots & gymnosperms

• Taproot develops from embryonic root

• Forms lateral roots (branch roots)

• Penetrate deep• Eudicot: most flowering

plants that have 2 embryonic seed leaves

• Gymnosperm: vascular plant that bears naked seeds

Roots in Monocots

• Grasses: no tap root• Roots called adventitious: grows

in unusual locations– Example: roots arising from stems

or leaves

Adventitious Root Systems

• Each small root forms its own lateral roots fibrous root system

Root Hairs

• Emerge near tips of roots• Increase surface area for

absorption of water and mineral ions (do not help anchor plant)

• are thin, tubular extensions of a root epidermal cell

Root Adaptations

• Prop Roots: support tall, top-heavy trees

• Pneumatophores: air roots, portion above water line allows them to get O2

Root Adaptations

• Buttress Roots: tallest trees in rain forest have shallow roots; buttress roots give support to trunks

• “Strangling” Aerial Roots: seeds of these trees (strangler fig) germinate in branches of host tree

Stems

• Plant organs that 1. raise or separate leaves

allowing them to receive more sunlight

2. Raise reproductive structures facilitating dispersal of seeds or pollen

Parts of a Stem

• Each stem has alternating system of:1. Nodes– Pts @ which leaves are attached

2. Internodes – Stem segments between nodes

3. Axillary Bud– Upper angle (axil) formed by each leaf

& stem– Structure that can form a lateral shoot

(branch)

Parts of a Stem

4.Apical Bud– Part of shoot tip– The terminal bud (where most of

growth occurs)

5. Apical Dominance– Inhibits growth @ axillary buds– If eaten by herbivore or if light

more intense @ side of a shoot axillary buds break apical dominance & grow

Why Pruning Makes a Plant Bushier

Parts of a Stem

Adaptations of Stems

1. Rhizomes: horizontal shoots that grow just below surface ;vertical shoots emerge from axillary buds

• Ex: Irises, Hops

Stem Adaptations

2.Bulbs: are vertical, underground shoots made mostly of enlarged bases of modified leaves that store food

• Ex: onion, tulips

Adaptations of Stems

3. Stolons: horizontal shoots that grow along surface; aka “runners”

• Enable plant to reproduce asexually: plantlets form @ nodes

• Ex: strawberries, some grasses

Adaptations of Stems

4. Tubers: enlarged ends of rhizomes or stolons specialized for storing food. “Eye” of potato is cluster of axillary buds that mark the nodes

Ex: potato, dahlias

Leaves

• In most plants leaf is main photosynthetic organ

• General Structure:– Blade– Petiole • not on grasses or most monocots

– Veins • Patterns differ monocots & eudicots

Structure of a Leaf

Types of Leaves

Leaf Adaptations

• Tendrils: modified leaf used to support plant

• Ex: pea plants

• Some plants have tendrils that are modified stems (grapevines)

Leaf Adaptations

2. Spines: leaves adapted for protection

In cacti, stems are main photosynthetic organ

Leaf Adaptations

3. Storage Leaves: most succulents have leaves adapted to store water

Leaf Adaptations

4. Reproductive Leaves: leaves of some succulents produce adventitious plantlets which fall off & take root in soil

Ex: some succulents

Hens and Chicks

Leaf Adaptations

5. Bracts: modified leaves surrounding the real flower; function: attract pollinators

The yellow portion of poinsettia is the flower; the red leaves are bracts

Tissue Systems

• Are functional units connecting all of the plants organs

Dermal Tissue System

• Plant’s outer protective covering:

• Epidermis: nonwoody plants: tightly packed cells

• Cuticle: waxy covering on epidermal surface prevents water loss

• Periderm: in woody plants: replaces in older regions of stems & roots

Vascular Tissue System

Carries out long-distance transport of materials between the root & shoot systems

1. Xylem– H2O & dissolved

materials roots shoots

2. Phloem– Sugars roots &

sites of growth

Ground Tissue System

• Tissue that isn’t dermal or vascular tissue

• Pith: Internal to vascular tissue

• Cortex: external to vascular tissue

Plant Cells

• Cell differentiation involving changes in:– Cell walls– Cytoplasm–Organelles

Parenchymal Cells

• Mature cells have thin, slightly flexible cell walls (only 1)& large central vacuole

• Functions:– Perform most of metabolic

functions of plant• Chloroplasts• Plastids: store starch, found in roots• Make up most of fleshy part of fruits• Most able to divide & differentiate into

other cell types

Collenchyma Cells

• Come grouped in strands just below epidermis

• Support young parts of plant shoot w/out interfering with growth

• Elongated cells with thicker cell walls (compared to parenchymal cells) which can be irregularly thickened

• Remain living cells thru out plant life

Sclerenchymal Cells

• Supportive role but more rigid than collenchymal cells in regions of plant that have stopped growing

• 2º cell walls thick, contain lignin (>1/4 dry mass of wood)Lignin in all vascular plants, not in bryophytes

• Many dead at plant maturity: rigid cell walls support plant

Sclerenchymal Cells

• 2 types: (both for support & strength)

1. Sclereids– Boxy, irregularly shaped cells– Thick lignified 2º cell walls– Hardness in nutshells/grittiness in pear

2. Fibers – Grouped in strands– Long, slender, tapered– Hemp fibers for rope/flax fibers for linen

Water-Conducting Cells of Xylem

• 2 types: both elongated, tubular cells/dead at plant maturity: form tubular conduit for water flow; have pits thru which water can move laterally

1. Tracheids– In nearly all vascular plants

2. Vessel Elements– In some vascular/most

angiosperms/few gymnosperms

Sugar-Conducting Cells of the Phloem

• 4 types all alive @ plant maturity1. Sieve Cells: in seedless vascular plants &

gymnosperms2. Sieve Tubes: chains of cells/ enucleated,

no ribosomes, vacuole, or cytoskeleton sugars can diffuse thru cell more easily

3. Sieve Plates: pores for flow of sap fluid cell-to-cell

4. Companion Cells: nonconducting cells connected to sieve tube cells by plasmodesmata/their ribosomes & nucleus serve both cells

Growth in Plants

Stem Growth

Root Growth

Cross-Section of a Leaf

Anatomy of a Tree Trunk

• bark includes all tissues external to the vascular cambian (2º xylem, wood, and phloem)

• Sapwood = “living wood” has lighter color than heart wood (center) which is made of dead cells

Development of a Plant• Definitions:• Development: specific series of

changes by which cells form tissues, organs, & organisms

• Growth: irreversible increase in size

More Definitions

• Morphogenesis:cellular & tissue-based processes by which an organism takes shape, depends on cells responding to positional information from neighboring cells

• Differentiation : process by which a cell or group of cells become specialized in structure & function

Plant Cell Division

• Preprophase band made of microtubules develops in late interphase determines where cell plate will form

Asymmetrical Cell Division

• not all plant cells divided equally duting M Phase of cell cycle

• When occurs: usually signals a key event in development

• Example:– Epidermal cell divides • 1 large epidermal cell• 1 small guard cell

Cell Polarity

• The condition of having structural or chemical differences at opposite ends of an organism

• Typical plant has axis with a shoot end & a root end– 1st division of fertilized plant ova

asymmetrical which initiates polarization of plant body into shoot & root

• Cell division enhances possibility of plant growth but it is cell elongation that is responsible for plant growth

Cell Elongation

Cell Elongation

• Controlled by microtubule orientation controls the orientation of cellulose microfibrils in cell wall

Cell Differentiation

• Arises from differential gene activation

• Enables cells w/in plant to assume different functions

• Way any particular cell differentiates depends on its position in developing plant

Pattern Formation• Is the development of specific

structures in specific locations• 2 hypothesis to explain1. Lineage-based mechanism– Daughter cells have instructions

from early cells in plant development

2. Position-based mechanism– Cell’s position in emerging organ

determines what kind of cell it will become

Pattern Formation

• Position-based hypothesis– By destroying cells during

development have shown that cell’s fate determined late in development & depends mostly on signals from neighboring cells

– Cell fate in animals mostly lineage-dependent involving transcription factors

Pattern Formation

• Hox genes–Homeotic genes that code for

transcription factors – Critical for proper # & placement of

embryonic structures (legs, antennae in Drosophila)

Pattern Formation

• Knotted-1 homologous to Hox gene found in maize

• Important in development of leaf morphology

Control of Cell Differentiation

• Depends on control of gene expression: which genes are transcribed protein

• But fate of a particular cell is determined by its final position in the developing organ, not by cell lineage– If undifferentiated cell is misplaced

it will differentiate into cell type appropriate to its position

Activation of Genes

• Depends on signals from neighboring cells

Phase Changes

• Cues from plant itself or from its environment cause plant to switch from 1 developmental stage to another: called Phase Changes

• Most obvious changes in leaf shape & size

Genetic Control of Flowering

• Flower formation involves phase change from vegetative growth reproductive growth

• transition triggered by:1. Environmental cues– Length of daylight2. Internal signals– Plant hormones

Genetic Control of Flowering

• Production of a flower by a shoot apical meristem stops the primary growth of the shoot

• Is ass’c with the switching on of floral meristem identity genes

• Meristem: plant tissue that remains embryonic as long as plant live, allowing for indeterminate growth

Meristem Identity Genes

• Code for transcription factors that regulate genes needed for conversion of the indeterminate vegetative meristems determinate floral meristems

Organ Identity Genes

• A plant homeotic gene that uses positional information to determine which emerging leaves develop into which type of floral organs

Organ Identity Genes

• Provide model system for studying pattern formation:– The development of a multicellular

organism’s spatial organization: arrangement of organs & tissues in their characteristic places in 3-D

Organ Identity Genes

• 3 classes identified by studying mutants with abnormal flowers

• ABC Hypothesis• A model of flower formation

identifying 3 classes of organ identity genes that direct formation of the 4 types of floral organs

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