Plant

• Three basic organs evolved: roots, stems, and leaves

• They are organized into a root system and a shoot system

Reproductive shoot (flower)

Terminal bud

NodeInternode

Terminalbud

Vegetativeshoot

BladePetiole

Stem

Leaf

Taproot

Lateral roots Rootsystem

Shootsystem

Axillarybud

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Roots

• A root– Is an organ that anchors the vascular plant– Absorbs minerals and water– Often stores organic nutrients

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• In most plants– The absorption of

water and minerals occurs near the root tips, where vast numbers of tiny root hairs increase the surface area of the root

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Many plants have modified rootsMany plants have modified roots

(a) Prop roots (b) Storage roots (c) “Strangling” aerialroots

(d) Buttress roots (e) Pneumatophores

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Stems

• A stem is an organ consisting of – An alternating system of nodes, the points

at which leaves are attached– Internodes, the stem segments between

nodes

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Buds

• An axillary bud– Is a structure that has the potential to form

a lateral shoot, or branch

• A terminal bud– Is located near the shoot tip and causes

elongation of a young shoot

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Many plants have modified stemsMany plants have modified stems

Rhizomes. The edible base of this ginger plant is an example of a rhizome, a horizontal stem that grows just below the surface or emerges and grows along thesurface.

(d)

Tubers. Tubers, such as these red potatoes, are enlarged ends of rhizomes specializedfor storing food. The “eyes” arranged in a spiral pattern around a potato are clusters of axillary buds that markthe nodes.

(c)Bulbs. Bulbs are vertical,underground shoots consistingmostly of the enlarged bases of leaves that store food. You can see the many layers of modified leaves attached to the short stem by slicing an onion bulb lengthwise.

(b)

Stolons. Shown here on a strawberry plant, stolons are horizontal stems that grow along the surface. These “runners”enable a plant to reproduce asexually, as plantlets form at nodes along each runner.

(a)

Storage leaves

Stem

Root Node

Rhizome

Root

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Leaves

• The leaf– Is the main photosynthetic organ of most

vascular plants

•Leaves generally consist of

–A flattened blade and a stalk

– The petiole, which joins the leaf to a node of the stem

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Some plant speciesSome plant speciesHave evolved modified leaves that serve various Have evolved modified leaves that serve various

functionsfunctions(a) Tendrils. The tendrils by which this

pea plant clings to a support are modified leaves. After it has “lassoed” a support, a tendril forms a coil that brings the plant closer to the support. Tendrils are typically modified leaves, but some tendrils are modified stems, as in grapevines.

(b) Spines. The spines of cacti, such as this prickly pear, are actually leaves, and photosynthesis is carried out mainly by the fleshy green stems.

(c) Storage leaves. Most succulents, such as this ice plant, have leaves modified for storing water.

(d) Bracts. Red parts of the poinsettia are often mistaken for petals but are actually modified leaves called bracts that surround a group of flowers. Such brightly colored leaves attract pollinators.

(e) Reproductive leaves. The leaves of some succulents, such as Kalanchoe daigremontiana, produce adventitious plantlets, which fall off the leaf and take root in the soil.

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The Three Tissue Systems: Dermal, Vascular, and Ground

• Each plant organ– Has dermal,

vascular, and ground tissues

Dermaltissue

Groundtissue Vascular

tissue

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• The dermal tissue system– Consists of the epidermis and periderm

•The vascular tissue system

–Carries out long-distance transport of materials between roots and shoots

–Consists of two tissues, xylem and phloem

•Ground tissue

–Includes various cells specialized for functions such as storage, photosynthesis, and support

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Common Types of Plant Cells• Like any multicellular organism

– A plant is characterized by cellular differentiation, the specialization of cells in structure and function

•Some of the major types of plant cells include

–Parenchyma

–Collenchyma

–Sclerenchyma

–Water-conducting cells of the xylem

–Sugar-conducting cells of the phloem12

Vascular tissue

• Xylem– Conveys water and dissolved minerals

upward from roots into the shoots

• Phloem– Transports organic nutrients from where

they are made to where they are needed

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In most monocot stemsThe vascular bundles are scattered throughout the ground tissue, rather than forming a ring

Groundtissue

Epidermis

Vascularbundles

1 mm

A monocot stem. A monocot stem (maize) with vascularbundles scattered throughout the ground tissue. In such anarrangement, ground tissue is not partitioned into pith andcortex. (LM of transverse section) 14

XylemPhloem

Sclerenchyma(fiber cells)

Ground tissueconnecting pith to cortex

Pith

Epidermis

Vascularbundle

Cortex

Key

Dermal

Ground

Vascular1 mm

(a) A dicot stem. A dicot stem (sunflower), withvascular bundles forming a ring. Ground tissue towardthe inside is called pith, and ground tissue toward theoutside is called cortex. (LM of transverse section)

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Difference in monocot and dicot

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Keyto labels

DermalGround

Vascular

Guardcells

Stomatal pore

Epidermalcell

50 µm

Surface view of a spiderwort(Tradescantia) leaf (LM)

(b)Cuticle

Sclerenchymafibers

Stoma

Upperepidermis

Palisademesophyll

Spongymesophyll

Lowerepidermis

Cuticle

Vein

Guard cells

Xylem

Phloem

Guard cells

Bundle-sheathcell

Cutaway drawing of leaf tissues(a)

Vein Air spaces Guard cells

100 µmTransverse section of a lilac(Syringa) leaf (LM)

(c)

Leaf anatomy

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Tissue Organization of Leaves

• The epidermal barrier in leaves– Is interrupted by stomata, which allow CO2

exchange between the surrounding air and the photosynthetic cells within a leaf

• The ground tissue in a leaf– Is sandwiched between the upper and lower

epidermis

• The vascular tissue of each leaf– Is continuous with the vascular tissue of the stem

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Meristems in Dicotyledonous plant

• Meristems generate cells for new organs

• Apical meristems– Are located at the tips of roots and in the

buds of shoots– Elongate shoots and roots through primary

growth

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• Lateral meristems– Add thickness to woody plants through

secondary growth

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APICAL MERISTEM

• Primary growth lengthens roots and shoots

• Primary growth produces the primary plant body, the parts of the root and shoot systems produced by apical meristems

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Primary Growth of RootsPrimary Growth of Roots• The root tip is covered by a root cap, which protects

the delicate apical meristem as the root pushes through soil during primary growth

Dermal

Ground

Vascular

Key

Cortex Vascular cylinder

Epidermis

Root hair

Zone ofmaturation

Zone ofelongation

Zone of celldivision

Apicalmeristem

Root cap

100 m

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Primary Growth of Shoots• A shoot apical meristem

– Is a dome-shaped mass of dividing cells at the tip of the terminal bud

– Gives rise to a repetition of internodes and leaf-bearing nodes

Apical meristem Leaf primordia

Developingvascularstrand

Axillary budmeristems

0.25 mm

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Secondary growth

• Secondary growth– Occurs in stems and roots of woody plants

but rarely in leaves

• The secondary plant body– Consists of the tissues produced by the

vascular cambium and cork cambium

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An overview of primary and secondary growthAn overview of primary and secondary growth

In woody plants, there are lateral meristems that add secondary

growth, increasing the girth of

roots and stems.

Apical meristemsadd primary growth,or growth in length.

Vascularcambium

Corkcambium

Lateralmeristems

Root apicalmeristems

Primary growth in stems

Epidermis

Cortex

Primary phloem

Primary xylem

Pith

Secondary growth in stems

PeridermCorkcambium

CortexPrimary phloem

Secondaryphloem

Vascular cambium

Secondaryxylem

Primaryxylem

Pith

Shoot apicalmeristems(in buds)

The corkcambium addssecondarydermal tissue.

The vascularcambium addssecondaryxylem andphloem.

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Xylem

Xylem vessels - heavily lignified to withstand the pressure of carrying water.

• Dead and hollow so minimum resistance for water flow.

• Stacked on end to end, with no end walls.

• Perforated with holes where there used to be plasmodesmata, for sideways/radial transport.

• Transport of water and mineral ions dissolved in water.

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Pattern of lignin in xylem vessels

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Tracheids• Tracheids - similar to xylem vessels,

• heavily lignified,

• Dead,

• tapering ends so water only passed sideways through the holes.

• Fibres - heavily lignified, only for support.

• Xylem parenchyma - normal metabolic activity of the cell, packing tissue.

- Involved in radial transport. Alive, with cellulose cell walls. May store starch 30

Phloem• Phloem sieve tube element - alive, but with

only a thin cytoplasmic strand along the sides

• no Golgi Apparatus, nucleus or ribosomes.

• Stacked on end to end, end walls are modified to form sieve tube plates with pores in them.

• Transport of organic solutes such as sucrose, amino acids.

• Companion cells - usually one per sieve tube element, fully functional cell with lots of mitochondria.

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• Involved in translocation by unloading/loading sucrose into the sieve tube element and for metabolic support.

• Linked to the sieve tube element by plasmodesmata.

• Phloem fibre - same as xylem fibre

• Phloem parenchyma - same as xylem parenchyma.

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Xylem vessel element Xylem Tracheid

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•

• This is a file from the Wikimedia Commons

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summary

• In dicot stems, the cambium layer gives rise to phloem cells on the outside and xylem cells on the inside.

• All the tissue from the cambium layer outward is considered bark,

• while all the tissue inside the cambium layer to the center of the tree is wood.

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Phototropism

• Is growth response or movement of a plant in response to light coming from a specific direction.

• +ve phototropism

• -ve phototropism

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Plant Hormones

• Auxin

• Indole-3-acetic acid

• Works together with other hormones-

• Gibberellins

• Abscisic Acid

• Ethylene

• Cytokinins38

Two shoots-one in dark and other was exposed to blue light

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Second experiment- covering the tip with foil

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• Mica is impermeable

• Both plants grew straight

Conclusion-

Chemical compd. is produced in tip,

transported down

Mica is blocking

Tip is the source

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Block of agar usedplant grew towards the light

-block of agar used-plant grew towards the light-Auxin was able to diffuse thru the agar

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AUXIN• Produced in apical bud

• Transported down the stem

• Accumulates at the shaded side of the plant

• Stimulate growth- cell division and cell stretching.

• Plants grow towards light

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