Topic 9: Plant ScienceTopic 9: Plant Science

9.1 Plant Structure & Growth9.1 Plant Structure & Growth

Plan diagram: STEM of Plan diagram: STEM of dicotyledondicotyledon

Distribution of tissues in a Distribution of tissues in a stemstem

StructureStructure FunctionFunction

EpidermisEpidermis Thin layer of Thin layer of cells for cells for protectionprotection

XylemXylem For water For water transporttransport

PhloemPhloem For For transporting transporting organic organic moleculesmolecules

PithPith Storage and Storage and supportsupport

CambiumCambium Dividing tissueDividing tissue

CortexCortex Storage and Storage and supportsupport

Differences: Differences: monocotyledonous vs. monocotyledonous vs. dicotyledonous plantsdicotyledonous plants

Monocotyledons Dicotyledons

ParallelParallel veins in veins in narrownarrow leavesleaves

BranchingBranching, net-like veins , net-like veins in in broadbroad leaves leaves

Embryo has ONE Embryo has ONE cotyledoncotyledon

Embryo has TWO Embryo has TWO cotyledonscotyledons

Flower parts arranged in Flower parts arranged in threesthrees

Flower parts arranged in Flower parts arranged in foursfours or or fivesfives

Fibrous adventitious Fibrous adventitious rootsroots

Tap root with lateral Tap root with lateral branchesbranches

Eg. maize, wheat, oatEg. maize, wheat, oat Eg. bean, daisy, Eg. bean, daisy, sunflowersunflower

Distribution of tissues in a Distribution of tissues in a leafleaf

Relationship between tissue Relationship between tissue distribution in a leaf and distribution in a leaf and

functionfunction Palisade cells contain lots of chloroplasts Palisade cells contain lots of chloroplasts

for absorbing light.for absorbing light. Palisade cells arranged “end-on” to Palisade cells arranged “end-on” to

ensure each cell receives maximum ensure each cell receives maximum strength light.strength light.

No chloroplasts in upper epidermis to No chloroplasts in upper epidermis to ensure light reaches palisade layer.ensure light reaches palisade layer.

Chloroplasts present in spongy mesophyll Chloroplasts present in spongy mesophyll cells to ‘mop up’ any unused light.cells to ‘mop up’ any unused light.

Relationship between tissue Relationship between tissue distribution in a leaf and distribution in a leaf and

functionfunction Waxy cuticle to prevent water loss from Waxy cuticle to prevent water loss from

epidermal cells by evaporation.epidermal cells by evaporation. Air spaces in spongy mesophyll layer to Air spaces in spongy mesophyll layer to

ensure adequate supply of CO2 to ensure adequate supply of CO2 to photosynthesizing cells and ease of photosynthesizing cells and ease of removal of O2.removal of O2.

Stomata located on leaf lower surface to Stomata located on leaf lower surface to allow gases in and out of leaf.allow gases in and out of leaf.

Presence of vascular bundle to supply Presence of vascular bundle to supply water from roots and method for removal water from roots and method for removal of products of photosynthesis.of products of photosynthesis.

Modifications of roots, stems, Modifications of roots, stems, leaves for different functionsleaves for different functions

BulbsBulbs – these are – these are underground storage underground storage structures that structures that contain reserves of contain reserves of nutrients to ensure nutrients to ensure survival.survival.

Fleshy leaf bases Fleshy leaf bases closely packed on a closely packed on a short stem.short stem.

Eg. onion, garlicEg. onion, garlic

Modifications contd.Modifications contd.

Stem tubersStem tubers – – swollen tip of swollen tip of rhizome or rhizome or underground stemunderground stem

Storage of Storage of carbohydrate for carbohydrate for growth of new growth of new plantsplants

Eg. potatoEg. potato

Modifications contd.Modifications contd.

Storage rootsStorage roots – – these contain stores these contain stores of carbohydrate for of carbohydrate for the plant to use the plant to use later. later.

They are usually They are usually swollen tap roots swollen tap roots (primary roots)(primary roots)

Eg. carrot, sweet Eg. carrot, sweet potatopotato

Modifications contd.Modifications contd.

TendrilsTendrils – slender – slender stem-like structure stem-like structure to wrap around or to wrap around or hook a support.hook a support.

Sensitive to touch, Sensitive to touch, so faster growth on so faster growth on the opposite side the opposite side occurs.occurs.

Eg. vine, ivyEg. vine, ivy

Apical v Lateral MeristemsApical v Lateral Meristems Dicotyledonous plants have Dicotyledonous plants have apicalapical (primary) (primary)

and and laterallateral (cambium) meristems. (cambium) meristems. Meristems generate new cells for growth of Meristems generate new cells for growth of

the plant.the plant. Apical meristems found at the tips of stems Apical meristems found at the tips of stems

and roots increase the length of these and roots increase the length of these sections.sections.

Stems and roots may also grow in thickness Stems and roots may also grow in thickness or in diameter through cell divisions in lateral, or in diameter through cell divisions in lateral, or secondary, meristems, found just under the or secondary, meristems, found just under the surface along the length of the stem or root. surface along the length of the stem or root.

PhototropismPhototropism

Phototropism is generally described Phototropism is generally described as a plant's response to light. as a plant's response to light.

There are two kinds of phototropism: There are two kinds of phototropism: positive phototropismpositive phototropism, or the growth , or the growth of a plant stem towards light, and of a plant stem towards light, and negative phototropismnegative phototropism, or the growth , or the growth of a plant's roots away from light.of a plant's roots away from light.

Phototropic responses are caused by Phototropic responses are caused by auxinsauxins, which modify cell walls. , which modify cell walls.

More on phototropismMore on phototropism

Auxins concentrate on the side of a Auxins concentrate on the side of a plant stem plant stem away from lightaway from light, in order , in order to stretch rigid plant cell walls into to stretch rigid plant cell walls into growth growth toward lighttoward light..

Auxin is produced in the Auxin is produced in the tipstips of of shoots and roots.shoots and roots.

Yet more on phototropismYet more on phototropism

Auxin acts as a Auxin acts as a growth promoter, growth promoter, possibly by causing possibly by causing HH++ ions to be secreted ions to be secreted into cell walls.into cell walls.

This causes a This causes a loosening of the loosening of the connections between connections between cellulose fibres, cellulose fibres, allowing cell allowing cell expansion.expansion.

Phototropism diagramPhototropism diagram

http://www.biology-online.org/3/7_mehttp://www.biology-online.org/3/7_meristems.htmristems.htm

http://www.biology-online.org/3/5_plahttp://www.biology-online.org/3/5_plant_hormones.htmnt_hormones.htm