PlantsPlantsChapters 21-23Chapters 21-23

Evolution of PlantsAdapting to Land• Nothing lived on land until an ozone

layer formed – the ozone layer offered protection from the sun’s intense UV radiation. With life on land came adaptations for survival:– Preventing water loss– Reproducing by spores & seeds– Abilities to transport materialsthroughout the plant

Preventing Water Loss• Cuticle – a waxy protective

covering that prevents water loss

• The cuticle kept water in, but kept O2 and CO2 out.

• Plants developed a stomata (little mouth) – a small opening in the leaf that allows for gas exchange.

Reproduction• Reproduction by spores and seeds

helps protect reproductive cells from drying out.

• spore – haploid (n) reproductive cell surrounded by a hard outer wall

• Seed – embryo, diploid (2n) surrounded by a protective coat– endosperm – tissue that provides

nourishment for the developing embryo

Transporting materials throughout the plant

• Vascular tissue – transports water and dissolved substances from one part of the plant to another

• Xylem – carries water and inorganic nutrients from the roots to the stems and leaves

• Phloem – carries organic compounds and some inorganic nutrients in any direction

• Some plants developed woody tissue and grew to great heights– This gave them an advantage in

gathering light.

• Woody tissue– Several layers of xylem– Usually brown, woody stems

• Non-woody plants– Herbaceous– Soft, green stems

Classifying Plants

Classifying Plants

Vascular

Non-VascularBryophytes

Seeds Mosses Liverworts HornwortsSeedless

Ferns ClubMosses

HorsetailsGymnospermUnprotected

seeds

AngiospermsProtectedseeds

FloweringPlants

Cone-Bearing

Monocots Dicots

Floweringplants

Cone-bearingplants

Ferns andtheir relatives

Mosses andtheir relatives

Green algaeancestor

Flowers; SeedsEnclosed in Fruit

Seeds

Water-Conducting(Vascular) Tissue

A Cladogram of Plant Groups

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Bryophytes, Seedless Plants, Gymnosperms &

Angiosperms• When we classify plants, what

question do we first ask?– Vascular or non-vascular?

• Let’s look at non-vascular first• Bryophytes do not have vascular

tissue

Bryophytes – Non-Vascular

• Bryophytes include:– Mosses (most common)– Liverworts– Hornworts

• Help! I Need Water!!!– Bryophytes have life cycles that

depend on water for reproduction– Since they don’t have vascular tissue,

the plants draw up water by osmosis.

• Dependence on water keeps bryophytes small in size

• Bryophytes are low-growing plants found in moist, shaded areas.

• Lack of vascular tissue means that bryophytes do not have true roots.

• Bryophytes have rhizoids – long, thin cells that anchor them to the ground and absorb water and minerals from the soil (by osmosis).

The gametophyte stage of moss

Liverworts

Hornworts

Reproduction

• For fertilization to occur, the sperm must swim to an egg.

• Because of this limiting factor, bryophytes must live near water.

• Remember, bryophytes need water to reproduce !!!

Vascular Plants• Vascular plants have vascular tissue• Xylem – carries water upward from

the roots to every part of the plant• Phloem – transports solutions of

nutrients and carbohydrates produce by photosynthesis (all directions)

• Produce lignin – makes cell walls rigid.– Allows vascular plants to grow

upright and reach great heights

• Vascular plants have true roots, leaves and stems

• Roots – underground organs that anchor plants, and absorb water and minerals

• Leaves – photosynthetic organs – one or more bundles of vascular tissue gathered into veins made of xylem and phloem

• Stems – supporting structures that connect roots and leaves, carrying water and nutrients between them.

• Vascular plants have a life cycle in which the sporophyte is the dominant stage.

• Remember, first we ask vascular or non-vascular?

• Next, we ask seeds or seedless?

Seedless vascular plants

• Include:– Whisk ferns– Club mosses– Horsetails– Ferns (most common)

• Seedless vascular plants have true vascular tissue, strong roots, rhizomes (underground stems), fronds (large leaves)

The whisk fern are rare. They are usually found in tropical and subtropical regions.

The club moss is sometimes known as a ground pine.

Horsetails

Tree ferns

Seed Plants• We have asked vascular or not-vascular?• We then asked seeds or seedless?• Seed plants can be further classified:

– Gymnosperms (unprotected seeds on cones)• Includes conifers, cycads, ginkoes,

gnetophytes– Angiosperms (flowering plants with

protected seeds)• Includes grasses, flowering trees and

shrubs, wildflowers and cultivated flowers

Reproduction free from water

• Adaptations that allow reproduction without water– Flowers or cones– Transfer of sperm by pollination– Protection of embryos in seeds

•A seed coat surrounds the embryo. It also contains a stored food supply for the embryo.

• Angiosperms can be further classified:– Named for the number of seed

leaves (cotyledons) in the plant embryo

– Monocots have one seed leaf– Dicots have two seed leaves

Monocots Dicots

Seeds

Leaves

Flowers

Stems

Roots

Single cotyledon

Parallel veins

Floral parts often in multiples of 3

Vascularbundlesscattered throughout stem

Fibrous roots

Two cotyledons

Branched veins

Floral parts often in multiplesof 4 or 5

Vascularbundlesarranged ina ring

Taproot

Comparison ofMonocots and Dicots

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Roots, Stems and Leaves

•The three principle organs of seed plants are:–Roots–Stems–Leaves

Roots• Two main types of roots

– Taproots – found mainly in dicots

– Fibrous roots – found mainly in monocots

• Two functions of roots– Anchor a plant in the ground– Absorb water and dissolved

nutrients from the soil

Stems

• Three functions of stems:– Produce leaves, branches and flowers– Hold leaves up in the sunlight– Transport substances between roots

and leaves•The vascular tissue of stems lifts water from the roots to the leaves (xylem) and sends the products of photosynthesis from the leaves to other parts of the plant (phloem)

Leaves• Main function is photosynthesis

– The broad, flat surface of leaves helps increase the amount of sunlight absorbed by plants

• Other functions of leaves:– Transpiration– Gas exchange

• Connects with exterior through stomata – allows CO2 and O2 to diffuse in and out of the leaf

• Each stoma consists of two guard cells that control the opening and closing of the stomata by responding to changes in water pressure.

Transpiration•Transpiration is the loss of water from a plant through its leaves.

•The water is replaced by water drawn into the leaf through the xylem vessels in the vascular tissue.

Evaporation of water molecules out of leaves.

Pull of water molecules upward from the roots.

A B

Gas Exchange• Plants keep their stomata open just

enough to allow photosynthesis to take place, but not so much that they lose an excessive amount of water.

• Guard cells control the stomata through changes in water pressure.– when water pressure in guard cells is

high, stoma is open– When water pressure in guard

cells is low, stoma is closed.

•In general, stomata open in the daytime, when photosynthesis is active, and close at night to prevent water loss.

•A plant will close its stomata any time water conservation is an issue.

Reproduction of Seed Plants• Reproduction in gymnosperms

takes place in cones.• Pollen cones – male cone –

produce pollen grains (male gametophytes)

• Seed cones – female cone – produce female gametophytes

• Gymnosperm pollen is carried by the wind to female cones (pollination)

Angiosperms

• Flowers are reproductive organs composed of four kinds of specialized leaves:• Sepals• Petals• Stamen• Carpels

• Sepals – resemble leaves. Enclose the bud before it opens; protect the flower while it is developing.

• Petals – often brightly colored (the pretty part). It attracts insects and other pollinators.

• Stamen (boy parts)• anther – produce male gametophytes

(pollen)• filament – supports anther

• Carpel (girl parts) sometimes called pistils•ovary – produces female

gametophytes•style – stalk•stigma – sticky portion where

pollen lands

FilamentAnther

StigmaStyle

Ovary

Carpel

PetalSepal

Ovule

Stamen

 The Structure of a Flower

Pollination• Most gymnosperms and some

angiosperms are wind pollinated• Most angiosperms are pollinated by

animals (beneficial to both plant and animal)

• For fertilization to occur, pollen grains land (or are carried) on the stigma of an appropriate flower.

• Pollen (male gametes) reach the ovary (where female gametes are made) and fertilization occurs

Seed Development & Germination

• Once fertilization occurs, nutrients flow into the flower tissue to support the growing embryo within the seed.

• As seeds mature, ovary walls thicken to form a fruit that encloses developing seeds

• A fruit is a ripened ovary that contains angiosperm seeds.

• The term “fruit” applies to any seed (even vegetables) enclosed within its embryo wall

• Why do angiosperms produce fruit?• To disperse seeds

• Two ways seeds can be dispersed• animals

• Typically contained in fleshy, nutritious fruits

• wind and water• Typically lightweight

• Allows them to be carried in the air or float on the surface of the water

• Some plants develop right away, others “wait”•Environmental factors like

temperature and moisture determine when a seed develops

•dormancy – embryo is alive but not growing

•germination – early growth stage of the plant embryo

Seed Germination

• Seed absorbs water• Absorbed water causes the

endosperm to swell, cracking open the seed coat

• Through the cracked seed coat, the young root emerges and begins to grow

Three categories of plant life spans

• annuals – flowering plants that complete a life cycle within one growing season

• biennials – complete their life cycle in two years

• perennials – flowering plants that live for more than two years

are categorized as

that completetheir life cycle in

that completetheir life cycle in

that completetheir life cycle in

Concept Map

Plants

Annuals Biennials Perennials

1 growingseason 2 years

More than2 years

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Plant Responses•The response of plants to environmental stimuli are called tropisms.•Tropisms demonstrate the ability of plants to respond effectively to conditions in which they live

Light & Gravity

•The response of a plant to light is known as phototropism

• Phototropism causes a plant to grow toward a light source

•The response of a plant to gravity is known as gravitropism

Gravitropism• Gravitropism causes the shoots of a germinating seed to grow out of the soil - against the force of gravity• It also causes the roots of a plant to grow with the force of gravity and into the soil

Response to Touch

•The response of plants to touch is called thigmotropism.

•An example of thigmotropism is the growth of vines and climbing plants.

Any Questions ???