Chapter 26 lecture presentation

Seed Plants

Chapter 26

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The Evolution of Seed Plants

• Seed plants first appeared 305–465 mya• Success attributed to evolution of seed

– Seed includes embryo, food for the embryo, and a seed coat

– Allows the “clock to be stopped” to survive harsh periods before germinating

– “Naked” seeds first, then later development of fruits enhanced dispersal

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• Seed plants produce 2 kinds of gametophytes• Male gametophytes

– Pollen grains– Dispersed by wind or a pollinator– No need for water

• Female gametophytes – Develop within an ovule– Enclosed within diploid sporophyte tissue in

angiosperms

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Five Phyla of Extant Seed Plants

• Coniferophyta• Cycadophyta• Gnetophyta• Ginkophyta• Anthophyta

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Gymnosperms

• Plants with “naked seeds”• There are four living groups

• Coniferophytes• Cycadophytes• Gnetophytes• Ginkgophytes

• All lack flowers and fruits of angiosperms• All have the ovule exposed on a “scale”

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Ang

iosp

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Gym

nosp

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Fern

s an

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llies

Conifers (phylum Coniferophyta)

• Most diverse and familiar gymnosperm phylum

• Pines, spruces, firs, cedars, and others• Found in colder and sometimes drier

regions of the world• Conifers are sources of important products

– Timber, paper, resin, and taxol (anti-cancer)

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• Pines– More than 100 species, all

in the Northern hemisphere

– Produce tough needlelike leaves in clusters

– Leaves have thick cuticle and recessed stomata to retard water loss

– Leaves have canals with resin to deter insect and fungal attacks

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• Pine reproduction• Male gametophytes (pollen grains)

– Develop from microspores in male cones by meiosis

• Female pine cones form on the upper branches of the same tree– Female cones are larger, and have woody

scales– Two ovules develop on each scale– Each contains a megasporangium

• Each will become a female gametophyte

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Microsporemother cell

Microspores PollenAir bladder

Pollination

Pollen tube

SpermMITOSIS

Zygote

Section of seed (second year),showing embryo embeddedin megagametophyte

MITOSIS

Pineseed

MITOSIS

Seedling

Sporophyte

Pollen-bearingcone

Scale

Megasporemother cell

Ovulate(seed-bearing)cone

Megaspore

Pollen tube

n2n

(15 monthsafter pollination)

MEIO

SIS

FERTILIZATION

• Female cones usually take 2 or more seasons to mature• During the first spring, pollen grains drift down between open

scales– Pollen grains drawn down into micropyle– Scales close

• A year later, female gametophyte matures– Pollen tube is digesting its way through– Mature male gametophyte has 2 sperm

• 15 months after pollination, pollen tube reaches archegonium and discharges contents– One sperm unites with egg = zygote– Other sperm degenerates

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Cycads (phylum Cycadophyta)

• Slow-growing gymnosperms of tropical and subtropical regions

• Sporophytes resemble palm trees

• Female cones can weigh 45 kg

• Have largest sperm cells of all organisms!

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a.© Luca Invernizzi Tetto/agefotostock


Gnetophytes (phylum Gnetophyta)

• Only gymnosperms with vessels in their xylem

• Contain three (unusual) genera– Welwitschia– Ephedra– Gnetum

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b.© Juan Carlos Muñoz/agefotostock


Ginkgophytes (phylum Ginkgophyta)

• Only one living species remains– Ginkgo biloba

• Flagellated sperm• Dioecious

– Male and female reproductive structures form on different trees

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c. © Robert Gustafson/Visuals Unlimited


c.c.

Angiosperms

• Flowering plants • Ovules contain eggs,

and become seeds after egg fertilization

• Carpel, a modified leaf that covers seeds, develops into fruit

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Ferns and Allies

Angiosperms

Gymnosperms


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(bottom right): © Goodshoot/Alamy RF

Fusion ofleaf margins

Modified leafwith ovules

Cross section

Ovules(seeds)

Carpel(fruit)

Folding of leafprotects ovules

Ovules

Angiosperm origins are a mystery

– Origins as early as 145–208 MYA

– Oldest known angiosperm in the fossil record is Archaefructus

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Fruits

© David Dilcher and Ge Sun

Pairedstamens

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Archaefructus may be the sister clade to all other angiosperms

Gymnosperms AngiospermsGinkgo Gnetophytes Conifers Cycads Archaefructus

(extinct)Amborella Water lillies MonocotsMagnoliidsEudicotsStaranis


Flowers contain the gametophyte generation

• Flower morphology– All flower parts have evolved either from

stems or leaves– Flower parts are organized in circles called

whorls

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• Flower whorls– Outermost whorl – sepals– Second whorl – petals– Third whorl – stamens (androecium)

• Each stamen has an anther (where the pollen is) and a filament (stalk)

• Pollen is the male gametophyte– Fourth whorl - gynoecium

• Consists of one or more carpels• Contain the female gametophyte

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• Carpel has 3 major regions– Ovary – contains the ovules

• The ovary later develops into a fruit, and the ovules inside become the seeds

– Stigma – tip where pollen lands– Style – neck or stalk

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Carpel

Stigma

Style

Ovule

Ovary wall

StamenAntherFilament

PetalSepal

Ovary

a.


Megaspore (n)

GenerativecellTubenucleus

Pollen tubeSpermTubenucleus

Style

Pollen (n)

Formation ofpollen tube (n)

Polarnuclei

Sperm

Egg

Polarnuclei

Egg

MIT

OSI

S

MIT

OSI

S

Zygote

Embryo (2n)

Cotyledons

Seed (2n)

Endosperm (3n)

Endosperm

Anther

Ovary

Stigma Anther (2n)

Ovule

2n

DO

UB

LEFE

RTI

LIZA

TIO

N

MIT

OSIS

GERMINATIO

N

Microsporemother cells (2n)

Megasporemother cell (2n)

Adult sporophytewith flower (2n)

Youngsporophyte (2n)

Seedcoat

MITOSIS


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• Pollination – Mechanical transfer of pollen from anther to

stigma– May or may not be followed by fertilization– Pollen grains develop a pollen tube that is

guided to the embryo sac– The “generative cell” divides to produce two

sperm cells• No flagella on sperm

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• Double fertilization – One sperm unites with egg to form the diploid

zygote (which becomes the new sporophyte embryo)

– Other sperm unites with the two polar nuclei to form the triploid endosperm

• Provides nutrients to new sporophyte embryo

• Seed may remain dormant for many years– Germinate (grow by mitosis) when conditions

are favorable

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Seeds

• In many angiosperms, development of the embryo stops soon after meristems and cotyledons differentiate

• Integuments develop into a relatively impermeable seed coat

• Encloses the seed with its dormant embryo and stored food

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Shoot apical meristem

Root apicalmeristem

Root cap

Endosperm

Procambium

Seed coat(integument)

Cotyledons

• Seeds are an important adaptation1. They maintain dormancy under unfavorable

conditions2. They protect the young plant when it is most

vulnerable3. They provide food for the embryo until it can

produce its own food4. They facilitate dispersal of the embryo

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• Once a seed coat forms, most of the embryo’s metabolic activities cease

• Germination cannot take place until water and oxygen reach the embryo

• Seeds of some plants have been known to remain viable for thousands of years

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• Specific adaptations ensure that seeds will germinate only under appropriate conditions– Some seeds lie within tough cones that do not

open until exposed to fire

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a: © Ed Reschke; b: © David Sieren/Visuals Unlimited

Fruits

• Most are simply defined as “mature ovaries” (carpels)

• At the same time seeds are forming, the flower ovary begins to develop into a “fruit”

• However, it is also possible for fruits to develop without seed development– Bananas are propagated asexually

• Not all fruits are something you would want to eat!

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True Berries

The entire pericarp isfleshy, although theremay be a thin skin.Berries have multipleseeds in either one ormore ovaries. The tomato flower had four carpels that fused.Each carpel containsmultiple ovules thatdevelop into seeds.

Outer pericarpSeedFused

carpels

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Drupes

Seed

PericarpExocarp (skin)MesocarpEndocarp (pit)

Single seedenclosed in a hard pit;peaches, plums,cherries. Eachlayer of the pericarp hasa different structureand function, withthe endocarpforming the pit

Samaras

PericarpNot split andwith a wingformed from theouter tissues;maples, elms,ashes.

Seed

(left): © Kingsley Stern; (right): Courtesy of Robert A. Schisling


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Aggregate Fruits

Sepals of asingle flower

Ovary

Seed

Derived from manyovaries of a singleflower; strawberries,blackberries.Unlike tomato,these ovariesare not fusedand coveredby a continuouspericarp.

Multiple Fruits

Individual flowers form fruitsaround a single stem. The fruitsfuse as seen with pineapple. Pericarp of

individual flower

Main stem

(left): Courtesy of Robert A. Schisling; (right): © Charles D. Winters/Photo Researchers, Inc.


Fruit Dispersal

• Occurs through a wide array of methods– Ingestion and transportation by birds or other

vertebrates– Hitching a ride with hooked spines on birds

and mammals– Burial in caches by herbivores– Blowing in the wind– Floating and drifting on water

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Education

Chapter 26 lecture presentation