Seed Plants: Angiosperms

Chapter 23

Outline Introduction Phylum Magnoliophyta – The Flowering Plants• Development of Gametophytes• Pollination• Fertilization and Development of the Seed• Apomixis and Parthenocarpy• Trends of Specialization and Classification in Flowering

Plants

Pollination Ecology

Introduction Angiosperms = flowering plants

Seeds enclosed in carpel – resembles folded over leaf and fused at margins• Pistil composed of single carpel, or >2 united carpels

Seed develops from ovule within carpel

Ovary becomes fruitBleeding hearts

Introduction Angiosperms = Phylum Magnoliophyta Divided into 2 large classes:• Magnoliopsida - Dicots

−DNA and cladistic evidence suggest 2 groups of dicots should be recognized

• Liliopsida - Monocots Flower = modified stem bearing modified leaves• Most primitive flower

−Long receptacle−Many spirally arranged flower parts that are

separate and not differentiated into sepals and petals (= tepals)

−Flattened and numerous stamens and carpels

Phylum Magnoliophyta – The Flowering Plants

Heterosporous

Sporophytes dominant

Female gametophytes wholly enclosed within sporophyte tissue and reduced to only few cells

Male gametophytes consist of germinated pollen grain with 3 nuclei

Phylum Magnoliophyta Development of gametophytes - Female:• 2n megasporocyte differentiates in ovule

−Undergoes meiosis and produces 4 1n megaspores3 degenerate

• Remaining cell enlarges and nucleus divides to produce 8 nuclei (without walls)

• Outer 2 layers of ovule differentiate into integuments that later become seed coat−Micropyle at one end of ovule

Phylum Magnoliophyta Development of gametophytes – Female cont’d.:• 8 nuclei form 2 groups, 4 near each end of cell• 1 nucleus from each group migrates to cell middle and

form central cell• Cell walls form around

remaining 6 nuclei−Egg and 2 synergids

closest to micropyle−3 antipodals at

opposite end• Female gametophyte =

megagametophyte or embryo sac

Phylum Magnoliophyta Development of gametophytes - Male:• Formation of male gametophytes takes place in

anthers• 4 patches, corresponding to pollen sacs, of

microsporocyte cells differentiate in anther• Each

microsporocyte undergoes meiosis to produce 4 1n microspores

Anther with microspores

Phylum Magnoliophyta Development of gametophytes – Male cont’d.:• Microspores undergo 3 changes:

− Divide once by mitosis to form small generative cell inside larger tube cellNucleus of tube cell = vegetative nucleus

−Members of each quartet of microspores separate−Wall becomes 2-layered

Outer layer (= exine) finely sculptured and contains chemicals that react with chemicals in stigma

• Generative nucleus divide to produce 2 sperm

Pollen grain

Phylum Magnoliophyta Pollination:• Pollination - transfer of pollen grains from anther to

stigma−Self-pollination - pollen grains germinate on

stigma of same flower

• Fertilization - union of sperm and egg

• Pollination by insects, wind, water, animals or gravity

Phylum Magnoliophyta Fertilization and development of the seed:• After pollination, further development of male

gametophyte may not take place unless pollen grain:−From different plant of same species−From variety different from that of receiving flower

• Pollen tube grows between cells of stigma and style until reaches ovule micropyle

• Vegetative nucleus stays at tips of pollen tube, while generative cell lags behind and divides into 2 sperm

• Pollen tube enters female gametophyte, destroying synergid in process, and discharges sperms

Phylum Magnoliophyta Fertilization and development of the seed cont’d.:

• Mature male gametophyte = germinated pollen grain with vegetative nucleus and 2 sperms within tube cell

Phylum Magnoliophyta Fertilization and development of the seed cont’d.:• Double fertilization:−1 sperm unites with egg, forming zygote,

then embryo

−Other sperm unites with central cell nuclei, producing 3n endosperm nucleus developing into endosperm tissueEndosperm tissue = nutritive tissue for

embryo

Phylum Magnoliophyta Fertilization and development of the seed cont’d.:

Endosperm becomes extensive part of seed in some monocots (i.e., corn and other grasses)

Endosperm absorbed into cotyledons in most dicots

−Ovule becomes seed, ovary matures into fruit, integuments harden into seed coat

Phylum Magnoliophyta Fertilization and development of the seed cont’d.:• Other types of (female) gametophyte

development:−Female gametophyte can have from 4 to 16

nuclei or cells at maturity−Endosperm may be 5x, 9x or 15x

Phylum Magnoliophyta Apomixis and parthenocarpy: • Apomixis - without fusion of gametes but with

normal structures otherwise being involved−Embryo from 2n nutritive cell or other 2n

cell of ovule, instead of from zygoteResults in vegetatively propagated plant

−Parthenocarpy - fruits develop from ovaries with unfertilized eggs.

−Results in seedless fruitsNavel oranges and bananas

Phylum Magnoliophyta Trends of specialization and classification in

flowering plants:• 1st historical classifications for convenience• Modern botanists group plants according to natural

relationships based on evolution• Fossil record suggests flowering plants 1st appeared

about 160 mya during late Jurassic• Flowering plants developed during Cretaceous and

Cenozoic• Dominant plants today

Phylum Magnoliophyta Trends of specialization and classification in flowering

plants cont’d.:• 1st pistil from leaflike structure with ovules along

margins = carpel−Edges of blade rolled inward and fused together

• Separate carpels of primitive flowers fused together to form compound pistil consisting of several carpels

Phylum Magnoliophyta Trends of specialization and classification in flowering

plants cont’d.:• Inferior ovary (epigynous

flower) - receptacle or other flower parts fused to ovary and grown up around it

• Superior ovary (hypogynous flower) - ovary produced on top of receptacle

• Perigynous flowers - flower parts attached to corolla tube of fused petals, creating floral tube not attached to ovary

Phylum Magnoliophyta Trends of specialization and classification in flowering

plants cont’d.:• Complete flower - has

calyx, corolla, stamens and pistil

• Incomplete flower - corolla or other flower parts missing

• Perfect flower - both stamens and pistil present

Male flower

Female flower with inferior ovary

Phylum Magnoliophyta Trends of specialization and classification in flowering

plants cont’d.:• Imperfect flower - either

stamens or pistil missing– Monoecious species -

male and female imperfect flowers on same plant

– Dioecious species - plant bears only male flowers and other plants bear only female flowers

Male flower

Female flower with inferior ovary

Phylum Magnoliophyta Trends of specialization and classification in flowering plants

cont’d.:• Primitive flowering plants

−Simple leaves−Flower with numerous, spirally arranged parts, not fused

to each other• Flowers radially symmetrical = regular.−Flowers complete

and perfect−Superior ovary

(hypogynous flower)

Magnolia

Phylum Magnoliophyta Trends of specialization and classification in flowering

plants cont’d.:• Specialized flowering plants:

−Flower parts fewer and definite in #−Spiral arrangements compressed to whorls−Bilaterally symmetrical flowers = irregular

Orchid

Phylum Magnoliophyta Trends of specialization and classification in flowering

plants cont’d.:• Specialized flowering plants cont’d.:

−Reduction and fusion of parts Incomplete or imperfect flowers

− Inferior ovary

Orchid

Pollination Ecology

Pollinators co-evolved with plants• 20,000 bee species among current-day pollinators• Bee-pollinated flowers:

−Generally brightly colored, mostly blue or yellow−Often have lines or distinctive markings, may

function as guides to lead bees to nectar Bees see UV light (humans do not)

In ordinary light In UV light

Pollination Ecology

Beetle-pollinated flowers:• Strong, yeasty, spicy or fruity odor• White or dull in color• Some do not secrete nectar, but furnish pollen or

food on petals in special storage cells

Fly-pollinated flowers:• Smell like rotten meat• Dull red or brown

Pollination Ecology

Butterfly- and moth-pollinated flowers:• Often have sweet fragrances• White or yellow for night-flying moths• Sometimes red, often blue, yellow or orange for

butterflies• Nectaries at bases of corolla tubes or spurs for

long tongues

Pollination Ecology

Bird-pollinated flowers (hummingbirds and sunbirds):• Often bright red or yellow• Little if any odor - Birds don’t have keen sense of

smell• Large and part of sturdy inflorescence• Copious amounts of nectar - Birds highly active• Long floral tubes

Pollination Ecology

Bat-pollinated flowers:• Primarily in tropics• Open at night when bats

foraging• Dull in color• Large enough for bat to

insert head or consist of ball-like inflorescence containing large numbers of small flowers

Pollination Ecology

Orchid flowers:• Have pollinators among all types mentioned• Some adaptations between orchid flowers and

pollinators extraordinary.• Pollen grains produced in little sacs called pollinia

(singular: pollinium) with sticky pads at base

Ophrys

Review Introduction Phylum Magnoliophyta – The Flowering Plants• Development of Gametophytes• Pollination• Fertilization and Development of the Seed• Apomixis and Parthenocarpy• Trends of Specialization and Classification in Flowering

Plants

Pollination Ecology