Seed Plants

The Evolution of Seed Plants

Seed plants first began to diversify from their seedless ancestor 319 MYA

• Evolved from spore-bearing plants known as progymnosperms

Success attributed to evolution of seed

• Protects and provides food for embryo

• Allows the “clock to be stopper” to survive harsh periods before germinating

• Later development of fruits enhanced dispersal

Seeds & the Embryo

Embryo protected by integument

• An extra layer or two of sporophyte tissue

• Hardens into seed coat

Megasporangium divide meiotically inside ovule to produce haploid megaspore.

Megaspore produces egg that combines with sperm to form zygote.

Also contain food supply for embryo.

Two Kinds of Gametophytes

Male Gametophyte

• Pollen Grains

• Dispersed by wind or a pollinator

• No need for water

Female Gametophyte

• Develop within an ovule

• Enclosed within diploid sporophyte tissue in angiosperms

• Ovule and protective tissue are the ovary

• The ovary develops into fruit

Five Phyla of Extant Seed Plants

• Coniferophyta

• Ginkgophyta

• Cycadophyta

• Gnetophyta

• Anthophyta

Gymnosperms

Plants with “naked seeds”

There are four living groups:

• Coniferophytes

• Cycadophytes

• Gnetophytes

• Ginkgophytes

All lack flowers and fruits of angiosperms

All have ovule exposed on a scale

Conifers (Phylum Coniferophyta)

• Most familiar gymnosperm phylum

• Pines, spruces, firs, cedars, and others

  • Coastal redwood - Tallest living vascular plant

  • Bristlecone pine - Oldest living tree

• Found in colder and sometimes drier regions of the world

• Conifers are sources of important products

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

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

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 contain a megasporangium that is known as the nucellus

Female Pine Cones

The nucellus is surrounded by the integument

• Micropyle : small opening at end of integument

• Seed coat forms from a layer of integument

One megaspore mother cell within each megasporangium forms 4 megaspores via meiosis

• 3 megaspores breaks down

• 1 slowly develops into a female gametophyte via mitosis

Female Gametophyte

• Female gametophyte has archegonia at the micropylar end

• Each archegonium has a large egg

Fertilization

• Female cones usually take 2 or more seasons to mature

• During the first spring, pollen grains drift down between open scales

  • Pollen grains drown down into microphyle

  • 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

  • 1 sperm unites with egg = zygote

  • Other sperm degenerates

Cycads (Phylum Cycadophyta)

• Slow-growing gymnosperms of tropical and subtropical regions

• Sporophytes resemble palm trees

• Female cones can weigh 45kg

• Have largest sperm cells of all organisms

Gnetophytes (Phylum Gnetophyta)

Contain 3 (unusual) genera

• Welwitschia

• Ephedra

• Gnetum

Ginkgophytes

Only one living species remains

• Ginkgo biloba

Flagellated sperm

Dioecious

• Male and Female reproductive structures form on different trees

Angiosperms

• Flowering plants

• Ovules are enclosed in diploid tissue at the time of pollination

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

Angiosperm Abundance

• The emergence of angiosperms changed the terrain of Earth

  • Previously dominated by ferns, cycads, and conifers

• Unique angiosperm features aided abundance

  • Flower production, insect pollination, broad leaves with thick veins

Angiosperm origins are a mystery

Oldest known angiosperm in the fossil record is Archaefructus

• 125 million years old

• Unlikely to have been the first angiosperm

• Lack sepals and petal

Flowers house the Gametophyte Generation

Flower Morphology

• Modified stems bearing modified leaves

• Primordium develops into a bud at the end of a stalk called the **pedice**l

• Pedicel expands at the tip to form a receptacle, to which other parts attach

• Flower parts are organized in circles called whorls

Flower Whorls

Outermost whorl - sepals

Second whorl - petals

Third whorl - stamens

Innermost whorl - Pistil (aka Carpel)

The Carpel

Carpel has 3 major regions

• Ovary - swollen base containing ovules

  • Later develops into a fruit

• Stigma - tip where pollen lands

• Style - neck or stalk

Double Fertilization

A single diploid megaspore mother cell in ovule undergoes meiosis

• Produces 4 haploid megaspores

  • 3 disappear

  • Nucleus of remaining megaspore divides mitotically

Embryo Sac

Daughter nuclei divide to produce 8 haploid nuclei in 2 groups of 4

• 2 nuclei ( 1 from each group of 4 ) migrate toward center

  • Function as polar nuclei - may fuse

• Cell closest to the micropyle becomes the egg

• 2 other cells are synergids

• Antipodals are the 3 cells at the other end - they have no function

-Integuments become the seed coat

-The 8 haploid nuclei in 7 cells make up the female gametophyte

• Also known as the embryo-sac

Pollen Production

Pollen production occurs in the anthers

• It is similar but less complex than female gametophyte formation

• Diploid microspore mother cells undergo meiosis to produce 4 haploid microspores

• Binucleate microspores become pollen grains

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

• One of the 2 pollen grain cells lags behind

  • This generative cell divides to produce two sperm cells

  • No flagella on sperm

Double Fertilization & Seed Formation

• One sperm unites with egg to form the diploid zygote

  • New Sporophyte

• Other sperm unites with the 2 polar nuclei to form the triploid endosperm

  • Provides nutrients to embryo

• Seed may remain dormant for many years

• Germinate when conditions are favorable

Seeds

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

• Integuments develop into a relatively impermeable seed coat

• Encloses the seed with its dormant embryo and stored food

How Seeds Protect Embryos

• They maintain dormancy under favorable conditions

• They protect the young plant when it is most vulnerable

• They provide food for the embryo until it can produce its own food

• They facilitate dispersal of the embryo

The Importance of the Seed Coat

• 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

• Specific adaptations ensure that seeds will germinate only under appropriate conditions

Fruits

• Most simply defined as mature ovaries ( carpels )

• During seed formation, the flower ovary begins to develop into fruit

• It is possible for fruits to develop without seed development

  • Bananas are propagated asexually

Fruit Development

The ovary wall is termed the pericarp

• 3 layers : exocarp, mesocarp and endocarp

• Their fate determines the fruit type

Fruits contain 3 genotypes in 1 package

• Fruits and seed coat from prior sporophyte generation

• Remnants of gametophyte generation produced egg

• Embryo represents next sporophyte generation