Chapter 30 Flashcards
Seed plants have come to dominate the terrestrial landscape over the last
several hundred million years
Over 400,000 species of seed plants today
Evolved from spore-bearing plants known as progymnosperms.
Success attributed to evolution of seed
Protects and provides food for embryo.
Allow embryos to pause development and growth and germinate after a harsh winter or dry season has passed
Seeds and the Embryo
Embryo protected by integument
An extra layer or two of sporophyte tissue.
Hardens into seed coat.
Megasporangium divides meiotically inside ovule to produce haploid megaspore
Megaspore divides by mitosis to produce a female gametophyte carrying the female gamete, an egg
Also contain food supply for embryo
Two kinds of gametophytes
Male gametophytes
Pollen grains.
Carried to female gametophyte by wind or a pollinator.
No need for water.
Female gametophytes
Develop within an ovule.
Enclosed within diploid sporophyte tissue in angiosperms.
Ovule and protective tissue are the ovary.
The ovary develops into fruit.
Gymnosperms
Plants with “naked seeds”
Encompass four of the five lineages of seed plants
Coniferophytes.
Cycadophytes.
Gnetophytes.
Ginkgophytes.
All have ovule exposed on a scale
All lack flowers and fruits of the fifth lineage, angiosperms
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 temperate 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 minimize water loss
Leaves have canals with resin to deter insect and fungal attacks
Wood lacks some of the more rigid cell types found in other trees, which is why they are sometimes referred to as “soft” wood
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 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 four megaspores via meiosis
3 megaspores break down.
1 slowly develops into a female gametophyte via mitosis.
Female Gametophyte
At maturity, the female gametophyte may consist of thousands of cells
Female gametophyte has 2 to 6 archegonia at the micropylar end
Each archegonium has a single large egg
Three mitotic divisions of the megaspore nucleus produces 8 haploid nuclei in 2 groups of 4
2 nuclei (1 from each group of 4) migrate toward center.
Cell wall forms around the polar nuclei to form the central cell.
Cell closest to the micropyle becomes the egg.
2 other cells in that group become synergids.
Antipodals are the 3 cells at other end – they have no apparent function.
Integuments become the seed coat
The 8 haploid nuclei in 7 cells make up the female gametophyte
Also known as the embryo sac
Conifer Pollination
Female cones usually take two or more seasons to mature
During the first spring, pollen grains drift down between open scales
Pollen grains drawn down into micropyle.
Scales close.
Conifer Fertilization
A year later, female gametophyte matures
Pollen tube is digesting its way through nucellus
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.
Cycads (phylum Cycadophyta)
Slow-growing gymnosperms of tropical and subtropical regions
Sporophytes of most 250 known species resemble palm trees
Individuals make either pollen-bearing or ovule-bearing cones, not both
Female cones are huge (up to 45 kg)
Sperm is formed in pollen tube but released within ovule to swim to archegonium
Gnetophytes (phylum Gnetophyta)
Only gymnosperms with vessels in their xylem
Contain 65 species in three (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 encapsulates seeds, develops into fruit
Angiosperm Abundance
Today there are some 300,000 species of flowering plants
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
At least 125 million years old.
Unlikely to have been the first angiosperm.
Multiseeded carpels (fruits) and stamens
Lack sepals and petals.
Closest living relative to the original angiosperm is Amborella
Horizontal gene transfer explains the presence of moss mitochondrial genes in the genome of Amborella
Flowers House Gametophytes
Flowers are modified stems bearing modified leaves.
Originate as primordium that develops into a bud at the end of a stalk called a pedicel.
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 (androecium)
Pollen is the male gametophyte.
Each stamen has a pollen-bearing anther and a filament (stalk).
Innermost whorl – gynoecium
Consists of one or more carpels.
House the female gametophyte.
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 connecting stigma and ovary
Ovule
A single diploid megaspore mother cell in ovule undergoes meiosis to produce 4 haploid megaspores.
3 disappear.
Nucleus of remaining megaspore divides mitotically to form female gametophyte
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 four haploid microspores.
All four at first remain together as a tetrad
The nucleus of each one divides once by mitosis
Binucleate microspores become pollen grains.
Pollination
Mechanical transfer of pollen from anther to stigma
May or may not be followed by fertilization
If the stigma is receptive, pollen grains develop a pollen tube that is guided to the embryo sac
One of the two pollen grain cells lags behind
This generative cell divides to produce two sperm cells.
No flagella on sperm.
