Biology, SEED Plants CH30

Raven Biology CH30 Seed Plants

The Evolution of Seed Plants, Seed plants have come to dominate the terrestrial landscape

over the last several hundred million years

The Evolution of Seed Plants, Success attributed to evolution of seed

protects and provides food for embryo, allows embryos to pause development and germinate after harsh conditions

Seeds and the Embryo, Embryo protected by integument

an extra layer or two of sporophyte tissue that hardens into seed coat

Seeds and the Embryo, Megasporangium divides meiotically inside ovule to produce

haploid megaspore which divides by mitosis to produce a female gametophyte carrying an egg and food supply

Two kinds of gametophytes, Male gametophytes

are pollen grains, carried by wind or pollinator, no need for water

Two kinds of gametophytes, Female gametophytes

develop within an ovule enclosed within diploid sporophyte tissue in angiosperms, the ovule and protective tissue are the ovary which develops into fruit

Five Phyla of Extant Seed Plants, Coniferophyta

heterosporous seed plants, sperm not motile, leaves needle-like or scalelike, produce seeds in cones, ~630 species

Five Phyla of Extant Seed Plants, Cycadophyta

heterosporous, sperm flagellated but confined within pollen tube, palmlike plants with pinnate leaves, seeds in cones, ~306 species

Five Phyla of Extant Seed Plants, Gnetophyta

heterosporous, sperm not motile, only gymnosperms with vessels in vasculature, ~65 species

Five Phyla of Extant Seed Plants, Ginkgophyta

heterosporous, sperm flagellated but accesses egg by pollen tube, deciduous tree with fan-shaped leaves, one species

Five Phyla of Extant Seed Plants, Anthophyta

flowering plants, seeds enclosed within a fruit, most significant plant contributor to human and animal food, >300,000 species

Gymnosperms, Plants with “naked seeds”

encompass four of the five lineages of seed plants, all have ovule exposed on a scale, lack flowers and fruits

Conifers (phylum Coniferophyta), Most familiar gymnosperm phylum

includes pines, spruces, firs; found in colder temperate regions; sources of timber, paper, resin, taxol

Conifers (phylum Coniferophyta), Coastal redwood is

tallest living vascular plant; bristlecone pine is oldest living tree

Pines, there are

100+ species in Northern hemisphere, produce needlelike leaves in clusters

Pines, Leaves have

thick cuticle and recessed stomata to minimize water loss, canals with resin to deter pests

Pines, Wood

lacks rigid cell types found in other trees, so referred to as "soft" wood

Pine reproduction, Male gametophytes (pollen grains)

develop from microspores in male cones by meiosis

Pine reproduction, Female pine cones

form on upper branches, are larger with woody scales, two ovules develop on each scale

Female Pine Cones, The nucellus is surrounded by the integument

micropyle is small opening at end, seed coat forms from a layer of integument

Female Pine Cones, One megaspore mother cell within each megasporangium forms four megaspores via meiosis

three break down, one develops into female gametophyte

Female Gametophyte, At maturity consists of thousands of cells

has 2 to 6 archegonia at micropylar end, each archegonium has a single large egg

Conifer Pollination, Female cones take two or more seasons to mature,

first spring

pollen grains drift between open scales, drawn into micropyle, scales close

Conifer Pollination, 15 months after pollination pollen tube reaches archegonium

one sperm unites with egg to form zygote, other sperm degenerates

Cycads (phylum Cycadophyta), Slow-growing gymnosperms of tropical regions

sporophytes resemble palm trees, individuals make either pollen or ovule cones, female cones huge

Cycads (phylum Cycadophyta), Sperm is formed in pollen tube but released within ovule to

swim to archegonium

Gnetophytes, Only gymnosperms with vessels in their xylem

contain 65 species in three genera: Welwitschia, Ephedra, Gnetum

Ginkgophytes, Only one living species remains

Ginkgo biloba, has flagellated sperm, is dioecious (male/female on different trees)

Angiosperms, Flowering plants

ovules enclosed in diploid tissue at pollination, carpel (modified leaf) encapsulates seeds and develops into fruit

Angiosperm Abundance, There are some 300,000 species of flowering plants

emergence changed terrain previously dominated by ferns, cycads, conifers

Angiosperm Abundance, Unique features aided abundance

flower production, insect pollination, broad leaves with thick veins

Angiosperm origins, Oldest known angiosperm in fossil record is Archaeofructus

at least 125 million years old, had multiseeded carpels and stamens, lacked sepals and petals

Closest living relative to original angiosperm is Amborella

horizontal gene transfer explains presence of moss mitochondrial genes in its genome

Flowers House Gametophytes, Flowers are modified stems bearing modified leaves

originate as primordium developing into a bud at end of pedicel, pedicel expands to form receptacle

Flower Whorls, Outermost whorl

sepals

Flower Whorls, Second whorl

petals

Flower Whorls, Third whorl

stamens (androecium), pollen is male gametophyte, each stamen has anther and filament

Flower Whorls, Innermost whorl

gynoecium, consists of one or more carpels housing female gametophyte

The Carpel, Carpel has three major regions

ovary (swollen base with ovules, develops into fruit), stigma (tip for pollen), style (stalk connecting stigma and ovary)

Ovule, A single diploid megaspore mother cell undergoes meiosis

produces 4 haploid megaspores, 3 disappear, remaining divides mitotically to form female gametophyte

Ovule, Three mitotic divisions of megaspore nucleus produces 8 haploid nuclei

two become polar nuclei forming central cell, one becomes egg, others become synergids and antipodals, integuments become seed coat

Pollen Production, Occurs in the anthers

diploid microspore mother cells undergo meiosis to produce four haploid microspores, each divides once by mitosis to become binucleate pollen grains

Pollination, Mechanical transfer of pollen from anther to stigma

may or may not be followed by fertilization, pollen tube grows if stigma is receptive

Pollination, One of two pollen grain cells lags behind

generative cell divides to produce two non-flagellated sperm cells

Double Fertilization and Seed Production, Double fertilization

one sperm unites with egg to form diploid zygote, other sperm unites with two polar nuclei to form triploid endosperm

Seeds, In many angiosperms embryo development is arrested soon after

meristems and cotyledons differentiate, integuments develop into impermeable seed coat enclosing dormant embryo and stored food

Seeds, Functions include

maintain dormancy, protect young plant, provide food for embryo, facilitate dispersal

Dessication of a Seed, Once seed coat forms

most metabolic activities cease, mature seed has 5-20% water, seed is stable, germination needs water and oxygen

Germination, Specific adaptations ensure seeds germinate only under appropriate conditions

some need fire, some need chemicals leached, some need to pass through animal intestines

Fruits, Most simply defined as mature ovaries (carpels)

during seed formation ovary develops into fruit, fruits can develop without seeds (e.g., bananas)

Fruit Structure, The ovary wall is termed the pericarp

has 3 layers: exocarp, mesocarp, endocarp; their fate determines fruit type

Fruit Structure, Fruits contain three genotypes in one package

fruit/seed coat from

prior sporophyte, embryo is next sporophyte, endosperm is triploid product of fertilization

Fruit Dispersal, Occurs through wide array of methods

ingestion by vertebrates, hooked spines, burial by herbivores, wind, wate