Transcript Unit 8 Notes (Plants)

Alternation of Generations
and Plant Life Cycles
• Haploid and Diploid generations alternate in plant life
cycles.
• Mosses have a dominant gametophyte.
• Ferns, like most plants, have a dominate sporophyte.
• A pine tree is a sporophyte with tiny gametophytes in its
cones.
– Cones are a significant adaptation to land.
• They harbor all of a conifer tree’s reproductive structures.
– Diploid sporangia, which produce haploid spores by meiosis.
» Haploid female and male gametophytes.
Alternation of Generations
Gymnosperms- Pine tree
• Female cone has many hard, radiating scales, each
bearing a pair of ovules = (sporangium and a
covering)
• Males cones are generally much smaller than
females cones.
– Each scale on a male cone produces many sporangia,
each of which makes numerous haploid spores by
meiosis.
– Male gametophytes or pollen grains, develop from the
spores.
• When the male cone mature, the scales open and
release a cloud of pollen.
Alternation of Generations
Gymnosperms- Pine tree
• Pollen grains house the cells that will develop into
sperm.
• Pollination occurs when a pollen grain lands on
and enters an ovule.
• After pollination, meiosis occurs in the ovule.
• A haploid spore cell begins developing into the
female gametophyte.
– Not until months later do eggs appear within the female
gametophyte.
– It also takes months for sperm to develop in the pollen
grain.
Alternation of Generations
Gymnosperms- Pine tree
• A tiny tube grows out of the pollen grain and
eventually releases a sperm into the egg.
– Fertilization does not occur until more than a year after
pollination.
• Following fertilization, the zygote develops into a
sporophyte embryo and the whole ovule
transforms into the seed.
– In a typical pine, seeds are shed from cones about two
years after pollination.
Alternation of Generations
Gymnosperms- Pine tree
• The seed falls to the ground, or is dispersed
by wind or animals, and when conditions
are favorable, it germinates and grows into
a tree.
Alternation of Generations
Gymnosperms- Pine tree
• Summary
– All the reproductive stages of conifers are
housed in cones borne on sporophytes.
– The ovule is a key adaptation- a protective
device fro all the female stages in the life cycle,
as well as the site of pollination, fertilization,
and embryonic development.
– The ovule becomes the seed, a major factor in
the success of the conifers and flowering plants
on land.
Angiosperms
• Dominate most landscapes today, except the northern parts
of the globe.
• 400 times more species of angiosperms than
gymnosperms.
• Nearly 80% of all plants are angiosperms.
• Gymnosperms supply most of our lumber.
• Angiosperms supply nearly all our food and much of our
fiber for textiles.
– Cereal grains, including wheat, corn, oats, and barely, are
flowering plants, as are citrus and other fruit trees, garden
vegetables and cotton.
– Fine hardwoods from flowering plants, such as oak, cherry,
and walnut trees, supplement the lumber from conifers.
Angiosperms
The flower is the centerpiece of angiosperm reproduction
• A flower is actually a short stem with four kinds of
modified leaves called sepals, petals, stamens, and
carpels.
• Sepals
– Bottom of the flower and are usually green.
– They enclose the flower before it opens.
• Petals
– Above the sepals, which are usually the most striking
part of the flower and are often important in attracting
animal pollinators.
Angiosperms
The flower is the centerpiece of angiosperm reproduction
• Stamen
– Reproductive structure
– Consists of a stalk bearing a sac called anther.
• The male organ in which pollen grains develop.
• Carpal
– Consist of a stalk with an ovary at the base and a
sticky tip known as the stigma, which traps pollen.
– Ovary is a protective chamber containing one or
more ovules, in which the eggs develop.
Angiosperms
The angiosperm plant is a sporophyte
with gametophytes in its flowers
• Meiosis occurring in the anthers of the flower
leads to the haploid spores that undergo mitosis
and form the male gametophytes, or pollen grains.
• Meiosis in the ovules leads to haploid spores that
undergo mitosis and form the female
gametophytes, each produce an egg.
• Pollination occurs when a pollen grain lands on
the stigma.
– As in gymnosperms, a tube grows from the pollen grain
to an egg and a sperm fertilizes the egg creating a
zygote.
• Fertilization occurs about 12 hrs after pollination.
Angiosperms
Structure of a fruit reflects its function in seed dispersal
• Dandelion fruit acts like a kite.
– Carried away by wind currents.
• Cockleburs attach to the fur of animals
– May be carried for miles before they open and release
their seeds.
• Fleshy, edible fruits eats by animals.
– Seeds pass through the digestive tract and deposited
some distance from where they were eaten.
• Fruit of flowering plants usually develop and
ripens quickly, so seeds can be produced and
dispersed in a single growing season.
Angiosperms
The angiosperm plant is a sporophyte
with gametophytes in its flowers
• A seed develops from each ovule.
– Each seed consists of an embryo (a new sporophyte)
surrounded by a store of food and a seed coat.
• While the seeds develop, the ovary’s wall
thickens, forming the fruit that encloses the seeds.
• When conditions are favorable, the seed
germinates and the embryo grows into a mature
sporophyte, completing the life cycle.
Fertilization in Angiosperms
• Pollen lands on the sticky stigma.
– Pollen grain contains tube nucleus and generative
nucleus
• Pollen tube grows down to the ovule
– Generative nucleus divides into 2 sperm cells in the
pollen tube
• Ovules are within the ovary.
– Embryo sac
• Micropyle end: 1 egg and 2 synergids
• End opposite of the micropyle: 3 antipodal cells
• In the middle: 2 haploid nuclei, the polar nuclei
Fertilization in Angiosperms
• When the pollen tube enters the embryo sac
– One sperm cell fertilizes the egg, forming a
diploid zygote
– Second sperm fuses with both polar nuclei,
forming a triploid nucleus
• Divides by mitosis to produce the endosperm
• Double fertilization