Alternation of Generationsand 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 GenerationsGymnosperms- 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 GenerationsGymnosperms- 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 GenerationsGymnosperms- 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 GenerationsGymnosperms- 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 GenerationsGymnosperms- 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.

AngiospermsThe 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.

AngiospermsThe 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.

AngiospermsThe 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.

AngiospermsStructure 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.

AngiospermsThe 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