Plant Reproduction: Life Cycles & Generations

Plant Reproduction and Life Cycles

Haploid vs. Diploid

• Mitosis makes copies of chromosomes, while meiosis reduces a diploid cell to a haploid.
• In mitosis, a haploid cell produces a haploid cell, and a diploid cell produces a diploid cell.
• In meiosis, a diploid cell is reduced to a haploid cell.

Generalized Plant Life Cycle

• Involves the alternation of generations between a haploid gametophyte and a diploid sporophyte.
• Meiosis produces spores, which undergo mitosis to form the gametophyte.
• The gametophyte produces gametes through mitosis, which fuse during fertilization to form a zygote.
• The zygote undergoes mitosis to develop into the sporophyte.

Bryophytes (Mosses)

• Characterized by a life cycle dominated by the gametophyte generation.
• The sporophyte generation is a temporary reproductive structure.
• Mature gametophytes form antheridia, which produce sperm, and archegonia, which produce eggs.

Ferns

• Distinguished by a large sporophyte generation.
• Sori (singular sorus), located on the underside of the frond, contain sporangia.
• Sporangia produce spores via meiosis.
• Spores disperse to form heart-shaped gametophytes called prothallia.
• Prothallia are gametophytes that are held to soil by rhizoids.
• Prothallia produce antheridia (sperm) and archegonia (eggs).

Gymnosperms

• Dominated by the sporophyte generation (the tree).
• The gametophyte generation is greatly reduced and occurs inside the cones.
• Gymnosperms have male and female cones.
• Microsporangia in the male cones produce male spores called microspores via meiosis.
• Microspores develop into mature pollen grains (the male gametophyte).
• Megasporangia inside the female cone (ovulate cone) produce megaspores via meiosis.
• Megaspores develop into the female gametophyte, which produces eggs in the archegonia.
• Pollen grains contain wing cells for dispersal, a generative cell, and a tube cell.
• Wing cells aid in pollen dispersal.
• A generative cell produces sperm.
• A tube cell transports sperm to the egg and is a temporary structure.
• The tube cell elongates toward the archegonium to form the pollen tube.
• The generative cell undergoes mitosis to produce sperm.
• Sperm moves down the pollen tube and fertilizes the egg.

Angiosperms

• Possess flowers that produce pollen and seeds and fruits that disperse seeds.
• The male part of the flower is the stamen, composed of the anther and filament.
• The female part of the flower is the carpel, composed of the stigma, style, and ovary.
• A Tepal is when the sepal and petal are fused together.
• The anther is made of four sac-like structures called microsporangia.
• Microsporangia produces haploid microspores.
• Microspores grow into the male gametophyte called pollen grains.
• Angiosperm pollen grains contain a generative cell and a tube cell, similar to gymnosperms.
• The tube cell grows the pollen tube once a pollen grain has been deposited on the stigma.
• The generative cell then divides to form two sperm cells, which travel down the pollen tube to the ovule.
• Ovules contain cells that divide by meiosis to produce haploid megaspores.
• Megaspores grow into the female gametophyte in the ovule; this gametophyte contains the egg.
• The egg is fertilized by one sperm cell to produce a diploid zygote.
• The other sperm cell fertilizes the cells surrounding the egg.
• This forms the endosperm in a process called double fertilization (unique to angiosperms).
• The zygote develops into an embryo, and the ovule develops into a seed along with the endosperm (the embryo's food supply).
• A grass flower is known as an inflorescence.

Fruits and Seeds

• Fruit is the ovary plus any accessory parts like the receptacle.
• The ovary wall is called the pericarp.
• The purpose of fruit is to protect seeds and aid in their dispersal.
• Mature fruits can be fleshy (tomatoes, apples, oranges) or dry (peanuts, beans, grains).
• Seeds gain nutrients from a stalk-like funiculus, which is like an umbilical cord for developing seeds.
• The string connecting the seed to the pumpkin wall in a pumpkin is the funiculus.
• Endosperm stores starch.
• The embryo is composed of root, embryonic leaves, and cotyledons.
• Monocots have one cotyledon, dicots have two cotyledons.
• Monocot leaves grow from the base and dicot leaves grow from the shoot.