Growth & Reproduction in Plants

18. 4 – 18.8

Flowers have several roles in plant reproduction.

18.4 The flower is the chief structure for sexual reproduction

18.4 The flower is the chief structure for sexual reproduction

18.4 The flower is the chief structure for sexual reproduction

Sepalsleaf-like structures located at point where

flower is connected to main support structure

grow in a ring around outside of flowersurround and protect flower bud during

developmentusually green but some flowers are brightly

colored and closely resemble flower’s petals

18.4 The flower is the chief structure for sexual reproduction

Petalsusually brightly coloredhelps flower attract pollinatorslong and thin short and broad

18.4 The flower is the chief structure for sexual reproduction

Stamensmale reproductive partsfilament: long, thin stalk  anther: head-like top where pollen grains

are produced

18.4 The flower is the chief structure for sexual reproduction

Carpelfemale reproductive structure stigma

◦flat, sticky surface at top of carpel◦functions as landing pad for pollen

style◦holds the stigma high out of the center of the flower ◦ long thin structure that leads down to the ovary

ovules◦produce female gametes (eggs) ◦after egg is fertilized, ovule develops into a seed

18.4 The flower is the chief structure for sexual reproduction

Take-home message 18.4

Flowers are plant structures specialized for sexual reproduction.

Most flowers have the same fundamental structures: sepals, petals, stamens, and a carpel.

18.5 The male reproductive structure produces pollen grains.

18.5 The male reproductive structure produces pollen grains.

spores: haploid cells produced by meiosis in plants

microspores: spores produced in anthersmegaspores: spores produced in ovules

18.5 The male reproductive structure produces pollen grains.

as anther grows, 4 chambers form—sometimes called  “spore sacs”

each is filled with diploid cells called microspore mother cells

18.5 The male reproductive structure produces pollen grains.

microspore mother cells divide by meiosis; each produces 4 haploid microspores.

microspores then quickly divide by mitosis two-cell grain of pollen w/complex, water-tight, sticky surface

18.5 The male reproductive structure produces pollen grains.

two-celled structure is the pollen grain containing 2 haploid cells

one cell will eventually grow to form a pollen tube, aiding in fertilization.

other will divide once to produce two sperm cells. 

18.5 The male reproductive structure produces pollen grains.

18.5 The male reproductive structure produces pollen grains.

A pollen grain contains two haploid cells. One forms a tube used as a conduit for fertilization. The other cell divides once and forms two sperm cells.

18.5 The male reproductive structure produces pollen grains.

The pollen on this nose hair may be flushed from the body by mucus (or a sneeze)!

Sometimes the reaction is extreme.

Take-home message 18.5

The male reproductive structure produces pollen grains, each grain a two-cell structure that is water-tight and has a sticky surface.

One of the cells in the pollen grain will form a pollen tube, and the other will divide to produce two sperm cells.

18.6 Female gametes develop in embryo sacs.

18.6 Female gametes develop in embryo sacs.

Within the ovary1 or more diploid cells differentiate into

ovuleseach ovule is made of outer protective

cells that surround megaspore mother cell (diploid egg-producing cell

megaspore mother cell undergoes meiosis to produce haploid megaspores

18.6 Female gametes develop in embryo sacs.

Within a flower1 of haploid megaspores undergoes mitosis

several times to produce embryo sac (structure in which fertilization will occur) 

embryo sac: 7 cells6 of these cells—including one that is the

egg—have haploid nucleicentral cell: 7th cell w/2 distinct haploid nucleiembryo sac waits for male gamete to arrive

18.6 Female gametes develop in embryo sacs.

Take-home message 18.6

Within the ovary, diploid cells differentiate into ovules, each of which is a group of outer protective cells around a diploid egg-producing cell, which undergoes meiosis to produce haploid megaspores.

One of these megaspores undergoes mitosis several times to produce the embryo sac, the structure that contains the egg and is the place where fertilization will occur.

Pollination, fertilization, and seed dispersal often depend on help from other organisms.

18.7 Plants need help getting the male gamete to the female gamete for fertilization.

green algae release gametes into watermosses and ferns must rely on the

presence of water for transporting male gametes to female gametes, otherwise gametes dry out

mosses and ferns must live in moist habitats or reproduce when moisture is available

18.7 Plants need help getting the male gamete to the female gamete for fertilization.

process involves enlisting animals to carry the male gametes

plant attracts animal with its flower ◦visual cues (color, shape)◦olfactory cues (smell) ◦tactile cues (soft, bristly, hard, rough, smooth)

pollination: pollen grain from a plant must journey to the stigma of another plant of the same species

18.7 Plants need help getting the male gamete to the female gamete for fertilization.

~10% of plant species achieve pollination by wind (grasses, pine trees) or water (eelgrass)

slim chance that—through random luck—some of pollen will land on female reproductive organs of another plant of same species

astronomically low probability of any one pollen grain actually doing that plants respond by producing ~ 10,000,000 pollen grains/plant

18.7 Plants need help getting the male gamete to the female gamete for fertilization.

wide variety of pollinators: birds (mostly hummingbirds), bees, flies, beetles, butterflies, moths, and even some mammals (mostly bats)

strong coevolution between plants and pollinators◦plants have become more and more effective at

attracting the pollinators and deterring other species from visiting the flower

◦pollinators have become more and more effective at exploiting resources offered by plants

Take-home message 18.7

Plants usually utilize trickery or bribery to get the assistance of animals in carrying the male gametes to the female gametes.

There has been strong coevolution between plants and their animal pollinators.

18.8 Fertilization occurs after pollination.

pollination brings male and female gametes close to each other, but it isn’t quite fertilizationfertilization requires that male and female

gametes fuse so that genetic material can be combined

18.8 Fertilization occurs after pollination.

Within 12 to 36 hours of pollen landing on stigma1 cell in pollen grain starts to grow into a

pollen tube by an elongation of tube cell (not mitosis)

stretches and pushes through stigmatakes sperm-producing cell farther and

farther down style toward ovary

18.8 Fertilization occurs after pollination.

cells within the style, which is part of the female reproductive structure (the carpel), test the tube cell to check whether it is too closely related to the plant on which the pollen has landed (as it would be if the plant self-pollinated). If the tube cell is too closely related, it is killed—often by a chemical reaction initiated by the cells of the style, breaking down the cytoskeleton of the pollen tube. If it is not too closely related, the pollen tube is allowed to continue growing toward the ovary.