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Plant Reproduction Reproductive Development

• Once plants are competent to reproduce (mature), a combination of factors determines when a flower is produced:

• Temperature

• Light

• Internal signals: inhibitors or promoters

• Competent: reached maturity. Plants are “competent” to reproduce.

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Phase Change• Internal developmental changes allow plants to

respond to signals (external or internal) that trigger flower formation: phase change.

• The plant matures enough to respond to signals.

• Can be obvious or very subtle.

• In oak trees, lower branches (juvenile phase) cling to their leaves in the fall

• Only juvenile ivy makes adventitious roots

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Light-Dependent Pathway

• Keyed to amount of dark in the daily 24-hr cycle (day length).

• Short-day plants flower when daylight becomes shorter than a critical length. (ex: Fall blooming)

• Long-day plants flower when daylight becomes longer. (ex: Summer blooming)

• Day-neutral plants flower when mature regardless of day length.

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Light-Dependent Pathway

• Using light as a cue allows plants to flower when conditions are optimal.

• Manipulation of photoperiod in greenhouses ensures that short-day poinsettias flower in time for the winter holidays.

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Flower Structure• Floral organs evolved from

leaves.

• A complete flower has four whorls:

• 1. Sepals (Calyx)

• 2. Petals (Corolla)

• 3. Stamens (Androecium)

• 4. Carpels (Gynoecium)

• An incomplete flower lacks one or more of the whorls.

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Male & Female flower parts

• Male: Stamen

• Anther

• Filament

• Female: Carpel

• Stigma

• Style

• Ovary

• Ovule

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Male

structureFemale

structure

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Plant life cycle

• Alternation of generations in which haploid (n) and diploid (2n) generations take turns producing each other. Two phases of a lifecycle.

• Haploid: containing a single set of chromosomes.

• Diploid: containing two homologous sets of chromosomes; one set inherited from each parent.

• Sporophyte: diploid plant body. Dominant stage of plant life cycle.

• Gametophyte: haploid reproductive spores.

• Male gametophyte – pollen grains

• Female gametophyte – embryo sac

Gamete Production

• Gametes are produced in separate, specialized structures of the flower (ovule and anther).

• Reproductive organs of angiosperms differ from those of animals in two ways.

1. Both male and female structures often occur together in the same individual, but not always.

2. Reproductive structures are not permanent parts of the adult individual.

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Pollen Formation

• Anthers contain four microsporangia which produce microspore mother cells (Diploid 2n).

• Microspore mother cells produce microspores (haploid n) through meiosis.

• Microspore develops by mitosis into pollen (gamete).

• Generative cell in the pollen grain will later divide to form two sperm cells.

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Embryo Sac Formation

• Within each ovule, a diploid microspore mother cell undergoes meiosis to produce four haploid megaspores.

• Usually only one megaspore survives.

• Enlarges and undergoes repeated mitotic divisions to produce eight haploid nuclei enclosed within a seven-celled embryo sac.

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Endosperm

• Food-storing tissue of the seed: endosperm

• Provides nutrients to the developing embryo.

• Endosperm varies between plants

• In coconuts it includes the liquid “milk”

• In corn it is solid

• In peas and beans it is used up during embryogenesis

• Nutrients are stored in thick, fleshy cotyledons

Polar nuclei

Egg

Micropyle

Sperm

Pollen tube

3n endosperm 2n zygote

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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• During embryogenesis, angiosperms undergo three other critical events

1. Development of a food supply

2. Development of seed coat

3. Development of fruit surrounding seed

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Pollination

• Outcrossing is highly advantageous for plants and for eukaryotic organisms generally.

• 2 basic reasons for frequency of self-pollination.

1. Self-pollination is favored in stable environments.

2. Offspring are more uniform and probably better adapted to their environment.

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• Floral morphology has coevolved with pollinators.

• Early seed plants wind pollinated.

• Among insect-pollinated angiosperms, the most numerous groups are those pollinated by bees.

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Pollination

• Flowers that are visited regularly by butterflies often have flat “landing platforms.”

• Flowers that are visited regularly by moths are often white or pale in color.

• Also tend to be heavily scented.

• Easy to locate at night.

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Pollination

• Flowers that are visited regularly by birds must produce large amounts of nectar.

• Often have a red color.

• Usually inconspicuous to insects.

Pollination• Some angiosperms are wind-pollinated.

• Flowers are small, green, and odorless, with reduced or absent corollas.

• Often grouped and hanging down in tassels.

• Stamen- and carpel-containing flowers are usually separated between individuals (promotes outcrossing).

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Seeds

• In many angiosperms, development of the embryo is arrested soon after meristems and cotyledons differentiate.

• Develops a relatively impermeable seed coat.

• Encloses the seed with its dormant embryo and stored food.

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• Once a seed coat forms, most of the embryo’s metabolic activities cease.

• Germination cannot take place until water and oxygen reach the embryo.

• Seeds of some plants have been known to remain viable for thousands of years.

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• Seeds are an important adaptation:

1. They maintain dormancy under unfavorable conditions.

2. They protect the young plant when it is most vulnerable.

3. They provide food for the embryo until it can produce its own food.

4. They facilitate dispersal of the embryo.

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Seed dispersalSeed dispersal

• Wind • Water

Seed dispersal

• Animals & Humans • Expulsion

Fruits

• Most simply defined as mature ovaries (carpels).

• During seed formation, the flower ovary begins to develop into fruit.

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Dry fruits (nuts)

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Fruit Dispersal

• Occurs through a wide array of methods

• Ingestion, transportation, and deposition by birds or other vertebrates.

• Hitching a ride with hooked spines on birds and mammals.

• Burial in caches by herbivores.

• Blowing in the wind.

• Floating and drifting on water.

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Germination

• Defined as the emergence of the radicle (first root) from the seed coat.

• Germination begins when a seed absorbs water and metabolism resumes.

• Oxygen must be available.

• May requires additional environmental signals such as specific wavelength of light, appropriate temperature, or stratification (period of low temperature exposure).

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• As the sporophyte pushes through the seed coat, it orients with the environment such that the root grows down and shoot grows up.

• Shoot becomes photosynthetic.

• Postembryonic phase is under way.

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Asexual Reproduction

• Produces genetically identical individuals because only mitosis occurs. Natural clones.

• Vegetative reproduction• New plant individuals are

cloned from parts of adults.

• Comes in many and varied forms:• Runners or stolons

• Rhizomes

• Suckers

• Adventitious plantlets

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