Reproduction in Angiospermophytes

Topic 9.3

Assessment Statements

9.3.1 Draw and label a diagram showing the structure of a dicotyledonous animal-pollinated flower.

9.3.2 Distinguish between pollination, fertilization and seed dispersal.

9.3.3 Draw and label a diagram showing the external and internal structure of a named dicotyledonous seed.

9.3.4 Explain the conditions needed for the germination of a typical seed.

9.3.5 Outline the metabolic processes during germination of a starchy seed.

9.3.6 Explain how flowering is controlled in long-day and short-day plants, including the role of phytochrome.

Flowers

• Reproductive structures of angiospermophytes

• Dependent upon animals for pollination

• Can grow as large as 3 feet in diameter and weigh as much as 14.5 lbs.

• Rafflesia arnoldii• When mature, this flower

smells like rotting meat thus attracting flies that transfer pollen from the male reproductive structures to the female structures

Flower structure and function

Flower part Function

Sepals Protect the developing flower while in the bud

Petals Often are colorful to attract pollinators

Anther Part of stamen which produces the male sex cells, pollen

Filament Stalk of stamen that holds up the anther

Stigma Sticky top of carpel on which pollen lands

Style Structure of the carpel that supports the stigma

Ovary Base of carpel in which the female sex cells develop

• Carpel (entire female part)• Stamen (entire male part)• Complete – contain

sepals, petals, stamen, and carpal

• Incomplete – lack at least one part

• Staminate – have only stamens

• Carpellate – have only carpels

Alternation of generations• All plants show two different generations in their life cycle:

• Gametophyte generation which is haploid• Sporophyte generation which is diploid

• Gametophyte generation produces plant gametes by mitosis

• Sporophyte generation produces spores by meiosis• For example, a cherry tree is in the sporophyte form (it

grew from a zygote and produces new cells by mitosis). When the cherry tree produces flowers, haploid spores are formed and develop into haploid bodies referred to as gametophytes. Sperm form within the male gametophytes, and eggs form within the female gametophytes.

Pollination

• Process by which pollen (containing male sex cells) is placed on a female stigma

• First step towards fertilization and the production of seeds

• Common vectors: wind, insects, birds, water, and other animals

• Means of attraction• Red flowers are

conspicuous to birds• Yellow and orange flowers

are noticed by bees• Heavily scented flowers

are easily found by nocturnal animals

• Plants that rely on wind have inconspicuous, odorless flowers

Types of pollination

Self-pollination

• Pollen from the anther of the same plant falls onto its own stigma

• Form of inbreeding and results in less genetic variation within a species

Cross-pollination

• Pollen is carried from the anther of one plant to the stigma of a different plant

• Increases variation and may result in offspring with better fitness

• Problem: distance

Fertilization

• Pollen grain adheres to the stigma, which is covered by a sticky, sugary substance

• Pollen germinates to produce a pollen tube• Pollen tube grows down the style of the carpel

• Within the growing pollen tube is the nucleus that will produce the sperm

• Pollen tube completes its growth by entering an opening at the bottom of the ovary

• Sperm moves from the tube to combine with the egg of the ovule to form a zygote

• Zygote develops with the surrounding tissue into the seed• As the seed is developing, the ovary around the ovule matures into a

fruit

• The fruit encloses and helps to protect the seed

The seed (means by which an embryo can be dispersed to distant locations)

Seed part Function

Testa Tough, protective outer coat

Cotyledons Seed leaves that function as nutrient storage structures

Micropyle Scar of the opening where the pollen tube entered the ovule

Embryo root (radicle) and embryo shoot (epicotyl)

Become the new plant when germination occurs

Plumule will become first leaves.Hilum is where the seed was attached to the ovary.Endosperm provides nutrition for growing embryo.

Maturation

• Dehydration until water content of the seed is about 10% - 15% of its weight

• Seed enters dormancy (low metabolism, not growth or development)

• Adaptation to overcome harsh environmental conditions

• Conditions needed for germination:• Water

• Oxygen for aerobic respiration

• Appropriate temperature for enzyme action

• Other (testa disrupted, fire exposure)

• Most will not become a functional plant so plants produce large numbers of seeds

Seed metabolism during germination

1. Uptake of water

2. Gibberellin is released

3. Gibberellin (growth hormone) triggers the production of the enzyme amylase

4. Amylase causes the hydrolysis of starch into maltose. The starch is present in the seed’s endosperm

5. Maltose is further hydrolyzed into glucose that can be used for cellular respiration or may be converted into cellulose by condensation reactions.

6. Cellulose is then used to produce the cell walls of new cells being produced

Control of flowering in angiosperms

• Photoperiodism – plant’s response to light involving the relative lengths of day and night (a very important factor in the control of flowering)

• A plant must flower when pollinators are available and when necessary resources are plentiful.

Plant-type Flowering and light Examples

Long-day plants Bloom when days are longest and nights shortest (midsummer)

Radishes, spinach, lettuce

Short-day plants Bloom in spring, late summer, and autumn when days are shorter

Poinsettias, chrysanthemums, asters

Day-neutral plants Flower without regard to day length

Roses, dandelions, tomatoes

Phytochrome

• Phytochrome is a photoreceptor, a pigment that plants use to detect light.

• There are two forms: Inactive (Pr) and active (Pfr)

• When red light (wavelength of 660 nm) is present in available light, the inactive form Pr is converted into the active form Pfr which has the ability to absorb far-red light (wavelength of 730 nm).

• This Pfr is rapidly converted back to the inactive form in daylight.

• However, in darkness, the conversion is very slow.

• It is thought that this slow conversion of Pfr back to Pr allows the plant to time the dark period.

• In long-day plants, the remaining Pfr at the end of a short night stimulates the plant to flower. In this case, Pfr acts as a promoter

• In short-day plants Pfr appears to act as an inhibitor of flowering. For these short-day plants, enough Pfr has been converted to Pr to allow flowering to occur.

• Even though the names refer to day length, it is actually the length of night that controls the flowering process.