9.4 Plant Reproduction

Question 1

Phases in sexual reproduction in flowering plants

Answer 1

involves the transfer of pollen (male gamete) to an ova (female gamete)

This involves three distinct phases – pollination, fertilization and seed dispersal

Question 2

Pollination

Answer 2

The transfer of pollen grains from an anther (male plant structure) to a stigma (female plant structure)
Many plants possess both male and female structures (monoecious) and can potentially self-pollinate
From an evolutionary perspective, cross-pollination is preferable as it improves genetic diversity

Question 3

Fertilisation

Answer 3

Fusion of a male gamete nuclei with a female gamete nuclei to form a zygote
In plants, the male gamete is stored in the pollen grain and the female gamete is found in the ovule

Question 4

Seed dispersal

Answer 4

Fertilisation of gametes results in the formation of a seed, which moves away from the parental plant
This seed dispersal reduces competition for resources between the germinating seed and the parental plant
There are a variety of seed dispersal mechanisms, including wind, water, fruits and animals
Seed structure will vary depending on the mechanism of dispersal employed by the plant

Question 5

Cross-pollination

Answer 5

involves transferring pollen grains from one plant to the ovule of a different plant

Pollen can be transferred by wind or water, but is commonly transferred by animals (called pollinators)

Question 6

Pollinator and flowering plant relationship

Answer 6

Pollinators are involved in a mutualistic relationship with the flowering plant – whereby both species benefit from the interaction

The flowering plant gains a means of sexual reproduction (via the transference of pollen between plants)
The animal gains a source of nutrition (plants secrete a sugar-rich substance called nectar to attract pollinators

Question 7

Flowering

Answer 7

Flowers are the reproductive organs of angiospermophytes (flowering plants) and develop from the shoot apex

Changes in gene expression trigger the enlargement of the shoot apical meristem
This tissue then differentiates to form the different flower structures – sepals, petals, stamen and pistil

The activation of genes responsible for flowering is influenced by abiotic factors – typically linked to the seasons

Flowering plants will typically come into bloom when a suitable pollinator is most abundant
The most common trigger for a change in gene expression is day/night length (photoperiodism)

Question 8

Male parts of a flower

Answer 8

is called the stamen and is composed of:

Anther – pollen producing organ of the flower (pollen is the male gamete of a flowering plant)
Filament – slender stalk supporting the anther (makes the anther accessible to pollinators)

Question 9

Female parts of a flower

Answer 9

is called the pistil (or carpel) and is composed of:

Stigma – the sticky, receptive tip of the pistil that is responsible for catching the pollen
Style – the tube-shaped connection between the stigma and ovule (it elevates the stigma to help catch pollen)
Ovule – the structure that contains the female reproductive cells (after fertilisation, it will develop into a seed)

Question 10

Support structures in a flower

Answer 10

Petals – brightly coloured modified leaves, which function to attract pollinators
Sepal – Outer covering which protects the flower when in bud
Peduncle – Stalk of the flower

Question 11

Phytochromes

Answer 11

are leaf pigments which are used by the plant to detect periods of light and darkness
exist in two forms – an active form and an inactive form

Question 12

Photoperiodism

Answer 12

The response of the plant to the relative lengths of light and darkness

Question 13

Pr

Answer 13

The inactive form of phytochrome, is converted into the active form when it absorbs red light (~660 nm)
rapidly converted to the active form in the presence of light

Question 14

Pfr

Answer 14

The active form of phytochrome, is broken down into the inactive form when it absorbs far red light (~725 nm)
the active form will gradually revert to the inactive form in the absence of light (darkness reversion)
Only the active form of phytochrome (Pfr) is capable of causing flowering, however its action differs in certain types of plants

Question 15

Short-day plants

Answer 15

flower when the days are short – hence require the night period to exceed a critical length
In short-day plants, Pfr inhibits flowering and hence flowering requires low levels of Pfr (i.e. resulting from long nights)

Question 16

Long-day plants

Answer 16

flower when the days are long – hence require the night period to be less than a critical length

In long-day plants, Pfr activates flowering and hence flowering requires high levels of Pfr (i.e. resulting from short nights)

Question 17

Monoecious plants

Answer 17

have both male and female reproductive organs on the same plant

Question 18

Dioecious plants

Answer 18

have both male and female reproductive organs on different plants

Question 19

Methods to induce flowering of short-day plants

Answer 19

Short-day plants require periods of darkness to be greater than an uninterrupted critical length

These plants will traditionally not flower during the summer months when night lengths are short
Horticulturalists can trigger flowering in these plants by covering the plant with an opaque black cloth for ~12 hours a day
Crysanthemums are an example of a short-day plant

Question 20

Structures within a seed

Answer 20

Testa – an outer seed coat that protects the embryonic plant
Micropyle – a small pore in the outer covering of the seed, that allows for the passage of water
Cotyledon – contains the food stores for the seed and forms the embryonic leaves
Plumule – the embryonic shoot (also called the epicotyl)
Radicle – the embryonic root

Question 21

Germination

Answer 21

is the process by which a seed emerges from a period of dormancy and begins to sprout

Question 22

Factors needed for germination

Answer 22

Oxygen – for aerobic respiration (the seed requires large amounts of ATP in order to develop)
Water – to metabolically activate the seed (triggers the synthesis of gibberellin)
Temperature – seeds require certain temperature conditions in order to sprout (for optimal function of enzymes)
pH – seeds require a suitable soil pH in order to sprout (for optimal function of enzymes)

Question 23

Germination stages

Answer 23

absorption of water allows cells to become metabolically active
gibberellin hormone is produced in cotyledons
this stimulates the production of amylase enzyme which converts starch into simple sugars
glucose is used in aerobic respiration to produce ATP or is used to make cellulose and other substances needed for growth