angiosperms
largest taxonomic
group of plants in which fl owers
are
involved in their reproductive
process
pollination
transfer of pollen
from the anther to the stigma of a
fl
ower, occurs before fertilization
in angiosperms
pollen
produced by the anther of
the stamen and carries the
male
gametes in angiosperms
stigma
upper most part of the
fl ower’s female structure
which
receives the pollen
vegetative n
non-reproductive,
non-fl owering
repression
the act of preventing
or decreasing the chances
Environmental factors
which may be involved in the change from
vegetative to reproductive mode in
flowering plants include:
• day/night length • temperature.
photoperiodism
response of a
plant to light involving the relative
lengths
of day and night
long-day plants
plants which
fl ower when days are longest and
nights are shortest
short-day plants
plants which
fl ower when days are shorter and
nights are longer
phytochrome
blue-green
pigment occurring in plant leaves
and involved
in photoperiodism
Pr
inactive form of phytochrome
involved in photoperiodism
Pfr
active form of phytochrome
involved in photoperiodism
interconversion
the way a
molecule changes between its
possible forms
bracts
unique structures
produced in some plants which
are a form
of leaves
stamen
male reproductive
structure of the fl ower
pollen
produced by the anther of
the stamen and carries the
male
gametes in angiosperms
anther
stamen structure in which
pollen is produced and matures
self-pollination
pollen is
transferred from anther to stigma
of the same fl
ower or of the same
plant
cross-pollination
pollen
transferred from anther to stigma
of fl owers on
different plants
haploid
a cell that has only one
chromosome of each homologous
pair
diploid zygote
fertilized egg
with the diploid chromosome
condition
pollen tube
growth structure
from a germinating pollen grain
which
contains the male gametes
There are three major factors called vectors involved in pollination. They are:
• wind
• water
• animals.
Fertilization
occurs after pollination. It is the union of haploid male and female
sex
cells to form a diploid zygote.
Pollen which falls on
the stigma of a fl ower produces a pollen tube which
grows
through the style of the carpel to the ovary.
The
growing pollen tube carries the male gametes to the ovules (the
female
gametes) of the ovary.
Fertilization in fl owering
plants is actually a double fertilization. One sperm
combines
with an ovule egg nucleus. Another sperm from the same pollen
tube
combines with two polar nuclei in the ovule to produce the
endosperm. The
endosperm is 3n, triploid, and provides the
nutrients for the new plant that
develops from seed
germination.
Fertilization in fl owering plants results in the
formation of a seed.
style
structure which connects
the stigma to the ovary in
the
carpel of a flower
carpel
female sex organ of a
flower
gamete(s)
a sex cell, either a
sperm cell or an egg cell
ovules
female gametes which
occur in the ovary of the carpel,
will
become seeds when fertilized
by male gametes
double fertilization
plant
fertilization when one male
gamete combines with an
egg
nucleus of an ovule and another
combines with two polar nuclei
polar nuclei
occur in an ovule
and form the endosperm when a
male gamete
combines with them
endosperm
part of the seed
which provides the nutrients for
the
developing plant after seed
germination
seed
structure produced after
fertilization in plants which
allows
the formation of a new plant
seed dispersal
transport of seed
from the parent plant to a new
location
germination
early growth or
sprouting of a seed
embryonic plant
early plant
which occurs inside the seed
cotyledons
seed leaves which
provide nutrients early in a plant’s
life
dicotyledous
plant seed with
two seed leaves
monocotyledous
plant seed
with one seed leaf
Common pollinators of flowering plants include
bats, birds, and, especially, insects.
A mutualistic
relationship often develops between the pollinator and the fl
owering
plant. In a mutualistic relationship, both of the
organisms involved benefi t.
Pollinators gain food in the form of
nectar when they carry out pollination of many
fl owering plants.
An example of this is the honey bee. The honey bee gains food
in
the form of nectar while carrying out pollination of a fl
ower. Both honey bee and
plant are helped by this association.
pollinators
organisms which
are involved in the process of
pollination
mutualistic relationship
relationship between two
organisms in which both are
helped
nectar
high sugar substance
produced by plant fl owers which
is
benefi cial to pollinators
Plants can reproduce in a number of different ways:
- Vegetative propagation (asexual reproduction from a plant cutting)
- Spore formations (e.g. moulds, ferns)
- Pollen transfer (flowering plants – angiospermophytes)
Sexual reproduction in flowering plants involves the transfer of pollen (male gamete) to an ova (female gamete)
- This involves three distinct phases – pollination, fertilization and seed dispersal
Pollination:
- 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
Fertilisation:
- 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
Seed dispersal:
- 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
Cross-pollination 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)
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
Common examples of pollinators include birds, bats and insects (including bees and butterflies)
- Flowers may be structured to optimise access for certain pollinators (e.g. tube-shaped flowers for birds with long beaks)
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)
Flowers are the reproductive organs of angiospermophytes (flowering plants) and contain male and female structures
- Most flowers possess both male and female structures
(monoecious), but some may only possess one structure (dioecious)
Flower Structures The male part of the flower 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)
- 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)
- 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
Long-day plants require periods of darkness to be less than an uninterrupted critical length
- These plants will traditionally not flower during the winter and autumn months when night lengths are long
- Horticulturalists can trigger flowering in these plants by exposing the plant to a light source during the night
- Carnations are an example of a long-day plant
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
Photoperiodism
Only the active form of phytochrome (Pfr) is capable of causing flowering, however its action differs in certain types of plants
- Plants can be classed as short-day or long-day plants, however the critical factor in determining their activity is night length
Short-day plants 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)
Long-day plants 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)
Phytochromes
Phytochromes are leaf pigments which are used by the plant to detect periods of light and darkness
- The response of the plant to the relative lengths of light and darkness is called photoperiodism
Phytochromes exist in two forms – an active form and an inactive form:
- The inactive form of phytochrome (Pr) is converted into the active form when it absorbs red light (~660 nm)
- The active form of phytochrome (Pfr) is broken down into the inactive form when it absorbs far red light (~725 nm)
- Additionally, the active form will gradually revert to the inactive form in the absence of light (darkness reversion)
Because sunlight contains more red light than moonlight, the active form is predominant during the day
- Similarly, as the active form is reverted in darkness, the inactive form is predominant during the night
When fertilisation occurs, the ovule will develop into a seed (which may be contained within a fruit)
- The seed will be dispersed from the parental plant and will then germinate, giving rise to a new plant
A typical seed will possess the following features:
- 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
Germination is the process by which a seed emerges from a period of dormancy and begins to sprout
For germination to occur, a seed requires a combination of:
- 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)
Additionally, certain plant species may require additional conditions for germination:
- Fire – some seeds will only sprout after exposure to intense heat (e.g. after bushfires remove established flora)
- Freezing – some seeds will only sprout after periods of intense cold (e.g. in spring, following the winter snows)
- Digestion – some seeds require prior animal digestion to erode the seed coat before the seed will sprout
- Washing – some seeds may be covered with inhibitors and will only sprout after being washed to remove the inhibitors
- Scarification – seeds are more likely to germinate if the seed coat is weakened from physical damage
Experiments can be developed using any of these factors as an independent variable
- Germination can be measured by the rate of seed growth over a set period of time
Monocotyledons and dicotyledons can be differentiated according to a number of features:
- Cotyledons – monocots have one cotyledon within their seed, dicots have two cotyledons
- Leaf veins – monocots show parallel venation, whereas dicots display reticulated venation
- Roots – monocots have fibrous (adventitious) roots, dicots have a main tap root with lateral branches
- Floral organs – monocots have flower parts in multiples of three, dicots in multiples of four or five
- Stem vascularisation – the vascular bundles in monocots are scattered, whereas they form a ringed structure in dicots
- Pollen – monocot pollen has a single pore (monosulcate), dicot pollen has three pores or furrows (trisulcate)
The first step in the germination process is the metabolic activation of a dormant seed
- Germination begins with the absorption of water, which causes gibberellin to be produced
- Gibberellin triggers the synthesis of amylase, which breaks down starch into maltose
- Maltose is either hydrolysed (to glucose) for energy, or polymerised (to cellulose) for cell wall formation
- This energy and cellular building blocks is used to promote cell division and the growth of a nascent shoot
Once the seed is metabolically activated, germination proceeds according to the following stages:
- The seed coat (testa) ruptures and the embryonic root (radicle) grows into the ground to extract key nutrients and minerals
- The cotyledon emerges and produces the growing shoot’s first leaves
- The growing plant can be divided into the epicotyl (embryonic shoot), hypocotyl (embryonic stem) and developing roots
