REPRODUCTION IN PLANTS BIO NOTES
Asexual reproduction x Sexual reproduction
Sexual Reproduction
Asexual reproduction
Process involving the fusion of two gametes
to form a zygote, producing genetically
dissimilar offspring.
Advantages:
- Offspring may inherit beneficial qualities
from both parents
- Greater genetic variation in the offspring,
leading to species that are better adapted to
changes in the environment
Disadvantages:
- Two parents are required (except plants with
bisexual flowers)
- Fusion of gametes required
- Slower method of producing offspring
Process which produces genetically
identical offspring from one parent, without
the fusion of gametes.
Advantages:
- Only one parent is required
- Fusion of gametes not required
- All beneficial alleles passed on
- Faster method of producing offspring
- Allow organisms to colonise area rapidly
Disadvantages
- No genetic variation in the offspring
- Species not well adapted in the environment
Parts of a flower
Petal
Modified leaves
Insect pollinated flowers are bright and large enough for insects to
stand on
All the petals together make up the corolla
Sepal
Modified leaves
Enclose and protect other parts of flower in bud stage
All the sepals together make up the calyx
Stamen and carpel
Stamen is the male parts of flower (filament and anther)
Carpel or pistil is the female parts of flower (stigma, style and ovary)
Receptacle
Enlarged end of flower stalk (or pedicel) which bears the other parts of
flower
Anther
Each anther contains two lobes and each lobe consist of pollen grains
Contains vascular bundle
Filament
Stalk that holds the anther in a suitable position to disperse the pollen
grains.
Ovary
Structure that develops into a fruit after fertilisation
Produces and protects ovules
Ovule
Structure that develop into a seed after fertilisation
Produces ovum by meiosis
Ovum is hence haploid
Style
Stalk that connects stigma to the ovary
Holds the stigma in a suitable position to trap pollen grains
Pollen grains
Produced by meiosis, hence are haploid
Each contains a male gamete
Inflorescences
A cluster of flowers borne on the same stalk
Pollination
- Transfer of pollen grains from the anther to the stigma from female flower to male
flower.
- Self-pollination; Transfer of pollen grains from the anther to the stigma of the same
flower or of a different flower on the same plant.
- Cross-pollination: Transfer of pollen grains from one plant to the stigma of a flower in
another plant of the same species.
Self-pollination
Cross-pollination
Features
Flowers are bisexual with anthers
and stigmas maturing at the same
time
Stigma is situated directly below
the anthers
In some plants, some flowers never
open, so that only self-pollination
occurs
Dioecious flowers (male and female
flower on different plants) so that they
cannot undergo self-pollination
Anthers and stigmas may mature at
different times
Stigmas and anthers of a bisexual
flower are separated some distance
away
Advantages
Only one parent is required
Beneficial qualities are more likely
to be passed down to offspring
Not dependent on external factors
such as insect/wind pollination
Higher probability of pollination
Offspring may receive beneficial
qualities from both parents
Greater genetic variation,
increases the chance of species
surviving changes in environment
Higher chance of heterozygous
plants
Disadvantages
Less genetic variation in offspring,
Two parent plants are required
Depends on external factors for
pollination (insect or wind pollination)
Lower probability of pollination than
self-pollination
More energy and pollen wasted
Less adaptive to changes in the
environment
Insect-pollinated flower — Clitoria
Structure
- Calyx: Consists of five green sepals
- Epicalyx: Base of calyx enclosed by two green leaves (epicalyx)
- Corolla: Consists five brightly-coloured petals of different shape and sizes
- Standard petal: Large dark blue petal that has nectar guides
- Wing petals: Two smaller dark blue petal enclosed by standard petal
- Keel petals: Two small yellow green petal enclosed by wing petals
It encloses the reproductive organs
- Stamens: Nine stamens fused along their length, the tenth stamen is free
- Carpel: Stigma is small and compact, located at end of style
The style is a long, curved, hairy structure
Ovary is long and narrow, with single row of ovules
Pollination of Clitoria
- 1. Insect lands on standard petal and follows the nectar guides into the flower.
- 2. Insect forces its way in between the two wing petals and moves in to collect nectar.
- 3. Insect’s back forces keel petals upwards, exposing stigma and anthers.
- 4. Stigma and anthers brush against the hairy back of insect, causing pollen grains to
stick on the hairy back of insect.
- 5. At the same time, pollen grains from another flower on back of insect are transferred
to sticky stigma.
- 6. Insect leaves the flower after some time, then the keel petals spring back to its
original position, enclosing anthers and stigma again.
Wind-pollinated flower — Ischaemum muticum
Structure
- Flowers: Flowers occur in pairs
- Upper flower: Bisexual, consists of 1 ovary, 2 feathery stigmas and lodicules, 3 stamens
Both lodicules swell to force flowering bracts open so that anther and
stigma can emerge
- Lower flower: Male, consists of 3 stamens and 2 lodicules
- Spikelet: Each pair of flowers with a short stalk forms a spikelet
Each spikelet is protected by non-flowering bracts
Pollination of Ischaemum muticum
- 1. Mature stamens are long, pendulous, and hang out of the bracts, exposed to wind.
- 2. When blown by wind, the dust-like pollen is shaken free and dispersed by wind.
- 3. Mature stigmas project out of bracts and are large, extended and feathery, in order
to provide large surface area to receive any pollen floating in the wind.
Differences between insect-pollinated and wind-pollinated flowers
Features
Insect-pollinated
Wind-pollinated
Petals
Flowers are large and brightlycoloured petals
Small and dull-coloured, no petals
Nectar
Present
Absent
Scent
Fragrant and sweet-smelling
Absent
Stigmas
Small, compact and do not
protrude out of flower
Large, feathery and protrude out of
flower (to trap pollen grains)
Stamens
Not pendulous and do not protrude
out of flower
Long, pendulous and protruding
anthers
Pollen
Fairly abundant
Larger with rough surfaces so that
they can readily cling on insects
More abundant
Tinier with smooth surfaces so that
they are easily blown in the wind
Nectar guide
Nectar guides may be present
Nectar guides absent
Fertilisation
- 1. After pollination, the pollen grain germinates after it comes into contact with stigma,
in response to the sugary fluid secreted by the mature stigma.
- 2. A pollen tube grows out from each pollen grain, down the style and eventually into the
ovary. The tip of the pollen tube contains two male gametes.
- 3. As pollen tube grows, it secretes enzymes to digest the surrounding tissue of the
stigma and style.
- 4. The pollen tube enters the ovule through the micropyle.
- 5. In the ovule, the tip of pollen tube absorbs sap and bursts, releasing the two male
gametes.
- 6. The nucleus of one male gamete fuses with the nucleus of the ovum to form the
zygote.
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