16 — reproduction in plants

Question 1

Petal

Answer 1

Petal — modified leaf that forms the most obvious part of a flower

Insect-pollinated flowers:
1. Petal brightly coloured with conspicuous modified leaves to attract insects for pollination
2. Prov a landing platform for insects

(All petals tgt make up the corolla)

Question 2

Stamen

Answer 2

Male part of the flower that consists of an anther and a filament. Anther has 2 lobes, each contains 2 pollen sacs which produce pollen grains. The pollen grain contains 2 male gametes.

(All of stamens tgt make up the androecium (plural: androecia))

Question 3

Filament

Answer 3

Holds anther in a suitable position to disperse pollen grains

(Pollen grains contains male gametes)

Question 4

Anther

Answer 4

Produces pollen grains. It is made up of 2 lobes, each containing 2 pollen sacs. It consists of a vascular bundle made up of a phloem and a xylem.

Question 5

Carpel

Answer 5

Female part of the flower. Consists of an ovary which contains ovules, a style through which pollen tube grows, and one or more stigmas which capture pollen grains.

All of the carpels tgt make up the pistil or gynoecium (plural: gynoecia)

Question 6

Pollen grains

Answer 6

Hv a haploid set of chromosomes. Each pollen grain has 2 nuclei, the generative nucleus and the pollen tube nucleus or definitive nucleus.

Question 7

Stigma

Answer 7

Swollen structure that receives pollen grains. A mature stigma secretes sugary fluid to stimulate the germination of pollen grains.

Question 8

Style

Answer 8

Joins ovary to stigma. Holds stigma in position to receive pollen grains

Question 9

Ovary

Answer 9

Produces and protects the 1 or more ovules it contains. Becomes the fruit after fertilisation.

Question 10

Ovule

Answer 10

Becomes the seed after fertilisation. The ovule contains the ovum and the definitive nucleus. The ovum has a haploid set of chromosomes. The ovule is attached to the placenta by a stalk called called the funicle.

Question 11

Receptacle

Answer 11

Base of the flower where the other parts of the flower are attached

Question 12

Sepal

Answer 12

Protects the flower during the bud stage.

Question 13

Pollination

Answer 13

Pollination is the transfer of pollen grains from the anther of the male to the stigma of the female part of the flower

• Pollination is important to help bring together the male and female gametes to enable fertilisation to take place.
• Pollination can be brought about by insects or wind.
• There are two types of pollination:
  1) Self-pollination
  2) Cross-pollination

Question 14

Self-pollination

Answer 14

Self-pollination is the transfer of pollen grains from the anther to the stigma of the same flower or of a different flower on the same plant.

• Flowers are bisexual with anthers and stigmas maturing at the same time.
• The stigma is situated directly below the anther. (Anther higher stigma lower)
• Certain flowers in bisexual plants never open and only self-pollination can happen.

Question 15

Advantages and disadvantages of self-pollination

Answer 15

1. Beneficial qualities are passed down from parent plant to offspring.
2. Not dependent on external factors for pollination.
3. More likely to take place since stigmas are closer to anthers.
4. Less pollen and energy is wasted. [1]
5. Only one parent is required. [1]
6. Faster method of reproduction. [1]

Disadvantages:

1. Offspring produced are genetically similar to parents causing fewer varieties of offspring, which can be prone to diseases, thus less adaptable to changes in the environment. [1]
   Note: genetically similar not = genetically identical, not asexual.
2. Probability of harmful recessive alleles being expressed in offspring is higher as compared with cross-pollination. [1]

Question 16

Cross-pollination

Answer 16

Cross-pollination involves the transfer of pollen grains from the anther of one plant to the flower of another plant of the same species.

• Dioecious plants bear either male or female flowers making self-fertilisation impossible.
• In bisexual plants, cross-pollination can happen when:
• anthers and stigmas mature at different times.
• Stigmas of plants are situated at a distance away from the anthers. (Anther may be below stigma in this case)

Question 17

Advantages + disadvantages of cross-pollination

Answer 17

1. More varieties of offspring are produced as they’re genetically dissimilar to parents, which leads to greater genetic variation, which can be less prone to diseases/probability of harmful recessive alleles being expressed in offspring is lower. [2]
2. Increased probability of offspring being heterozygous. (Alleles r diff)
3. Seeds produced are capable of surviving longer before germination (Seeds-> sexual reproduction)
4. Probability of harmful recessive alleles being expressed in offspring is lower as compared with self-pollination.
5. Offspring can inherit beneficial qualities from both parents.

Disadvantages:

1. Two different plants of the same species are required. -> nd more energy
2. Dependant on external factors for pollination.
3. Less likely to occur as compared to self-pollination.
4. More energy and pollen are wasted.

Question 18

Insect pollination

Answer 18

• The insect lands on the standard petal and follows the nectar guide into the flower.
• The insect’s back forces the keel petal upwards to expose the anthers and stigma
• Pollen grains from the anther stick onto the hairy back of the insect and get transferred to a stigma and is deposited.
• When insect leaves flower, the keel petals spring back to its original position.

Question 19

Pollen allergy

Answer 19

A reaction by the immune system to fight against a specific pollen

Signs and symptoms:
Sneezing, stuffy nose, runny nose, watery eyes, wheezing, itchy
throat

Causes:
Breathing in pollen from some trees and wind-pollinated plants such as grass

Question 20

Outline the process leading to fertilisation after pollination in plants. [3] + Describe the process of fertilisation when the pollen lands on the stigma. [6]

Answer 20

1. After pollination, pollen grain germinates in response to sugary fluid secreted by mature
   stigma. [1]
2. A pollen tube grows out from the pollen grain which is controlled by the pollen tube nucleus, secreting enzymes to digest surrounding tissues of the stigma, style and ovary wall for the pollen tube to penetrate through the style and reach the ovary. [1]
3. The pollen tube enters the ovule in the ovary via the micropyle, its tip absorbs sap and bursts.
4. The generative nucleus divides and release 2 male haploid gamete, 1 which nucleus is released into the ovule and fuses with haploid female nucleus of the ovum to form the diploid zygote (fertilisation). [1]

——
After pollination, pollen grain germinates in response to sugary fluid secreted by mature
stigma. [1]
The growth of the pollen tube is controlled by the pollen tube nucleus, [1]
Which the pollen tube secretes enzymes to digest the surrounding tissue of the stigma, ovary walls and the style [1] for the pollen tube to penetrate through the style and reach the ovary.
The pollen tube enters the ovule in the ovary through the micropyle. [1] The generative nucleus divides into two male haploid gametes and pollen tube nucleus disintegrates. The tip of pollen tube absorbs sap and bursts to release the male gametes. [1]
One haploid male nucleus gamete fuses with the female haploid nucleus of the ovum to form the diploid zygote. [1]
The other male gamete fuses with the definitive nucleus to form the endosperm nucleus. [1]

Question 21

Discuss the differences in the process of fertilisation in a human and a flowering plant.

Answer 21

• Male and female nuclei are found in sperms and ova/eggs in
  humans while male and female nuclei are found in pollen grains
  and ovules in a flowering plant.
• Male gamete is delivered by pollen tube in a flowering plant while
  male gametes swims on their own/motile in humans.
• Fertilisation takes place in the oviduct/ fallopian tube in
  humans while it takes place in the ovule in the ovary in a
  flowering plant.
• Involves double fertilisation in a flowering plant but single
  fertilisation in humans.

Question 22

Compare and contrast between the adaptations of insect-pollinated and wind-pollinated flowers. [6]

Answer 22

Similarities:
Both Involve formation of flowers.[1]
Both need anther and stigma. [1]
Both involve production and transferring of pollen. [1]

Differences:

1. petals:
   I: Flowers are usually large with brightly-coloured petals.
   W: Flowers are usually small and dull-coloured, without petals
2. Stigmas:
   I: Stigmas are usually small and compact and do not protrude out of flowers.
   W: Stigmas are large and feathery and protrude out of flowers to provide a large surface area to trap pollen.
3. Stamens:
   I: Stamens are not pendulous and do not protrude out of flowers.
   W:Stamens have long pendulous filaments and protruding anthers.
4. Pollen:
   I: Pollen is larger with rough surfaces to attach to insects. Fairly abundant. Large and rough surface with spikes/hooks [1] that can cling to the backs of the animals [1] for dispersal
   W: More abundant. Pollen are smooth surfaces and are light and tiny to be carried by wind.
5. Nectar guides:
   I: Nectar guides may be present on the petals to guide insects.
   W: Nectar guides are absent.

Nectar:
I: nectar is present
W: nectar is absent

Scent:
I: fragrant and sweet-smelling flowers
W: scent is absent in flowers

Question 23

Compare and contrast between self-pollination and cross-pollination. [4]

Answer 23

Similarities (any 2)
1. Forms of sexual reproduction/Involve the production of male and female
Gametes/Involves formation of flowers/ Gives rise to genetic variation in offsprings

Differences (any 2)
1. Self-pollination involve only one plant whereas cross pollination involve 2 different plants
2. Self-pollination does not depend on external factors for pollination
whereas cross pollination needs
3. Self-pollination requires less energy and pollen wastage whereas cross pollination requires more energy and more pollen wastage.
4. Self-pollination gives rise to less genetic variation and less adaptability to environment whereas cross pollination gives rise to more genetic variation
and more adaptability to environment

Question 24

Ways to prevent self-pollination

Answer 24

1. Stamen and pistil mature at different times
2. Unisexual plant
3. Stigma of flower is above the anther
4. Stigma and anther are situated far apart from each other.

Question 25

Process of wind pollination

Answer 25

1. The pendulous filaments expose the anthers to the wind
2. When the wind blows, pollen grains are released and carried away by the wind
3. The pollen grains land on large feathery stigma, which increases surface area to trap the pollen grains.

Question 26

This pollen grain is dispersed by animal, explain how this pollen grain is adapted for dispersal.

Answer 26

This pollen grain has a large, rough and spiky surface [1] so that it can cling easily onto the animal’s body for dispersal. [1] It has a sticky surface to stick onto animal bodies and travel further away from the parent plant.

Question 27

Rice and wheat plants are wind-pollinated. Describe 2 ways in which the stamens of rice and wheat flowers are similar.

Answer 27

1.their stamens are large and pendulous
2. The stamens contain large amount of pollen grains.

Question 28

State the difference between the 2 flowers. [2]

Answer 28

In flower A, stigma is located above the anthers but in flower B, the anthers are located above the stigma.
In flower A, the ovary is located above the receptacle while in flower B, the ovary is sunken and embedded in the receptacle
Style for flower A is longer than that of flower B