REPRODUCTION IN PLANTS
Why do species reproduce?
To make sure a species is sustained, to maintain balance in life.
E.g. Dodos and mammoths reproduced at a rate slower than the rate at which they were hunted/used leading to their extinction. The same applies to plants.
Asexual Reproduction
Figure 4. Example of Fragmentation
Figure 5. Vegetative Propagation in a bulb
Figure 3. Budding in Hydra
Figure 1. Sporulation in mushrooms
Figure 2. Process of sporulation
MULTIPLE FISSION
BINARY FISSION
Asexual reproduction only uses the process of mitosis (resulting in identical offspring being produced). There are seven types of asexual reproduction.
1. Vegetative Propagation
2. Cloning
3. Sporulation
4. Budding
5. Parthenogenisis
6. Binary and multiple fission
7. Fragmentation and Regeneration
SPORULATION
- Small unicellular bodies called spores detach from parent and grow into new organisms
- Mushrooms produce 500,000 spores a minute at peak of production (once production is finished, mushroom bursts and spores are released. The wind carries these spores to different locations.).
- E.g. Bacteria, protozoan, algae, fungi, mosses, ferns.
Process:
1. DNA Replication
2. Forespore (membrane inside cell)
3. Endospore (Coating of spore)
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Endospore - A dormant, tough, non-reproductive structure produced by a small number of bacteria, algae, and fungi. Helps protect DNA from environmental stress (e.g. temperature and toxic chemicals)
Once environment is favorable, the spores germinate.
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BINARY AND MULTIPLE FISSION
- Bacteria and protozons (single celled organisms/prokaryotes)
- Simple mitosis (Order of cell division - DNA, Nuclear Material, Cell Itself)
- If cell divides into 3 or more, multiple fission
- If cell divides into 2, binary fission
- Under favorable conditions, daughter cells grow rapidly and divide
- Average life of a cell (before mitosis) is 20 minutes.
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BUDDING
- Outgrowth develops on parent, and later detaches and becomes independent
- Process occurs in flatworms (platyhelminthes), segmented worms (annelids), and hydra (cnidarians)
- Like binary fission, but the resultant cells are not of equal size, but comprises a smaller bud cell, becoming detached from the larger parent cell.
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FRAGMENTATION AND REGENERATION
- Organism is split into fragments, and each fragment grows into complete individual organisms that are clones of the original organism.
- Regeneration is a simple process of regrowth
E.g. Lizard tails
E.g. Human liver and skin
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PARTHENOGENESIS
- Development of a female gamete without fertilization (asexually)
- Parent is always diploid (paired chromosomes)
Through mitosis, offspring is always diploid
Through meiosis, offspring is always haploid
- Ovum starts reproducing into a new organism
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E.g. in honeybees
- Asexually - Male Bees
- Sexually - Female Bees
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VEGETATIVE PROPAGATION
- Plant being produced without seeds
- E.g. Onions and tulips (bulbs), Crocus (corn), potato (tuber)
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CLONING
- Clones are organisms that are exact genetic copies. Every single bit of their DNA is identical.
- Clones can happen naturally (e.g. identical twins)
- Cloning of organs
Process of Cloning
1. Take mature egg cell (foster mother)
2. Take out nucleus
3. Add nucleus of designated cell (to be cloned)
4. Give suitable conditions in lab (mimic uterus)
5. Embryo forms in petri dish
6. Embryo put back in foster mother
7. Birth naturally of a clone (is same age as original organism)
E.g. Dolly the sheep
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Figure 6. Process of cloning
ADVANTAGES OF ASEXUAL REPRODUCTION
- Only one parent required
- Fusion of gametes not required
- All beneficial qualities passed down to offspring
- Faster method of producing offspring compared to sexual reproduction
- Colonization of area can happen rapidly
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DISADVANTAGES OF ASEXUAL REPRODUCTION
- No genetic variation in offspring. Hence species are not well adapted to changes in the environment.
Asexual Reproduction
Does not involve the fusion of gametes
Only one parent required
Offspring are genetically identical
Relatively quicker method
Sexual Reproduction
Involves the fusion of a male gamete with a female gamete to form a zygote.
Requires two parents (except bi-sexual flower bearing plants)
Offspring are genetically different
Slower method
Sexual Reproduction in Plants
Sexual Reproduction is a process involving the fusion of two gametes to form a zygote. It produces generally dissimilar offspring.
ADVANTAGES
- Offspring may inherit qualities from both parents
- There is greater genetic variation in the offspring, leading to species that are better adapted to change in the environment
DISADVANTAGES
- Two parents are required (expect in plants with bisexual flowers)
- Fusion of gametes is required
- Slower method of producing offspring as compared with asexual reproduction
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Figure 7. Structure of a bisexual flower
A complete flower consists of the following parts:
Pedicel, receptacle, sepals, petals, stamens and carpels (pistil).
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Petal - Modified leaves forming the most obvious (conspicuous) part of the flower. All the petals together make up the corolla. In insect-pollinated flowers, petals:
- are brightly colored to attract insects for pollination; and
- provide a platform for insects to land
Sepal - Modified leaves which enclose and protect the other parts of the flower in the bud stage. All the sepals together make up the calyx. The sepals usually form the outermost layer of floral leaves. However, some flowers, for example, Hibiscus, have another layer of floral leaves outside the sepals, which make up the epicalyx of the flower.
Receptacle - The enlarged end of the flower stalk which bears the other parts of the flower
Pedicel - The flower stalk
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Stamen (Male Part)
Anther - The anther consists of two lobes. Each lobe contains two pollen sacs. In the pollen sacs are the pollen grains. The anther produces pollen grains. When the anther matures, it splits open to release the pollen grains. The anther also contains a vascular bundle.
Filament - The filament is the stalk that holds the anther in a suitable position to disperse the pollen.
Pollen grains - Produced by meiosis, hence haploid. Each pollen grain contains a male gamete. Note that the male gamete in flowering plants is the nucleus in the pollen grain.
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Carpel (Female Part)
Stigma - The stigma is a swollen structure at the end of the style. It receives the pollen grains. The mature stigma secretes a sugary fluid that stimulates the pollen grains to germinate.
Style - The style is a stalk that connects the stigma to the ovary. It holds the stigma in a suitable position to trap pollen grains.
Ovary - The ovary is the structure that will develop into a fruit after fertilization. It produces and protects one or more ovules.
The ovule is the structure that will develop into a seed after fertilization (if the carpel contains more than one ovule, more than one seed will be produced). It produces a female gamete or ovum by meiosis. The ovum is therefore haploid. The ovule is attached to a region by a stalk called the funicle.
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Pollination
Pollination is the transfer of pollen grains from the anther to the stigma.
In order for flowering plants to reproduce sexually, the pollen grains must be transferred from the anthers to the stigmas so that the male and female gametes can later be brought together. There are two types of pollination: self-pollination and cross-pollination.
Self-pollination - the transfer of pollen grains from the anther to the stigma of the same flower of a different flower on the same plant.
Features favoring self-pollination:
- Flowers are bisexual with anthers and stigmas maturing at the same time
- The stigma is situated directly below the anthers
- In certain plants with bisexual flowers, some flowers never open. Only self-pollination can occur in these flowers.
Advantages:
- Only one parent required
- Beneficial qualities will pass down from parent
- Does not depend on external factors such as insects or wind for pollination
- Since the anthers are close to the stigmas of the same flower, there is a higher probability that pollination will occur, as compared with cross-pollination.
- Less pollen and energy is wasted in self-pollination as compared with cross-pollination
Disadvantages
- Less genetic variation in the offspring. The species is less well adapted to change in the environment.
- The probability of harmful recessive alleles being expressed in the offspring is higher as compared with cross-pollination.
Cross-pollination - the transfer of pollen grains from one plant to the stigma of a flower in another plant of the same species.
Features favoring cross-pollination:
- Dioecious plants, e.g. papaya, bear either male or female flowers. This type of plants cannot undergo self-pollination.
- In many plants with bisexual flowers, the anthers and stigmas may mature at different times
- The stigmas of plants with bisexual flowers may be situated some distance away from the anthers so that self-pollination is unlikely.
Advantages:
- Offspring produced may inherit beneficial qualities from both parents
- More variety of offspring can be produced, that is, there is greater genetic variation. This increases the chance of the species surviving changes in the environment because any changes in the environment is less likely to destroy all the varieties in a species.
- Increased probability of offspring being heterozygous.
Disadvantages
- Two parent plants required
- Depends on external factors such as insects or wind for pollination
- Since the pollen grains have to be transferred from the anther of one plant to the stigma of another plant, there is a lower probability that pollination will occur.
- More energy and pollen is wasted as compared with self-pollination.
Figure 8. Insect Pollination of a flower
Figure 9. Structure of flower for wind pollination
Fertilization in Plants
1. After pollination, the pollen grains germinate after it comes into contact with the stigma, in response to the sugary fluid secreted by the mature stigma.
2. A pollen tube grows out from each pollen grain. The male gamete enters the pollen tube.
3. As the pollen tube grows, it secretes enzymes to digest the surrounding tissue of the stigma and the style. Thus, the pollen tube grows down the style into the ovary.
4. The pollen tube enters the ovule through an opening in the ovule wall called the micropyle.
5. Within the ovule, the tip of the 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.
