Unit 10: Reproduction in Plants — Long Questions

Definition
Reproduction is the process in which organism produce new organisms of their own kind.

Types
There are two main kinds of reproduction.

1. Asexual reproduction
(a) The reproduction that does not involve the fusion of gametes is called asexual reproduction.
(b) The offspring produced by asexual reproduction are genetically identical to the parents.

2. Sexual Reproduction
(a) The reproduction that involves the fusion of male and female gametes is sexual reproduction.
(b) In sexual reproduction, the offspring have variations among themselves and with the parents.

Definition
Some common methods of asexual reproduction in different organisms are given below.

1. Binary Fission / Explain the process of binary fission in bacteria and describe how it leads to the formation of two daughter bacteria?

Definition
Binary fission means division into two. It is the usual method of reproduction in bacteria.

Examples

a. Binary Fission in Bacteria
i. During binary fission, the bacterial DNA replicates and the daughter DNA molecules move to opposite sides.
ii. The cell membrane pinches in.
iii. New cell's wall is synthesized in the middle and so two identical daughter cells bacteria are produced.

b. Binary Fission in Protists (Unicellular, Eukaryotes)
i) Many protists (unicellular eukaryotes e.g. Amoeba, Euglena etc.) also reproduce by binary fission.
ii) In protists the nucleus of parent organism divides into two.
iii) This is followed by the division of cytoplasm. So, two daughter protists are formed.

c. Some animals e.g. planarians also reproduce asexually by binary fission.

2. Budding

a. Budding in Yeast
i) This method is very common in yeast (a unicellular fungus)
ii) During budding a part of the parent organism grows out from its body. This part is called a bud.
iii) When the bud has grown big, it may separate from parent body or may remain attached.

b. Some animals e.g. hydra also reproduce asexually by budding.

3. Spore Formation

Introduction
Spores are thick-walled asexual reproductive cells.

Explanation

a) Sporangia
i) Most fungi (e.g. Rhizopus; bread mold) produce spores in special sac-like structures called sporangia (Singular: sporangium).
ii) When spores are mature, the sporangium bursts and spores are released.

b) Germination of Spores
Spores can tolerate unfavorable conditions due to their thick walls. When favorable conditions are available the spores germinate to produce new fungus.

c) Endospores Formation
Some bacteria reproduce by forming endospores (spores produced inside the cell). They form endospores in unfavorable environmental conditions. Even if the original cell dies, the endospore survives. When conditions improve, the endospore grown into a new bacterium.

4. Vegetative Propagation / What do you mean by vegetative propagation? Differentiate among different plant structures modified for vegetative propagation.

Introduction
It is a method of asexual reproduction in plants. In this method, new plant is produced from the vegetative part (root, stem or leaf) of the parent plant. Vegetative propagation takes much less time to produce new generation as compared to the sexual method. Secondly, the offspring are genetically identical to the parent plant.

Kinds of Vegetative Propagation
Vegetative propagation may be natural or artificial.

1. Natural Vegetative Propagation
Natural vegetative propagation is process where plants reproduce on their own, using structures like stems, roots or leaves.

2. Artificial Vegetative Propagation
Artificial vegetative propagation means the processes in which humans use the vegetative parts of plants for their reproduction by methods like cuttings, grafting, or layering.

Explanation

Natural Vegetation Propagation
In the natural vegetative propagation, plants use the following vegetative parts for producing new plant.

(a) Stem
The following types of stems take part in vegetative propagation in plants:

1. Stolon (runner)
i. It is a horizontal stem that grows above the ground.
ii. A stolon has nodes where new leaves and roots grow.
iii. The leaves grow upwards and roots grow down.
iv. In this way, a new plant is formed at the node.
v. Strawberry reproduces by using its stolon.

2. Tuber
i. It is fleshy stem that grows underground. It has "eyes" which are actually its buds.
ii. Eyes can grow into new plants. Potatoes reproduce by tubers.

3. Rhizome
i. It is a horizontal stem that grows below the ground.
ii. It has nodes where new leaves and roots grow. In this way, a new plant grows from each node.
iii. Ferns, ginger, and sugar cane reproduce by using rhizome.

4. Bulb
i. It is a very short stem that grows underground.
ii. It has bud and fleshy leaves.
iii. Bulbs grows naturally to produce new plants.
iv. Tulips, onions and lilies reproduce by bulbs.

5. Corm
i. It resembles the bulb but does not have fleshy leaves.
ii. Almost all of a corm consists of stem, with a few brown non-functional leaves on the outside.
iii. Dasheen and garlic reproduce by corms.

(b) Suckers
i. Suckers are new shoots that emerge from the base of the parent plant or from its underground roots.
ii. These shoots grow into new plants while still attached to the parent.
iii. When suckers develop their own root system, they become independent.
iv. Examples are banana and raspberry plants.

(c) Modified Leaves
i. The leaves of some plants (e.g. Bryophyllum) are modified for vegetative propagation.
ii. Such leaves have buds at their margins.
iii. When leaf falls on ground, the buds grow into new plants.

Advantages and Disadvantages of Vegetative Propagation

Advantages
i. Vegetative propagation allows to produce many new plants in a short time.
ii. The new plants are exactly like the parent plant, so they all have same good characteristics. This means useful qualities, like good fruit or strong growth, are passed on to the next generation.

Disadvantage
i. Plants produced through vegetative propagation do not have genetic differences.
ii. In other words all the offspring are identical. Due to it, they are equally sensitive to environmental changes and prone to the same diseases or pests.

Introduction
The major groups of plants have two types of generations during sexual reproduction which come one after the other. These are sporophyte generation and gametophyte generation. The sporophyte generation produces spores which grow and make the new gametophyte generation. The gametophyte generation produces gametes which unite and make the new sporophyte generation. This phenomenon is called alternation of generations.

Sporophyte Generation
The sporophyte generation is diploid (2n) and produces haploid (1n) spores by meiosis. The spores develop into haploid gametophyte generation.

Gametophyte Generation
The gametophyte produces haploid gametes by mitosis. The haploid gametes fuse to form diploid zygote, which develops into the next sporophyte stage.

Life Cycle of Angiosperms (Flowering Plants)/Explain the life cycle of flowering plants, focusing on the alternation between the gametophyte and sporophyte generations.

In angiosperms, flowers are the organs for sexual reproduction.

Parts of Flower
i. Receptacle
The receptacle is the swollen tip of a flower stalk where all the floral parts are attached. It serves as the base that supports the flower's structure.

ii. Floral Parts
Floral parts are in the form of the following four concentric whorls, or rings:
1. Calyx: It is the outermost whorl. It is made of green leaf-like sepals. Sepals protect the inner parts of a developing flower before it opens.
2. Corolla: It is the second whorl and made of petals. Most flowers have coloured petals.
3. Androecium: It is the third whorl and is made of male reproductive structures called stamens. Each stamen consists of an anther and a filament. Anther contains pollen sacs (microsporangia), which produce microspores. The stalk-like filament supports the anther
4. Gynoecium: It is the innermost whorl made of the female reproductive structures called carpels. A carpel consists of three parts.
i. The enlarged base of carpel is called ovary. It is the part where ovules are produced. Ovules produce megaspores during reproduction.
ii. The stalk-like part attached to ovary is called style.
iii. The tip of style is called stigma.
iv. In some flowers, one or more carpels are fused to form a structure called pistil.

Stages of the Life Cycle
An angiosperm plant represents the sporophyte generation. When a flower matures, it produces spores. The spores germinate and make female and male gametophytes. The gametophytes are small structures consisting of few cells only. They make gametes which combine to form zygote that develops into new sporophyte.

Following are the main stages in the life cycle of an angiosperm:

1. Development of Female Gametophyte (Embryo Sac) / Describe how the female gametophyte (embryo sac) develop within the ovule of a flower?
i. The ovule acts as megasporangium. It contains diploid megaspore mother cell which undergoes meiosis, and produces four haploid megaspores.
ii. Only the megaspore remains alive.
iii. Inside megaspore, eight haploid nuclei are formed by mitosis.
iv. Two nuclei migrate to the center and fuse to form a fusion nucleus.
v. One nucleus out of the remaining six forms the female gamete i.e., egg cell.
vi. The resulting structure, which contains seven cells (one egg cell, one fusion nucleus, and five non-functional cells), is the female gametophyte or embryo sac.

2. Development of Male Gametophyte (Pollen Grain)
i. The pollen sacs present in anther act as microsporangia.
ii. Each pollen sac contains many diploid microspore mother cells.
iii. Each microspore mother cell undergoes meiosis and produces four haploid microspores.
iv. A microspore undergoes mitosis.
v. The resulting two-celled structure is a pollen grain, which is the male gametophyte.
vi. One cell in pollen grain is the tube cell, which will form the pollen tube. The other cell is the generative cell, which will form two sperms.

3. Pollination
i. The male gametophyte (pollen grain) contains sperms while the female gametophyte (embryo sac) contains egg.
ii. The pollen grains are transferred from the anther to the stigma so that the sperms can fertilize the egg.
iii. It is called pollination i.e. the transfer of pollen grains from an anther to a stigma.

4. Fertilization
i. When pollen grain reaches stigma, its tube cell forms a pollen tube.
ii. This tube grows through the stigma and style towards the ovary.
iii. The pollen tube reaches the ovule and enters in it through the micropyle.
iv. The generative cell of pollen grain forms two sperms, which enter the embryo sac to reach the egg.
v. Formation of Zygote and Embryo: One sperm fuses with the egg, forming a diploid zygote. The zygote eventually develops into an embryo.
vi. Formation of Endosperm
The second sperm fuses with the fusion nucleus, producing a triploid (3n) nucleus. This nucleus then develops into tissue called endosperm. The endosperm provides nourishment for the embryo.
vii. Double Fertilization
This process of the fusion of two sperms (one with the egg and the other with the fusion nucleus) is called, double fertilization. It is a unique characteristic of angiosperms.

5. Seed and Fruit Formation
i. After fertilization, the zygote develops into embryo and the triploid nucleus develops into endosperm tissue.
ii. After these developments, the ovule is said to be matured and is now called seed. The ovary changes into fruit.

6. Development of Sporohytes
i. When seeds mature, they are dispersed.
ii. If seeds get suitable conditions, their embryos develop into new plants (the sporohytes of the next generation).