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REPRODUCTION IN PLANTS

Reproduction is the process by which mature individuals produce offspring.  Reproduction is an essential characteristic of all living organisms. This process is important for procreation and for bringing about variation among individual species. Without reproduction, individual species could easily become extinct.

There are two types of reproduction: sexual and asexual. Sexual reproduction involves the fusion of male and female gametes to form a zygote. The fusion of the nuclei of male and female gametes is known as fertilization.

 Asexual reproduction differs from sexual reproduction in that it does not require two parents, and that special cells are not required. This also means that the special mechanisms necessary to bring together sex cells, and permit fertilization, then to support development of the fairly helpless stages, from zygote to independent organism, are not required.  Asexual reproduction is simply mitosis, and results in a copy of the parent organism.

Asexual reproduction in plants is also called vegetative reproduction because it simply involves the growth of parts which eventually become detached to form new plants. Since the new parts are produced by ordinary cell division (mitosis), they are direct copies of the original, with no input from another individual as in sexual reproduction.

Types of asexual reproduction

1.Sporulation
• sporulation involves the formation of spores. Spores are small haploid cells produced by plants; Spores give rise to new haploid organisms. Sporulation takes place in moulds, ferns, bryophytes, pteridophytes
2.Budding
budding is a process whereby an outgrowth arises from a parent and drops off to develop into a new organisms.This process occurs in organisms such as hydra, jelly fish, sea anemones, yeast and some fungi

3.Binary fission

This is a process whereby cell splits into two new cells of equal size. Each daughter cell grows into anew organism. The process usually occur in simple unicellular organisms such as amoeba, euglena, paramecium, some fungi and bacteria.

Alternation of Generations

Plants have two distinct multicellular stages in their life cycles, a phenomenon called alternation of generations (in contrast to the haplontic and diplontic life cycles). These two stages are the multicellular, haploid gametophyte and the multicellular diploid sporophyte.  This is very different from most types of animal reproduction where there is only one multicellular stage: a diploid organism which produces single-celled haploid gametes.

A Gamete is  a mature haploid male or female germ cell that is able to unite with another of the opposite sex in sexual reproduction to form a zygote

 A Spore is  a minute, typically one-celled, reproductive unit capable of giving rise to a new individual without sexual fusion

Gametes are always haploid, and spores are usually diploid (spores are always haploid in the plant alternations of generations life cycle).

In the alternation of generations life cycle, illustrated below, there is a mature multicellular haploid stage and a mature mulitcellular diploid stage. The multicelluar haploid stage (the gametophyte) produces gametes via mitosis which fuse to form a diploid zygote. The zygote develops into a mature multicellular diploid individual (the sporophyte), which produces haploid spores via meiosis. The haploid spores then develop into a mature multicellular haploid individual. Note the multicellular stages are named for what they produce, not what they come from.  The gametophyte makes gametes, and the sporophyte makes spores.

Diagram showing Alternation of Generations.

Though all plants display an alternation of generations life cycle, there are significant variations in different lineages of plants, consistent with their evolutionary history:

In seedless non-vascular plants, or bryophytes (mosses), the haploid gametophyte is larger than the sporophyte (the plant structure that you see is the gametophyte); this is a gametophyte-dominated life cycle. The sporophyte is attached to and dependent on the gametophyte. (By “dominated” we mean “the stage of the plant you can see by eye.”)

In seedless vascular plants (ferns), the sporophyte is larger than the gametophyte (the plant structure that you see is the sporophyte), but the gametophyte is free-living and independent from the diploid sporophyte.

The life cycle of angiosperms (flowering plants) and gymnosperms (conifers) is dominated by the sporophyte stage (the plant structure that you see is the sporophyte), with the gametophyte remaining attached to and dependent on the sporophyte (reverse of bryophytes).

Though they both have sporophyte-dominated life cycles, angiosperms and gymnosperms differ in that angiosperms have flowers, fruit-covered seeds, and double fertilization, while gymnosperms do not have flowers, have “naked” seeds, and do not have double fertilization (more on this later).

Vegetative propagation in Plants

vegetative reproduction involves the growth of parts which eventually become detached to form new plants. There are many types of vegetative propagation. these includes

1. Rhizomes

Plants such as the grasses, cattails and sedges produce underground stems or rhizomes.  As these stems grow through the soil, they will periodically produce adventitious roots and a new above ground shoot.  If the rhizome subsequently dies, a new separate plant will have been formed.

2. Tubers

 Tubers are actually modified rhizomes.  They are formed in such plants as Irish potatoes.  They develop when specialized stem branches grow down into the ground and swell up with starch containing cells.  Buds on the tubers will grow into new plants. .

3. Runners (Stolons)

These are horizontally growing stems that produce few, if any, leaves.  At the spot where a leaf would normally develop a node, these plants will produce adventitious roots down into the soil, and new above ground shoots. 

4. Plantlets

 A few seed plants such as the duckweed and Kalanchoe sp. produce miniature plants on the margin of their leaves.  These drop off and develop into mature plants.  The duckweed, which is an aquatic plant, reproduces almost entirely by this method. 

5. Bulbs

Onions, chives and lilies over-winter in the form of a bulb.  Each bulb has a very short stem which is surrounded by fleshy leaves.  In the spring, the shoot apex begins to grow using the nutrients stored in the leaves. 

6. Corms

This structure is similar to bulbs except that there are no storage leaves.  The nutrients are, instead, stored in the swollen stem. 

Sexual Reproduction in plants

In flowering plants, a flower is the reproductive organ which is a specialized shoot consisting of modified stem and leaves. A typical flower has four “layers,” illustrated and described below from external to internal structures:

• The outermost layer consists of sepals, green, leafy structures which protect the developing flower bud before it opens.
• The next layer is comprised of petals, modified leaves which are usually brightly colored, which help attract pollinators.
• The third layer contains the male reproductive structures, the Stamens are composed of anthers and filaments. Anthers contain the microsporangia, the structures that produce the microspores, which go on to develop into male gametophytes. Filaments are structures that support the anthers.

The innermost layer contains one or more female reproductive structures, the carpel. Each carpel contains a stigma, style, and ovary. The ovaries contain the megasporangia, the structures that produce the megaspores, which go on to develop into female gametophytes. The stigma is the location where pollen (the male gametophyte) is deposited by wind or by insects

Parts of a flower

Receptacle
This is the expanded end of stalk which bears floral parts

Calyx

Calyx consists of sepals.

 Sepals are usually green and  they protect flower in bud

Corolla

The corolla consist of petals, they often colored or scented to attract insect
Androecium

This consist of  the male part of flower ie  the stamens. each stamen consists of an anther containing pollen sacs. The anthers produces pollen grains which contain male gametes

Gynaecium
This consist of the female part of flower ie the style  . they also may consists of one or more carpels. each carpel contains one or more ovules in an ovary. The style bearing a stigma extends from ovary. The ovary contains female gametes which when fertilized become seedspollinators. The style is a structure that connects the stigma to the ovary.

Picture showing floral  parts of a flower

Terms which describe flowers

Hermaphrodite(Bisexual)
 This are flowers in which  have both male and female floral parts  eg Hibiscus Most flowers are hermaphrodite/bisexual

 Unisexual
This types of flowers Have only one of the reproductive organ: carpel or stamen i.e. either male or female flower.

Carpelate they are also called pistilate. They contains only carpels hence a female flower

Staminate Also called a male flower. they contains stamens only.

Dioecious plants. these plants  have their  male  and female  floral parts  (staminate and pistilate) on different plants e.g. pawpaw. The plant can be distinguished into male or female plant.

Monoecius plants these type of plants have both  pistilate and staminate  located on one same  plant. However, pistilate and staminate occur at different parts of the plants e.g. maize

Complete flower These flower has  all four parts i.e. Calyx, corolla, androecium and gynoecium

Incomplete flower

This  flowers  have some floral parts missing on the flower ie they do not have all four  floral parts

Pollination

Pollination is the process that involve transfer of pollen grains from anther to stigma of a flower

Types of pollination. There are basically two types of pollination

1. Self pollinationtakes place when mature pollen grains of a flower fall on the stigma of the same flower
2. Cross pollinationtakes place when pollen grains of a flower fall on the stigma of another flower of the same species

Advantages of pollination

1. Healthy offspring
2. Leads to variation
3. Greater chances of dispersal

Agents of pollination

1. Wind
2. Insects
3. Animals

Adaptations of flowers to wind and insect pollination
Insect pollinated flowers (entomophilus)

• Are scented to attract insects
• Have stick stigma for pollen grains to stick on
• Are brightly coloured to attract insects
• Presence of nectar to attract insects
• Have nectar guides to guide insects to the nectarines
• Have nectarines to secrete nectar
• Stigma/ anthers located inside the flower/tubal/funnel shaped corolla to increase chances of contact by insects
• Sticky/spiny/spiky pollen grains which stick on the body of insects and on stigma
• Large/conspicuous flowers easily seen by/attract insects
• Anthers firmly attached to the filament for insects to brush against them
• Landing platform to ensure contact with anthers and stigma
• Mimicry to attract (male) insects.

Wind pollinated flower (anemophilus)

• Anthers/stigma hang outside the flower to increase chances of pollination
• The style/filament is long to expose stigma/anthers
• Stigma is hairy/feathery/branched to increase surface area over which pollen grains land/to trap pollen grains
• Pollen grains are smooth/dry/light/small to be easily carried by win Large amount of pollen grains to increase chances of pollination
  Anthers loosely attached to filaments to enable them to sway to release pollen grains
• Pollen grains may have structures which contain air to increase buoyancy
  Flowers have long stalks holding them out in the wind

Ways in which plants prevent(Hinder) self-pollination

• Protandry (anthers/stamens mature first)
• Protagyny  (pistils mature first)
• Monoecism (where male and female parts are on same plant but different parts)
• Dioecism (where male and female parts are on different plants)
• Incompatibility (self sterility)
• Heterostyly (styles at different heights)

Characteristics that ensure cross pollination takes place in flowering plants
      • Presence of special structures that attract agents of pollination

• Protandry/dichogamy
• Protagyny/dichogamy
• Monoecism
• Self sterility
• Heterostyly

Advantages of cross pollination

1. Hybrid vigour
2. Less prone to diseases
3. Promotes genetic variation
4. Greater evolutionary potential

Fertilization

Fertilization is  the fusion of male and female gametes to form a zygote . fertilization  follows pollination. During pollination, a pollen grain is deposited on the stigma. This pollen grain sticks to the surface of the stigma.

 The surface of the stigma produces a chemical substance which stimulates the pollen grain to produce a pollen tube/to germinate. The pollen tube grows through the style tissues on which it feeds until it enters the ovary.

The generative nucleus divides into two giving two male nuclei.  Embryo sac contains eight nuclei i.e. two synergids, egg cell, two polar nuclei and three antipodal cells

 The pollen tube enters the embryo sac through the micropyle and one of the male nucleus fuses with the egg cell/ovum to form a zygote.

The other male nucleus fuses with the two polar nuclei to form the triploid nuclei/endosperm)food storage used by developing embryo). The pollen tube nucleus in the pollen tube disintegrates soon afterwards. This process is referred to as double fertilization.

Zygote grows into an embryo containing plumule, radicle and cotyledons
 Double fertilization

This is where two male nuclei enters the embryo sac. One  of the nuclei fuses with the ovum to form a zygote, while the other fuses with the polar nuclei to form a triploid primary endosperm nucleus.

• Therefore there are two fusions at fertilization Hence Double Fertilization.
Changes that Occur in a flower after fertilization

• Petals, stamen, calyx and style wither
• Ovary wall changes into pericarp
• Intergument changes into seed coat/testa
• Zygote changes into embryo (by mitosis)
• Primary endosperm nucleus changes into endosperm
• Whole ovule changes into seed
• Ovary develops and grows into fruit(under the influence of gibberrellic hormone)

Fruit formation

 One of the organs that remains on the plant after pollination and fertilization is the ovary. Within the ovary, the developing embryo produces special chemical substances that stimulate the young ovary.  These substances also signal the start of the formation of the fruit, which is a mature ovary. The fruit may contain one or more seeds

During fruit formation the ovary increases in size while ripening or maturing.

 There are two types of fruits

1. A true fruit is formed from the ovary of a flower after fertilization. It has two scars(style scar and stalk scar) and contains seeds
2. False fruit . these are fruits  that develop from other parts of the flower  other than the ovary,

How is a seed formed?
• After fertilization, zygote grows into an embryo, primary endosperm nucleus developed into endosperm, interguments harden to form testa, hence the whole ovule becomes the seed
• the seed loses water to become drier

The main parts of a seed
A seed has plumule, radicle, seed leaves called cotyledons, a microphyle and a scar
Testa also called seed coat is a  tough outer covering which protects the seed from insects, bacteria etc
Hilum is  a scar or  a spot where the seed was attached to the fruit or pod
Micropyle is a s mall hole through which water and air enter the seed
Radicle is an embryonic root that grows into the root system

Plumule is an embryonic shoot that grows into shoot
Cotyledons are embryonic leaves that store food for the germinating seed i.e. for plumule and radicle
When plumule and radicle grow, they use food stored in the cotyledon. in some seeds food is stored in the endosperm

Seed Dormancy and Germination

Seeds enters a period of temporary development after fertilization; in most species, the seed then enters a period of stasis (inactivity), called dormancy. Dormancy is triggered by loss of up to 95% of the seed’s water content, which dehydrates the seed, causes extremely low metabolic activity, and “concentrates” the seed’s sugars to protect the cells from freezing during winter months. Dormancy can last months, years, or even centuries in some cases.

Once conditions are appropriate for seedling growth, the seed will then germinate or re-initiate development. The signal to initiate seed germination is indicator that conditions are favorable for growth and, depending on the species, can include:

• water, indicating the start of the rainy season and rehyrdating the seed
• specific wavelengths of light, indicating favorable sunlight conditions necessary for photosynthesis and the seed is not buried too far under the soil
• a sustained period of cold, indicating that the seed does not germinate until the cold season is over
• fire, typical of forest trees, indicating reduced competition from existing tall tree
• scarification, or chemical treatment with acids, indicating that the seed has passed through the digestive tract of an animal