REPRODUCTION IN PLANTS

Understanding Plant reproduction in Plants


Objectives

This  blog post provides readers with the following objectives. The reader will be able to:

o   Explain the concept of reproduction in plants.  
o   Describe the different methods of vegetative reproduction/propagation.
o   Explain the advantages and disadvantages of vegetative reproduction in plants.
o   Identify the various parts of a flower.
o   Determine and write the floral formulae of named flowers.
o   Explain the concept Pollination.   
o   Explain how agents e.g., animal and wind bring about pollination.
o   Outline the process of fertilization in a flower.
o   Describe how fruits and seeds are formed.

Reproduction in Plants

The creation of a life form, by a similar life is called reproduction. Reproduction is the process by which living organisms give rise to new individuals of the same species. Reproduction is one of the most important features of living beings.

There are basically two types of reproduction - Asexual and Sexual.


Asexual Reproduction in Plants

Asexual reproduction is the formation of new individuals from a single parent. It does not involve the fusion of gametes or sex cells. The offspring are genetically identical to the parent.


Vegetative Propagation (Reproduction)

Vegetative Propagation is a form of asexual reproduction in plants. A new plant grows from a vegetative part of the parent plant. Since no gametes are involved, the plants produced asexually have identical genomes and the offspring form what is known as a clone.               


Advantages of Vegetative Propagation

  1. Clonal Uniformity

Produces genetically identical offspring, maintaining desirable traits and characteristics.

  1. Faster Reproduction

Allows for quicker establishment of plants compared to growing from seeds.

  1. Disease-Free Plants

Methods like tissue culture ensure the propagation of disease-free plants.

  1. Preservation of Hybrid Varieties

Maintains hybrid varieties that do not reproduce true-to-type from seeds.


Disadvantages of Vegetative Propagation

  1. Parent plant and offspring compete for light and nutrients due to overcrowding.
  2. No new varieties are produced as there is no mixing of characters.
  3. They are more prone to diseases (less resistance to diseases), due to lack of variety
  4. Many individuals may be destroyed by disasters such as fire and flood
  5. Undesirable traits are easily transmitted to offspring
  6. Colonization of new localities is not possible as offspring are always produced close to the parent plant.


Methods of Vegetative Propagation: A Guide to Asexual Plant Reproduction

The methods of vegetative propagation may be classified into 2 types.

1.       Natural vegetative propagation 

2.      Artificial vegetative propagation


Natural Vegetative Propagation

In this method, a vegetative part of the plant, stem, root, leaf or bud, detached and gives rise to new plants under suitable environmental conditions. The vegetative part possesses stored food and sprouting buds for growth of new plant.

Vegetative propagative organs include: bulbs, corms, rhizomes, tubers, stolons and runners.


Bulb

1     short disc-like stem
2.     leaves are arranged in concentric circles round the stem
3.     buds are covered with fleshy leaves
4.     Fleshy swollen leaves
5.     Prominent scale leaves
6.     Adventitious roots.

 E.g. Onion, Garlic, Lilies, etc.


Adaptive features of Bulb

1.         Scale of leaves for protection
2.         Leaf modified for storage of food
3.         Adventitious roots for absorption of nutrients
4.         Axillary/terminal buds for propagation.


Corm

A corm is a swollen underground portion of an erect stem.

1.    Presence of nodes
2.    It bears scale leaves
3.    Presence of axillary buds
4.   Presence of aerial shoot  
5.    Presence of adventitious root
       E.g., Caladium) and (Xanthosoma)


Adaptive features of corm

1.   Buds for propagation
2.  Scale of leaves for protection
3.  Modified stem for storage of food
4.  Adventitious roots for absorption of nutrients

Vegetative propagation of plant

These images are credited to D G Mackean www.biology-resources.com

Rhizome

1.         Stem grows horizontally under the ground.
2.         Stem is modified for storage (stem swollen with food reserves)  
3.         It has nodes and internodes,
3.         Buds are covered with scale leaves,
4.         Axillary buds, and a terminal bud.
5.        adventitious roots

          E.g. Ginger, canna lily.


Adaptive features of Rhizome

1.      It is a perennating organ
2.      Adventitious roots for anchorage
3.      Brown scale leaves protect buds
4.      protecting outer cover prevents water loss to soil
5.      Modified stem for storage of food


Stolon or Runner

1.  horizontal creeping stem grows long the surface of the ground
2.  long internodes
3.  adventitious roots and lateral buds develop at a node along the stolon
4.  lateral buds grow into aerial shoots
5.  It does not store food
    e.g. grasses, strawberry, Desmodium

drawing of runner with labelling

rhizome

Stem Tuber  

1.   It is underground stem swollen with food.
2.   Presence of many leaf scars
3.   Leaf scar encloses an axillary bud forming an eye
4.   Eye is capable of producing new plants by vegetative propagation
5.   It is propagated by cutting into pieces, called setts and planting them under the soil.
          E.g., Irish potato, yam, tiger nut


Root Tuber

1.     The root is swollen near the top with food reserves  
2.     No buds and therefore, not reproductive structure
      E.g., sweet potato (Ipomoea batatas) and cassava (Manihot esculenta)


vegetative propagation asexual reproduction in plants


Vegetative propagation of yam tuber (Stem tuber)

These images are credited to D G Mackean www.biology-resources.com

diagram of root tuber (cassava) with labelling

Offsets and Suckers

Offsets and suckers are shoots that grow from the base of the parent plant. These can be separated and planted to grow new plants.

  1. Suckers grow horizontally underground for a short distance
  2. Terminal buds at the base of the underground stem produce a slender, leafy shoots
  3. Suckers does not store food
  4. Propagated by cutting the suckers

  • Offsets: Common in plants like aloe and hens-and-chicks.

  • Suckers: Common in plants like blackberries and bananas.


Artificial Vegetative Propagation

This method involves taking a piece of one parent plant and causing it to regenerate itself into a new plant. This is particularly useful to agriculturists and horticulturists in order to get the best crop and uniform yield every time. There are various ways of carrying out artificial propagation of plants: cutting, layering, grafting, budding and tissue culture.


Cutting  

It is a process in which a vegetative portion from a plant is taken and rooted in the soil to produce a new plant. The portion used is called a cutting. Cutting involves removing a piece of the parent plant - stem, root or leaf, and planting it in a soil with a few nodes below the soil. Adventitious root arises from the nodes. E.g. sugarcane, roses, citrus plants, bougainvillea

  • Learn more about Cuttings on the Royal Horticultural Society (RHS) website.


Grafting

It is the transfer of a part of one plant to the stump of another plant of different variety of the same species. The part taken from a plant is a portion of the stem with many buds. This portion is called scion and is selected for the quality of its fruit. The stump to which the scion is attached is called the stock. Stock is selected for qualities such as disease resistance and hardiness. Grafting is similar to budding but in grafting the scion is a twig.



Budding or Bud Grafting

In this method, the scion is a bud along with some bark. A 'T'-shaped cut is made on the stock into which the scion is inserted and bound with a tape. The bud, once fixed, gives rise to new branches. For example, bud grafting is done on roses, plums, peaches, pears, citrus, etc.


Layering

In this technique certain branches of parent plant are induced to produce roots. There are two methods of layering are mound layering and air layering.

Mound Layering: In this process, stem branch is bent close to the ground, pegged and covered with moist soil leaving the tips exposed. After a few days, the covered portion on the stem branch develops the adventitious roots. The branch that produces the roots is called the layer. The layer is then detached from the parent plant and grows into an independent plant. E.g. lemon, strawberry, goose berry, etc.

vegetative propagation in plant


Air Layering or Marcottage: In process, the stem is girdled, i.e. a ring of bark is removed from the basal region of a branch. It is covered with moist moss or cotton and wrapped in a polythene sheet to preserve the moisture. After a few weeks, adventitious roots arise from the injured part. The branch along with the roots is then cut from the parent plant and planted to grow into a new plant. E.g. rubber plant 

  • Explore more about Layering on Gardening Know How


Tissue Culture (Micropropagation)

Tissue culture, also known as micropropagation, is a method of plant propagation that involves the growth of plant cells, tissues, or organs in a sterile environment on a nutrient culture medium. This technique is widely used in research, horticulture, and agriculture to produce large numbers of identical plants in a relatively short period. Here, we explore the principles, steps, advantages, and applications of tissue culture.


Principles of Tissue Culture

Tissue culture is based on the totipotency of plant cells, which is the ability of a single cell to regenerate into a whole plant. By providing the appropriate conditions and nutrients, cells can be induced to grow, differentiate, and develop into complete plants.


Steps in Tissue Culture

  1. Selection and Preparation of Explants:

    • Explants: Small pieces of plant tissue, such as leaves, stems, roots, or meristems, are selected for culture.
    • Sterilization: Explants are sterilized to remove any contaminants that could hinder growth. This typically involves washing with a disinfectant solution.

  2. Initiation of Culture:

    • Placement on Culture Medium: Sterilized explants are placed on a culture medium containing essential nutrients, vitamins, and plant hormones (auxins and cytokinins).
    • Nutrient Medium: The medium is usually a gel-like substance (agar) enriched with minerals, sugars, and growth regulators.

  3. Callus Formation:

    • Induction of Callus: The explants grow and form an undifferentiated mass of cells known as callus under controlled environmental conditions (light, temperature, humidity).
    • Hormonal Balance: The ratio of auxins to cytokinins in the medium is adjusted to promote callus formation.

  4. Regeneration of Shoots and Roots:

    • Shoot Formation: The callus is transferred to a medium with a different hormonal balance to induce shoot formation.
    • Root Formation: Once shoots have developed, they are transferred to a medium that promotes root formation.

  5. Acclimatization:

    • Hardening Off: The regenerated plantlets are gradually acclimatized to outside conditions. This involves transferring them to soil or another growth medium and gradually exposing them to normal light and humidity conditions.

  6. Transplantation:

    • Planting: Fully acclimatized plantlets are transplanted into pots or fields for further growth and development.


Advantages of Tissue Culture

  • Rapid Multiplication: A large number of plants can be produced from a single explant in a relatively short time.
  • Disease-Free Plants: The sterile environment ensures that plants are free from pathogens, leading to healthier plants.
  • Genetic Uniformity: Plants produced through tissue culture are genetically identical to the parent plant, ensuring consistency in traits.
  • Conservation: Rare, endangered, and threatened plant species can be propagated and conserved through tissue culture.
  • Year-Round Production: Tissue culture can be performed independently of the season, allowing for continuous plant production.


Applications of Tissue Culture

  • Horticulture: Used for the mass production of ornamental plants, fruits, and vegetables.
  • Agriculture: Enables the propagation of crops with desirable traits, such as disease resistance and high yield.
  • Forestry: Facilitates the rapid production of tree species for reforestation and commercial purposes.
  • Research: Provides a controlled environment for studying plant growth, development, and genetic manipulation.
  • Pharmaceuticals: Produces plants that are sources of medicinal compounds and secondary metabolites.

For further reading, visit Cuttings on RHS, Layering on Gardening Know How, visit Plant Tissue Culture on Frontiers in Plant Science .and Tissue Culture on Britannica.


FLOWER

Flower is the reproductive organ of a plant. Flower is the modified vegetative shoot and is meant for sexual reproduction. Flower arises from a modified leaf called bract. Flowers produce seeds and fruits. Depending on the position of flower bud a flower may be describe as;

a.  terminal when it is located at the tip or apex of the stem
b.  axillary when it is found in the axil of a leaf
c.  cauline when it is directly on the stem.

A flower may be solitary (single) or clusters (group). A cluster of flowers with a common stalk is called inflorescence. The stalk of an inflorescence is called peduncle. Each flower has its own stalk called a pedicel. Some flowers lack a stalk and described as sessile flowers.

A flower bears floral leaves arranged in concentric circles or rings called whorl. The floral leaves are attached to a swollen portion of the pedicel called receptacle.


Structure of the Flower

Generally, a flower consists of four whorls: Calyx, Corolla, Androecium and Gynaecium


Drawing of a longitudinal section flamboyant flower

Calyx

Calyx is the outermost whorl of a flower. It consists of sepals that are green and are leaf-like in appearance. In some plants, the sepals may be brightly colored and are called petaloid. The sepals may be polysepalous (free from each other) or gamosepalous (united or fused sepals).


Functions of Calyx

1.         The calyx encloses and protects the inner whorls.

2.        Sepals contain chlorophyll and can synthesize food.


Corolla 

Corolla is found on the inside of the calyx and consists of petals. It is the most conspicuous part in the flower because it is usually large, white/brightly colored and often scented. It may possess nectaries which produce nectar. The petals may be separate from each other and are described as polypetalous or become partly/completely fused, described as gamopetalous. The calyx and corolla together are called the perianth. 


Functions of Corolla

  1. The corolla attracts agents of pollination such as insects and birds.
  2. It encloses and protects the stamens and pistil.


Androecium 

Androecium forms the third whorl inside the corolla. It consists of stamens. Stamens are the male part of the flower and produces pollen grains. Stamen consists of a sac called anther supported by a stalk called filament.


Functions of Androecium

  1. The anther produces pollen grains which contain the male reproductive cells (gametes)
  2. The filament bears and supports the anther in a position for pollen transfer to take place.


Gynoecium

Gynoecium is the fourth and the innermost whorl of the flower. It consists of one or more carpels or pistils. The carpel is the female reproductive organ of the flower. It is made up of the stigma, style, and ovary. The stigma is sticky, hairy or feathery bulb at the tip of the style which receives the pollen grains. The style is the stalk between the stigma and the ovary. Ovary is the enlarged base of the carpel containing the one or more ovules.

1.   A pistil is described as monocarpus if it has a single carpel. 
2.   Apocarpus pistil, has multiple carpels that are distinct, free, or unfused. 
3.   Syncarpous pistil, the multiple carpels are fused into a single structure


Functions of Gynoecium

  1. The ovary is a hollow cavity which contains the ovules.
  2. The elongated style bears the stigma in a position for receiving pollen during pollination.
  3. The sticky stigma can receive or trap the pollen grains.


drawing of sepal, petals, stamen and carpel

maize male and female flower

Learn more about the Functions of Flower Parts on byjus.


Some Terms Used in Describing Flowers

1.  Complete Flower: a flower with all the four whorls present.

2.  Incomplete flower: a flower with one or more floral whorls absent.  

3.  Essential whorls: stamens and pistils are described as the essential parts or whorls. They are the male and female reproductive organs of the flower.

4.  Non-essential whorls: the calyx and corolla are described as the non-essential whorls, since they are not responsible for the formation of gametes and seeds.

5.  Hermaphrodite or Bisexual or Perfect Flower: a flower that has both the male parts and female parts. Examples: roses, lilies, and dandelion.

6.   Imperfect or Unisexual Flower: a flower with either male parts or female parts. E.g., cucumbers, melons

7.  Staminate Flower: a unisexual flower bearing only male sex parts; i.e., "male flower".

8.   Carpellate Flower: a unisexual flower with only female sex parts. i.e., "female flowers".

9.  Monoecious: Plants that have carpellate and staminate flowers on a single individual plant. E.g., maize, palm oil, castor oil.

10.  Dioecious: Plants that have carpellate and staminate flowers on separate individual plants. E.g., pawpaw 

11.    Receptacle: Part of flower stalk bearing the floral parts, at base of flower.

12.  Regular or Actinomorphic flower: flower that is radially symmetrical i.e., it can be divided into several planes. E.g., orange, sweet potato

13.  Irregular or zygomorphic flower: flower that is bilaterally symmetrical. i.e., it can be divided into similar halves in one plane only. E.g. cassia, balsam


Types of Flowers

In some flowers the stamens, petals, and sepals are fused to form a floral tube called hypanthium

1.  Hypogynous Flower: a flower is said to be hypogynous, if the stamens, petals and sepals are all attached to the receptacle below the gynoecium, the ovary is said to be superior. Hypanthium is absent in hypogynous flower.

2.   Epigynous Flower:  a flower is described as epigynous if the ovary is embedded in the receptacles so that other the floral parts arise from a position above; the ovary is said to be inferior.

3.  Perigynous Flowers: in perigynous flower, the external whorls (sepals, petals, and stamens) are fused to one another forming a floral cup (hypanthium) called calyx-tube which surrounds the gynoecium. The whorls separate at the top of the cup. These flowers are said to have half-inferior ovary.


Placentation

Placentation refers to the arrangement of placentas inside the ovary OR the arrangement of the ovules in the ovary of a flower or fruit. The types of placentation are:

1. Basal: the placenta is at the base or bottom of the ovary. It can be seen in a simple or compound carpel. 

2.  Marginal: ovules are arranged along one edge of a monocarpous ovary. This type is conspicuous in legumes and simple carpels. There is only one elongated placenta on one side of the ovary.

3.  Apical: placenta is at the apex of the ovary; it is also seen in simple or compound carpel. 

4.  Axile: ovules are arranged on a central column. The ovary in axile is sectioned by radial spokes with placentas in separate locules. Axile placentation is seen in compound carpels.

5.  Central: is also known as free placentation. Here the placentae are arranged in a central column within a non-sectioned ovary and is seen in compound carpel.

6.  Parietal: it is found only in the compound carpel. The ovary is positioned in the ovary wall within a non-sectioned ovary.

drawing of kind of placentation in flower


Floral Formulas

A floral formula is a convenient way to store and retrieve information about plants. The basic floral formula summarizes as follows:

1.       Floral Symmetry: actinomorphic flower is symbolized by an asterisk *; zygomorphic by (z) 

2.       Calyx: symbolized by the letter "K". Fused sepals can be indicated by keeping the sepal number in braket. E.g. a flower with five fused sepals would be K(5).

3.       Corolla: symbolized by the letter "C". The number of fused petals is kept in bracket.

4.       Androecium: symbolized by the letter "A". The number of stamens and degree of fusion can be represented as described for the calyx.

5.       Gynoecium: symbolized by the letter "G". The number of carpels and degree of carpellary fusion is expressed as in calyx.

6.       Inferior or superior ovary: can be represented by a line drawn above "G" to represent inferior or below the "G" to represent superior ovary. E.g. a unicarpellate gynoecious with a superior ovary would be symbolized G1. An apocarpous gynoecious comprised of 3 separate carpels and a superior ovary would be represented G3.

floral formula

Interpretation: flower is actinomorphic; bisexual; calyx of 5 sepals, corolla of 5 distinct petals (or rarely apetalous), stamens 5-10 and distinct, gynoecium syncarpous with (2-5) carpels; ovary superior.


Inflorescences

Flowers may be single or occur in clusters. Clustered flowers are arranged on floral stem is known as inflorescence. The stalk holding the inflorescence is called a peduncle. The stalk of the flower is called pedicel. A flower in an inflorescence arrangement is known as a floret.

There are three main types of inflorescences in flowering plants:

Simple/Single Terminal Flower: is a type of inflorescence where there is no branching.

Racemose

Racemose is unbranched type of inflorescence. In raceme type, new flowers are generated at the tip of the inflorescence. Examples of racemose include spike, raceme, corymb, and umbel

1.  Simple Raceme: has long peduncle and bears on a number of flowers in acropetal succession.  E.g., Crotalaria.

2.  Spike: has a long peduncle which bears a number of sessile flowers in acropetal succession. E.g., Amaranthus.

3.  Catkin: is a type of spike inflorescence with a pendulous peduncle. The flowers in this type of inflorescene are generally unisexual. E.g., Mulberry.

4.  Spadix: is a type of spike with a fleshy peduncle. The flowers are usually unisexual. The spadix inflorescence is almost covered by a big bract called spathe. E.g., Aroids.

5.  Corymb: is a racemose inflorescence with a slightly shortened axis. The older flowers have the longer and the younger flowers have the shorter pedicels. As a result of this flowers the corymb inflorescence are found more or less at the same level of arrangement. E.g. Caesalpinia.

6.  Umbel: is also a type of racemose inflorescence whose main axis is shortened and at the tip bears a whorl of bracts. All the flowers are at the same level and they show centripetal arrangement.  E.g.  Carrot.

Cymose

Cymose is a branched type of inflorescence. In cymose inflorescence, axis terminate in a flower. Lateral branches of the flower develop below the terminal flower, each branch ends in a flower, and they also produce lateral branches. Every axis terminates in a flower. There are different types of cymose inflorescence:

1.   Dicahsium or Simple Cyme: is an inflorescence with a terminal flower that opens first and two opposite flowers below it

3.  Compound Dichasium or Compound Cyme: it is a branched cyme, with each ultimate unit three flowered.

4.   Monochasial cyme: it bears a terminal flower with one flower below. The branches may repeat many times to give a long-coiled inflorescence also known as helicoid cyme. Branch of this inflorescence can go in different directions.


Pollination

Pollination is the transfer of pollen grains from the anther of a flower to the stigma of the same flower or another flower of the same species. The process of pollination leads to fertilization and production of seed and fruit.

Learn more about Pollination and Its Agents on National Geographic.


Types of Pollination

1.  Self Pollination or Autogamy: is the transfer of pollen grains from the anther of a flower to the stigma of the same flower or another flower on the same plant.

2.  Cross Pollination or Allogamy: is the transfer of pollen grains from the anther of a flower to the stigma of a flower on another plant of the same species.


Advantages of self pollination

1.     Individual characteristics are not changed

2.      Efficient use of pollen grain

3.     Chances of failure of pollination are very less

4.      Self pollinated plants can grow in areas where there is less pollinators


Disadvantages of self-pollination

1.      No new characteristics are introduced.

2.      Promotes disease transfer

3.      Undesirable traits cannot be eliminated.

4.      Disease resistant capacity becomes less.

5.      Due to continuous self pollination the progeny shows less vigor.


Advantages of cross-pollination

1.      The plants which are produced through cross pollination are more disease resistant.

2.     Plants produced from cross pollination can survive any changes in environment.  

3.    New varieties of offspring or progeny are produced

4     The defective characters can be eliminated and be replaced by better characters.


Disadvantages of cross pollination

1.     Unlikely to occur when distances between plants involved are great

2.     Wastage of pollen grains

3.     Pollinating agents may not readily be available at the right time

 

Adaptation of flowers for self-pollination (i.e. prevent cross-pollination)

1.  The flower is bisexual. E.g. pride of Bardados, flamboyant.

2.  The anthers and stigma mature at the same times (Homogamy). E.g. tomato

3. The flower opens only after self-pollination has taken place (cleistogamy).

4.  The flowers are concealed in the ground.


Adaptations of flowers for cross pollination (i.e. prevent self pollination)

1.   Chemicals substance on the stigma prevents germination of the pollen grain from the same flower, each flower shows self-sterility
2.   Female and male flowers occur on separate plants; plants are dioecious. E.g. pawpaw
3.   Stamens and carpels of a flower mature/ripens at different times in bisexual flowers. This condition is known as dichogamy. Dichogamy exists in two forms: protandry, when the stamens ripen first and protogyny when the carpels ripen first. A protandrous flower is bisexual flower in which the stamens mature before the carpel. A protogynous flower is a bisexual flower in which the carpels mature before the stamen.
4.   Plants produce unisexual flowers that occur on different parts of the same plant. i.e. (plants are monoecious). E.g. maize, coconut
5.   The style of the gynoecium and filament of the androecium differ in length (heterostyly).
6.   There is a physical barrier between androecium and gynoecium, i.e. each flower is herkogamous.


Agent of Pollination

1.  Biotic Agents (bats, birds, insects)

2.  Abiotic Agents (wind  and water)


Characteristic Features of Wind-Pollinated (Anemophilous) Flowers

1.  Small and inconspicuous
2.  Petals are not brightly colored
3.  Large amount of pollen grains
4.  Pollen grains  are light in weight and powdery
5.  Anther are large and loosely attarched
6.  There are no nectaries
7.  Styles are long with large stigma
8.  Stamens hang outside flower because filaments are long
9.  Flowers are not scented


Characteristic Features of Insect-Pollinated (Entomophilous) Flowers

1.     Large and conspicuous
2.      Brightly colored and sweet-scented petals
3.      Nectaries present
4.      Short styles and stigmas often enclosed in flower
5.      Stamen hidden inside flower
6.     Anthers small, firmly attached to filaments
7.      Pollen grains large, heavy, sticky with spines on surface
8.     Few pollen grains are produced


Pollination by Birds (Ornithophily)

Birds like humming bird, sun bird and honey eater are common bird pollinators. These birds obtain nectar from flowers. The flowers that are pollinated by Birds (Ornithophilous flowers) show the following characteristics:

1.  Flowers are usually funnel shaped or have tubular corolla
2.  Brightly-colored,
3.  odorless  
4.  The floral parts are leathery and produce large amount of nectar and
5.  pollen grains are sticky.

As a bird seeks energy-rich nectar, pollen is deposited on the bird's head and neck and is then transferred to the next flower it visits.


Pollination by Bats (Chiropterophily)

In the tropics and deserts, bats are often the pollinators of nocturnal flowers such as agave, guava, and morning glory. The flowers that are pollinated by bats (chrioperophilous flowers) show the following characteristics:

1.  The flowers are usually large and white or pale-colored
2.  The flowers have a strong fruity fragrance
3.  They produce large amounts of nectar.
4.  They are naturally-large and wide-mouthed to accommodate the head of the bat.

As the bats seek the nectar, their faces and heads become covered with pollen, which is then transferred to the next flower.


Pollination by Water (Hydrophily)

This occurs mostly in aquatic plants which release pollen directly into the surrounding water. Not all aquatic plants are pollinated by water, most of the aquatic plants bear flowers above the surface of water and are pollinated by wind or by insects.

Pollination by Human (Anthropophily)

This method is often used in hybridization techniques; often done in cocoa research canters.


Differences between Entomophilous Flowers and Anemophilous Flowers

Entomophilous  

Anemophilous

Flower are large and conspicuous

Very small and inconspicuous

Brightly colored and sweet scented

Dull and scented

Nectaries present

Nectaries absent

Stigma enclosed

Stigma hanging outside

Pollen grain are large with sticky spines

Pollen grains are light and powdery

Few pollen grains are produced.

Many pollen grains are produced


Fertilization in Flowering Plants

Pollen grains containing the male gametes are release by repture of the anther and transported by insects or wind to the stigma of the plant species. Pollen grains absorbs water and nutrients secreted by stigma and swells up. The nucleus of the grain divides into a large generative nucleus and a small tube nucleus or vegetative nucleus. Outer wall of the pollen grain ruptures and pollen tube protrudes. The tube nucleus moves to the tip of the pollen tube which penetrates the stigma and grows through the style towards the ovary. The generative nucleus divides into two male nuclei behind the tube nucleus. The pollen tube on reaching the ovary grows towards and through the micropyle into the embryo sac where the tip dissolves. One male nucleus fuses with egg to form the zygote which divides to form the embryo and the cotyledons. This type of fertilization is called true fertilization or syngamy. The other male nucleus fuses with the polar nuclei to form the endosperm nucleus which develops into the endosperm of the seed. The process of fertilization is known as ‘tripled fusion’. The two types of fertilization is described as double fertilization.

double fertilization in flowering plants

Post fertilization changes

1.  The corolla, calyx and stamens fall off. In some flowers the calyx may persist, e.g. tomato, pepper and Tridax.  In Tridax the calyx develops into parachute-like structure called pappus.
2.  The fertilized ovule develops into seed
3.  The ovary develops into a fruit
4.  The integuments (sheaths enclosing the ovule) develops into Testa and tegmen of the seed
5.  The micropyle persists as a small hole in the Testa
6.  The ovary wall develops into pericarp or fruit wall
7.  The zygote develops into the embryo consisting of the plumule, radicle and cotyledon(s)
8.  The triploid endosperm tissue serves as a source of nutrition for the developing embryo.

Innature, a few types flowers do not require pollination and fertilization to produce fruits. These fruits are termed as parthenocarpic, and they do not contain seeds. 


Fruit

A fruit is a developed ovary that may contain seeds.

Parts of a Fruit

1.     Pericarp - the fruit wall. Exocarp or Epicarp is the outermost layer of the pericarp. Mesocarp is the middle layer of the pericarp. Endocarp is the innermost layer of the pericarp.

2.     Placenta - region of attachment of seeds on the fruit wall.

3.      Funiculus - stalk attaching the seed to the placenta. 

Section through tomato and orange (fresh fruits)


Functions of Fruit

Fruits serve several vital functions in the life cycle of plants:

  • Seed Protection: The fruit protects the seeds from physical damage, pathogens, and herbivores.

  • Seed Dispersal: Fruits aid in the dispersal of seeds through various mechanisms:

    • Wind Dispersal: Lightweight fruits or those with wings (e.g., maple seeds) can be carried by the wind.

    • Water Dispersal: Buoyant fruits (e.g., coconuts) can float and disperse via water bodies.

    • Animal Dispersal: Fleshy fruits attract animals, which eat the fruit and disperse the seeds through their feces. Some fruits have hooks or spines that attach to animal fur.

  • Nutrient Supply: The fleshy part of the fruit often provides nutrients to the developing seedling upon germination.


Types of Fruit

There are two types of fruit depending on the part of flower that form the fruit. They are

1.  True fruit: formed from the ovary of the flower only

2. False fruit: formed from the ovary and some other parts of the flower. E.g. cashew plants, apple,

Fruit are described as simple, aggregate or multiple (compound) depending on the number of flowers that form the fruit.

a. Simple fruit: these are formed from one flower only with either a single carpel (monocarpous), e.g. cowpea, sunflower, flamboyant or many carpels fused together (syncarpous), e.g. tomato, orange, pawpaw.

b.  Aggregate Fruit: are formed from a single flower, having many free carpels (apocarpous). Each carpel forms a simple fruit called fruitlets. Aggregate is therefore a collection of fruitlets. E.g. rose fruits, strawberry, cola fruit. 

c.  Multiple compound: these are formed from cluster of flowers or a whole inflorescence. E.g. pineapple, apple,


Simple Fruit

Simple fruits are further subdivided as dry or fleshy fruits.

Dry Dehiscent: the pericarp split open to released the seeds. They include the following:

1.  Follicle: It is formed from a single carpel and is a unilocular fruit. It splits just once to release the seeds. Examples: Sodom apple, Milkweed

2.  Capsule: It formed from flower which has more than one carpel. It splits into sections corresponding to the number of carpels. E.g. Okro, Cotton, Castor oil

3.  Silique: It consists of two fused carpels, which splits into two or four valves to release seeds. Example: Cruciferae

4.  Schizocarp: formed from flower which has only one carpel. Contains several seeds, but split into several parts each containing one seed. E.g. Desmodium, Cassia

5.  Legume: Developed from a single carpel. It splits along its seams/sutures on either side to release the seeds. They are generally referred to as pods. Examples: Peas, Beans, Peanut. 


Dry Indehiscent 

In Dry Indehiscent, pericarp does not split open (dehisce).

1.  Achene: small, one-seeded fruit; pericarp is easily separable from seed coat. Examples: Buttercup, Sunflower

2.  Samara: Winged, one-or two-seeded achene-like fruit; wing(s) form from outgrowth of ovary wall. E.g. Combretum,  Maples, Ashes

3.  Caryopsis: It is one-seeded fruit that is formed from a single carpel. The pericarp is attached or fused with the seed. Examples: Rice, Corn, Barley

4.  Nut: is a dry, hard fruit that does not split at maturity to release the seed. It develops from more than one carpel and has a tough woody wall. E.g.: Chestnuts, Acorns, Walnuts


Fleshy fruit

In Fleshy, all or parts of the pericarp is fleshy, succulent or juicy.

1.  Berry: formed from flower  containing two or more carpel, usually with many seeds. It contains a fleshy mesocarp and endocarp with a thin exocarp. E.g. Grape, Tomatoes

2.  Hesperidium: Berry with thick, leathery exocarp and mesocarp and juicy endocarp arranged in sections Juice sac from ovary wall. Orange, Lemon, Lime, Grapefruit

3.  Drupes: Usually formed from only one-carpel and with only one seed developing. The endocarp is hard and stony, fitting closely around seed. Mesocarp is fleshy, and fruit is thin skinned (thin, soft exocarp). E.g: Mango, Coconut

4..  Pome: Composed of one or more carpels surrounded by accessory tissue. It has a leathery endocarp surrounded by fleshy accessory tissue. Examples: Apple, Pear. 

For more detailed information on fruit biology and their roles, visit Fruit Biology on Britannica and Nutritional Benefits of Fruits on Healthline.


Seeds

Seeds are the vital units of reproduction and survival for many plants. They contain the embryonic plant, stored nutrients, and protective outer coatings, enabling them to develop into mature plants under favorable conditions.

A seed is a matured ovule. It consists of a tough coat or testa enclosing an embryo which is made up of a plumule, a radicle and one or two cotyledons. 


Structure of a Seed

A typical seed consists of three main parts:

  1. Embryo: The young plant that will develop into the mature plant. It consists of:

    • Radicle: The embryonic root, which will develop into the primary root system.

    • Plumule: The embryonic shoot, which will develop into the stem and leaves.

    • Cotyledons - seed leaves. It serves as food for the sprouting plant. It serves as photosynthetic primary organs after germination and before the development of foliage leaves in many plants.  It encloses and protect the other parts of the embryo. Monocot plant have only one cotyledon whilst dicotyledon plants have two cotyledons. Each cotyledon is attached to the embryo by a stalk. The part of embryo plant lying between its cotyledons and its radicle (i.e. upper part of the radicle or part beneath the cotyledons) is called hypocotyl. The lower part of the plumule or the part of embryo immediately above the cotyledon is called the epicotyl.

  2. Endosperm: A tissue that provides nourishment to the developing embryo. In some seeds, the endosperm is absorbed by the cotyledons before germination.

  3. Seed coat – is a tough, hard, outer coat. It’s derived from the wall of the ovule. The testa protects the seed from injury, fungi, bacteria and moisture loss. It consists of two layers; testa and tegmen. The testa is formed from the outer integuments while the tegmen is formed from the inner integument of the ovule.

  4. Hilum - is a scar left by the stalk which attached the ovule to the ovary wall.

  5. Micropyle - a tiny hole through the seed coat. Its function is to absorb water need for germination. 


Types of Seed

Types of Seeds

Seeds can be classified based on various criteria, including the number of cotyledons and the presence or absence of endosperm.

  1. Monocot Seeds: Contain one cotyledon. Examples include grasses, lilies, and orchids.
  2. Dicot Seeds: Contain two cotyledons. Examples include beans, peas, and sunflowers.

Based on the presence of endosperm:

  1. Endospermic Seeds: Retain the endosperm as a food reserve, such as maize and wheat.
  2. Non-Endospermic Seeds: The endosperm is absorbed by the cotyledons, such as beans and peas.

Differences between Monocotyledonous Seed and Dicotyledonous Seed

Monocotyledonous

Dicotyledonous

Endosperm present

Endosperm absent

Pericarp fused with testa

Pericarp and testa are separate

One cotyledon

Two cotyledons


External Differences between a Fruit and a Seed

Fruit

Seed

Has two scars

Has one scar

Has fruit wall/pericarp

Has seed coat/testa

Micropyle is absent

Micropyle is present

Has stalk

Has no stalk

Line of suture is present

Line of suture is absent



Difference between Sexual Reproduction and Vegetative Reproduction


Sexual Reproduction

Vegetative Reproduction

Two parents are needed

One parent is needed

Fusion of gametes

No fusion of gametes

Need of pollinating agents

No need of pollinating agents

Less over-crowding of offspring

More over-crowding of offspring

Offspring mature more slowly

Offspring mature faster

Offspring have the ability to withstand environmental hazards

Offspring have less ability to withstand environmental hazards

Reproductive structures have smaller food reserve

Reproductive structures have more food reserve

Needs of agents of dispersal 

No agents for dispersal may be needed





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