Objectives
This blog post provides readers with the following objectives. The reader will be able to:
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.
Methods of Vegetative
Propagation
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
E.g. Onion, Garlic, Lilies, etc.
Adaptive features of Bulb
1. Scale of leaves for protection2. Leaf modified for storage of food
3. Adventitious roots for absorption of nutrients
Corm
A corm is a swollen underground portion of an erect stem.
1. Presence of nodes2. It bears scale leaves
3. Presence of axillary buds
4. Presence of aerial shoot
Adaptive features of corm
2. Scale of leaves for protection
%20and%20corm%20(Xanthosoma,%20cocoyam%20and%20caladium)%20%20Vegetative%20propagation%20of%20plant.webp "drawing of bulb (onion and lily) and corm (Xanthosoma, cocoyam and caladium) with labelling")
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.
E.g. Ginger, canna lily.
Adaptive features of Rhizome
1. It is a perennating organ2. Adventitious roots for anchorage
3. Brown scale leaves protect buds
4. protecting outer cover prevents water loss to soil
Stolon or Runner
1. horizontal creeping stem grows long the surface of the ground2. 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
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
Root Tuber
1. The root is swollen near the top with food reserves
.webp "drawing of stem tuber (yam Plant)")
.webp "drawing of cassava with labelling (root tuber in vegetative propagation)")
Suckers
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
E.g. plantain, banana
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
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.
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
Tissue culture (Micropropagation)
In laboratory conditions, single plant cells can be induced to divide and grow into complete plants. It is the fastest way producing plant that are genetically identical to their parents.
1. In this technique, some tissue from a suitable part of the parent plant is excised and grown on a nutrient medium under aseptic conditions.
2. The tissue develops into mass of similar cells called callus.
3. A small portion of the callus is transferred to another suitable nutrient medium where they develop and differentiate into small plantlets or seedlings.
Advantages of Vegetative Propagation
1. The offspring are genetically identical to parent.
2. Only one parent is required.
3. It is a more rapid, easier and cheaper method.
4. Many plants are able to tide over unfavorable conditions. This is because of the presence of organs of asexual reproduction like the tubers, bulbs, etc.
5. The modern technique of tissue culture can be used to grow virus-free plants.
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
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
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
.webp "longitudinal section of flower with labeling (flamboyant)")
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.
Functions of
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.
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.
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.
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.
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).
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
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
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
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.
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.
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)
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.
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Function of Fruit
1. To protect the seed
2. To aid in seed dispersal
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.
Seeds
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.
1. 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.
2. Radicle - is the embryonic root which develops into the root system of the plant.
3. Plumule - is the embryonic shoot which develops into shoot system.
4. 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.
5. Endosperm – is a massive tissue which stores food. Its place and function is taken by the cotyledons in most seeds.
6. Hilum - is a scar left by the stalk which attached the ovule to the ovary wall.
7. Micropyle - a tiny hole through the seed coat. Its function is to absorb water need for germination.
Types of Seed
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 |
Dispersal of Fruits and Seeds
Dispersal is the removal and scattering of fruits and seeds away from their parent plant to other places
Importance of fruits and Seeds dispersal
1. It prevents overcrowding of seedlings
2. It enables plants to colonize new localities
3. It reduces competition for light and soil nutrients among seedlings
4. It minimizes the spread of epidemic diseases among crowded seedlings.
5. It prevents the parent plant from shading out its seedling
6. It enhances the chance of survival of species, since disasters such as flood and fire that destroy plants in one locality may not occurs in another locality.
Disadvantages
of Dispersal
1. Fruit or seed may be land in area where conditions are not favorable for germination
2. External agents needed for dispersal may not be available at right time
Agents of Dispersal
1. Animals (birds, bats, monkeys and humans)
2. Wind
3. Water
Features of Fruit/Seeds Dispersed by Water
1. Thick fibrous mesocarp with numerous air spaces. E.g., fruit of coconut
2. Seeds with a spongy seed coat which has large air spaces. E.g., white mangrove
Above features
reduces the relative density of the seeds/fruits and therefore float easily in
water.
Features of Fruit and Seed Dispersed by Animals
1. The fruits may be succulent, scented and brightly colored. Part of the fruit is eaten by animals and seed are left somewhere e.g., mango
2. The seed may be small and have their walls resistant to secretions of alimentary canal. Seeds therefore pass out with feces and left somewhere. E.g., guava, tomato, pepper
3. The fruits may have hooks or hairs that enable them to become attached to the fur and skin of animals or people’s clothing and brushed off elsewhere. Desmodium has hooks, fruit of Boerhavia has sticky hairs
Features of Wind-dispersed Fruit/Seeds
2. Seeds with floss (a mass of silk thread), e.g., Cotton and Silk cotton plants
3. Winged seeds e.g., Tecoma, Jacaranda, Dutchman’s pipe
4. Winged fruits e.g., Combretum
5. Light weight seeds/fruits
Features of Fruit/Seeds Dispersed by Explosive Mechanism
Unequal drying of fruit coat. Tension builds up in it due to unequal drying of the pericarp. The fruits split open suddenly with explosive noise and the seeds are catapulted away from the parent plant. E.g., pride of Barbados, cowpea, crotalaria, flamboyant.
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 |
Germination
Germination is the process by which the embryo grows and develops, eventually becoming a fully mature plant. It is the onset growth of a seed, often following a period of dormancy or the emergence of embryo from the seed coat due to onset of growth and development of the seed. The pattern of germination is similar in most dicotyledonous seeds.
Mechanism of Seed Germination
During seed germination, seed absorbs water through the micropyle. The tissues absorb water and swell and the testa becomes soft. The radicle grows first, pushing though the testa and entering the soil. Next, either the hypocotyl or the epicotyl, depending on the species, starts to elongate and carry the plumule upwards through the soil.
Dormancy is the resting period in which growth stops temporarily and metabolism is reduced to its barest minimum. Dormant seed may survive adverse conditions such as drought, flood low and high temperature.
Conditions for Germination
A viable seed can only germinate if the following environmental factors are available
1. Water - seeds need a supply of water. This softens the testa or fruit wall and allows the radicle to grow and push its way out. Water activates the enzymes which convert the starch stored in the cotyledons or endosperm into soluble sugars which are then transported in solution to the growing regions.
2. Oxygen - seeds also need a supply of oxygen. This gas is necessary for aerobic respiration, from which the seed derives its energy for all the chemical processes which contribute to growth.3. Suitable temperature (Warmth) – seed will not start to germinate if the temperature is too low. The minimum temperature needed varies with the species of seed.
Types of Germination
1. Epigeal – type of germination where the cotyledon appears above the ground. It causes by elongation of the hypocotyl of the radicle. E.g., groundnut, cowpea, castor oil.
Whichever pattern of germination occurs, the energy and raw materials required for growth come from the food (usually starch) stored in the cotyledon.
Differences between epigeal and hypogeal germination
|
Epigeal germination |
Hypogeal germination |
|
Testa split |
Pericarp split |
|
Hypocotyl emerges
uncovered |
Hypocotyl covered by the sheath/coleorhiza |
|
Cotyledons are carried above the soil |
Cotyledons remain in the soil |
|
Hypocotyl elongates |
Epicotyl elongate/straightens out |
|
Cotyledon is the first photosynthetic tissue until true foliage leaves
develops |
Cotyledons wither so true foliage leaves are first photosynthetic
tissue |
|
Energy obtained from food
stored in the cotyledon |
Energy obtained from food stored in the endosperm |
 |
| Hypogeal germination of seed |
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