DEVELOPMENT OF NEW SEEDS

Subject: 

BIOLOGY

Term:

FIRST TERM

WEEK 4

 

TOPIC: DEVELOPMENT OF NEW SEEDS

 

CONTENT:

  • Progress of development of zygote in the flowering plant
  • Germination of seeds

 

SUB-TOPIC I: PROGRESS OF DEVELOPMENT OF ZYGOTE IN THE FLOWERING PLANT

 

Meaning of seed

 

Seeds are plant parts containing embryo and this embryo give rise to new individual.

Pollination is the first step in the process of sexual reproduction in flowering plants. It is followed by fertilization and development of zygote. These steps change the flower into a fruit enclosing seeds, the potential next plant

 

Pollination: this is the process whereby mature pollen grains are transferred from the anthers to the mature stigmas or pollination is the transfer of pollen grains from the anther to the mature stigma of a flower.

There are two types of pollination, They are:

 

  1. Cross pollination 
  2. Self pollination

 

Bumblebee Pollinating

 

When a bee such as this bumblebee visits a flower, tiny grains called pollen stick to the bee. The bee then carries the pollen to another flower. This process, known as pollination, is necessary for many plants to make seeds and fruit.

 

Cross Pollination 

 

Cross pollination occurs when the pollen grains from one flower are transferred to the mature stigma of another plant of the same species.

 

Features Of Cross Pollination

 

  1. Male and female reproductive parts occur in different flowers i.e. the flowers are unisexual
  2. If the plant is dioecious, then the male and the female flower occur on different plants of the same species.
  3. In monoecious plants, where male and female flowers occur on the same plants, self pollination is prevented by:
    1. The stamens ripening before the stigmas (flower is described as being protandrous e.g. sunflower, sativa)
    2. The stigmas ripening before the stamen (protogynous e.g. grasses)
  4. Self sterility which may occur in some flowers. This means that the pollen will not fertilize the ovary of the same flower or another flower of the same plant because the pollen cannot grow or grows slowly on the stigma. The flower eventually dies before fertilization occurs. Examples of this are the flowers of many leguminous plants and Ixora
  5. Some upright cross pollinated flowers have their stamen arranged below the stigmas. This will prevent pollination and may ensure that the visiting insect touches the stigmas before the stamens.

 

Self Pollination

 

Self pollination occurs when the pollen grains from a flower are transferred to the mature stigma of the same plant of the same species.

 

 

 

 

Features of Self Pollinated Flowers

 

  1. In composite plants e.g. sunflower, the flowers first have a chance of cross pollination hence they are protandrous. If cross pollination fails, the self pollination occurs by the stigmas growing and curling backward to pick up pollen grains remaining in their styles.

Sunflower    Cosmos

 

  1. Some flowers do not open, so that self pollination occurs in them. Such cleistogamous flowers are often small and inconspicuous in some normal flowers e.g. cowpea, self pollination occurs before the flower opens.
  2. The structure of some flowers e.g. caladium (ornamental cocoyam) enables them to trap insects which brings about self pollination.

 

Agents of Pollination

 

These include:

  1. Insects, birds, bats, and other animals
  2. Wind
  3. Water

 

Characteristics of insect pollinated flowers

 

  1. they have brightly coloured parts e.g. corolla, bracts
  2. The pollen grains are often rough and stick to the insect’s body easily.
  3. They are often scented
  4. They often have broad or rounded sticky stigmas for the visit of particular kinds of insects
  5. The flowers have structural modifications suited for the visit of particular kinds of insects.
  6. They contain nectar, sugary liquid which serves as food for many insect pollinators.
  7. Some flowers have nectar guides (lines or patches) leading the insects to the position of the nectary gland.

 

Characteristics of wind pollinated flowers

 

  1. They are often unisexual e.g. grasses.
  2. Pollen grains are produced abundantly to make up for the loss of many in transit
  3. Pollen grains are light and smooth to float easily in the wind.
  4. Anthers are often large and hinged on flexible filament. This arrangement makes it easy for pollen grain to be easily shaken off.
  5. Styles and stamen are long extending beyond the flower.
  6. Stigmas are often feathery for easy trapping of pollen in the air.
  7. Flowers are dull, without scent and nectar.
  8. Perianth is often absent but becomes reduced if present.

 

DIFFERENCES BETWEEN SELF POLLINATION AND CROSS POLLINATION

 

SELF POLLINATION CROSS POLLINATION
1. Transfer of pollen grains from the anther 1. Transfer of pollen grain from the anther
to a stigma of the same flower or another to the stigma of another plant but of the
flower of the same plant. same species.
2. Only one parent is involved 2. two parents are involved
3. it does not result in new varieties 3. It results in new varieties.

 

EVALUATION:

 

  1. Define the following terms;
    • (a) Seed
    • (b) Pollination
  2. State three features each of:
    • (a) Self pollination
    • (b) Cross pollination
  3. State three differences between self pollination and cross pollination in a tabular form.
  4. Mention two agents of pollination. 

 

SUB-TOPIC 2:

FERTILIZATION AND ZYGOTE DEVELOPMENT PROCESS IN FLOWERING PLANT

 

Fertilization is the fusion of male and female gametes. After pollination, a pollen grain sends out a pollen tube through the stigma and style into the ovary, finally reaching the ovule. Within the pollen tube are two nuclei and a pollen tube nucleus which is responsible for the development of the pollen tube.

The moment the pollen tube reaches the ovule through the micropyle, the pollen tube nucleus disappears.

 

Each ovule consists of a large oval shaped cell called embryo sac which is surrounded by two protective coats called integuments. Integuments later give rise to seed coat. The mature embryo sac contains eight nuclei, two called secondary nuclei.

 

Flower Pollination and Fertilization

 

Flowers contain the structures necessary for sexual reproduction. The male component, or stamen, consists of a thin stalk called the filament, capped by the anther. The female component, the pistil, includes the stigma, a sticky surface that catches pollen; the ovary, which contains the ovule and embryo sac with its egg; and the style, a tube that connects the stigma and ovary (A). Pollen is produced in the anther (B), and is released when mature (C). Each mature pollen grain contains two sperm cells. In self-pollinating plants, the pollen lands on the stigma of the same flower, but in cross-pollinating plants—the majority of plants—the pollen is carried by wind, water, insects, or small animals to another flower. If the pollen attaches to the stigma of a flower from the same species, the pollen produces a pollen tube, which grows down the neck of the style, transporting the sperm to the ovule (D). Within the embryo sac of the ovule, one sperm cell fertilizes the egg, which develops into a seed. The second sperm cell unites with two cells in the embryo sac called polar nuclei, and this results in the development of the endosperm, the starchy food that feeds the developing seed. The ovary enlarges (E) and becomes a fruit.

Embryo sac and its fertilization

One of the two male nuclei fuses with the female nucleus (ovum) to form the zygote and the second male nucleus fuses with the secondary nucleus to form endosperm. The zygote later forms the embryo, the ovule form the seed and the ovary forms the fruit. A fruit which develops without fertilization is described as Pathenocarpic.

 

A seed is a dormant embryo sporophyte with stored food and protective coats.

 

A seed consists of;

 

  1. a plumule, made up of two embryonic leaves which will become the first true leaves of the seedling and a terminal (apical) bud.
  2. the hypocotyle and radicle which will grow into the stem and primary root.

 

Angiosperms that produce seeds with two cotyledons are called dicotyledons e.g. bean and those that have a single cotyledon are called monocotyledons e.g. maize grain and other grass seeds.

 

Seeds

 

Differences between Pollination And Fertilization

 

POLLINATION FERTILIZATION
1. No fusion of male and female gamete 1. Involves fusion of male and female gametes
2. Pollination takes place externally 2. Fertilization takes place internally
3. Germination tube not formed 3. Germination tube formed
4. Agent is required 4. Agent is not required
5. Involve the transfer of pollen grains 5. Pollen grains not involved.
6. Zygote not formed 6. Zygote formed
7. One gamete is involved 7. Two gametes are involved.

 

EVALUATION:

 

Describe the process of fertilization in flowering plants

 

SUB-TOPIC 3: GERMINATION OF SEEDS

 

Germination is the stage when embryo develops into a seedling and it also refers to all the changes that occur when the embryo in a seed resumes its growth and development to become an independent seedling.

When seed is about to germinate, it absorbs several times its own weight of water and swells up.

 

TYPES OF GERMINATION

 

There are two types of germination:

  1. Epigeal germination: This is when the hypocotyl grows rapidly and elongates carrying the cotyledon above the soil where they carry out photosynthesis before the first foliage leaves develop.

Epigeal germination is also referred to as a type of germination in which the cotyledon is carried above the soil.

 

Examples of plants with epigeal germination are castor and cowpea plant

Description of Epigeal Germination

 

  1. Seed absorbs/imbibes water and swells.
  2. Testa splits and the radicle emerges
  3. The hypocotyls of the germinating seed elongates
  4. It pushes the seed out of the soil.
  5. The cotyledon(s) become exposed to the sunlight
  6. Cotyledons turn green to photosynthesize
  7. The plumule develops into a green shoot

 

iix. The cotyledon(s) dry up/shrink and fall off.

 

  1. Hypogeal Germination

 

The epicotyls grow rapidly and elongates, leaving the cotyledons in the soil. Hypogeal germination is the type of germination in which the cotyledons remain in the soil. Examples of plants with hypogeal germination are maize, gram seed and yam bean.

 

Hypogeal and Epigeal germination

 

Differences between Epigeal And Hypogeal Germination

 

Epigeal germination Hypogeal germination
1. Hypocotyl elongates 2. Epicotyl elongates
2. Cotyledon emerges out of the soil 2. Cotyledon remains in the soil
3. Cotyledon(s) turns green/photosynthesis 3. Cotyledons do not photosynthesis
4. Plumule does not emerge simultaneously Plumule emerges simultaneously with the
with the radical radical
5. Food/energy derived from cotyledon 5. Food/energy derived from endosperm

 

CONDITIONS NECESSARY FOR GERMINATION External conditions are:

 

  1. Availability of water,
  2. Temperature/warmth and
  3. Availability of oxygen.

 

Internal conditions are:

  1. Enzymes
  2. Energy/food and
  3. Viability of seed

 

EXPERIMENT 1: to show the conditions necessary for germination.

 

Put some cotton wool at the bottom of four test tubes marked 1-4 and place five cowpea seeds in each test tube.

Test tube 1: Put in a warm place so that there is warmth and oxygen but no water.

Test tube 2: Add boiled but cooled water and cover the water surface with oil. Put in a warm place so that there is water and warmth but no air.

Test tube 3: Add some water just to moisten the cotton wool. Put in a warm place so that air, water and warmth are provided.

Test tube 4: set as in test tube 3 but keep in a refrigerator so that air and water are present but there is no warmth.

Germination will occur in tube 3, showing that air, water and warmth are necessary conditions for germination.

 

EXPERIMENT 2: To show that oxygen is necessary for germination

 

Prepare two gas jars as in test tube 3 of Experiment 1. Label the first jar A and the second jar B. Add some progallic acid dissolved in sodium hydroxide to jar A and leave the set up for a few days.

 

Note:

The pyrogallic acid dissolved in sodium hydroxide absorbs oxygen from the air in gas jar A Both gas jars are placed in a warm place and made air tight by applying grease.

Gas jar B is the control, having normal air, water, warmth but no pyrogallic acid. Germination will be observed only in gas jar B

 

SEED DORMANCY

 

This is an inactive period of a seed during which growth slows/completely ceases due to certain internal or external factors

 

EVALUATION

 

  1. Define the term germination
  2. State four differences between epigeal and hypogeal germination.
  3. What is seed dormancy?
  4. What are the conditions necessary for germination?

 

OBJECTIVE TEST

 

  1. A fruit which develops without fertilization is described as (a) simple (b) aggregate (c) multiple (d) compound (e) pathenocarpic
  2. Epigeal germination can be found in sorghum (b) maize (c) millet (d) groundnut (e) wheat
  3. Hypogeal germination is characterized by the (a) emergence of plumule out of the ground (b) provision of nourishment by the endosperm (c) elongation of the hypocotyls (d) elongation of the epicotyls elongation of the epicotyls and hypocotyls
  4. A seed has the following features except (a) embryo (b) testa (c) a helium (d) pericarp
  5. What is the fate of the ovary of a flower after fertilization? (a) becomes the seed (b) withers away (c) becomes the fruit (d) develops more nuclei

 

WEEKEND ASSIGNMENT

Carry out some planting exercise using cowpea and maize seeds and note your observations