Pollination and Fertilization in Plants

Posted on Oct 27, 2024 in Biology

Pollination

Types of Pollination

The transfer of pollen takes place with the intervention of various actors. Pollination is called differently depending on the agent involved:

  1. Anemophilous Pollination: The agent is wind. Plants with this type of pollination have many flowers that produce abundant pollen to offset the amount lost when the wind carries it.
  2. Ornithophilous Pollination: Birds are involved. This type is not common.
  3. Insect Pollination: Insects, preferably wasps and butterflies, are involved.
  4. Hydrophilic Pollination: Water is involved, such as in aquatic plants.

Insects and birds come to flowers to extract nectar from nectaries, which are small gland-like holes with sugar that are implanted at the base of the petals and stamens. The pollen sticks to the legs or hair of the insect’s head and body, or the bird’s beak. When they extract nectar from another flower, the pollen sticks to the stigmas.

Fertilization

Fertilization is the union of the oosphere (egg) and the anterozoide (pollen).

The Fertilization Process

Pollen consists of two membranes enveloping a portion of protoplasm with two cores. The egg consists of two membranes that surround a set of cells, including the embryo sac, inside which other cells are formed. When the anther dehiscence occurs and the pollen grain is transferred to the stigma of a carpel, the fertilization process begins.

  1. Pollen Attaches to the Stigma:
    • The roughness of the exine allows it to attach to the cells of the stigma, which secrete a sugary, viscous paste that traps the pollen grain.
    • The secretion of the stigma penetrates the pores of the exine and is absorbed by the intine.
  2. Pollen Germination: If the pollen grain is from the same plant species as the stigma, the liquid will be absorbed and the grain will germinate. If it does not germinate, fertilization will not occur.
  3. Pollen Tube Formation: As the pollen grain absorbs the secretion of the stigma, it increases in volume. Since the exine is inextensible, the intine emits a prolongation through one of the pores of the exine, forming the pollen tube.
  4. Descent of the Pollen Tube: The tube is inserted between the cells of the stigma, then between the style, and continues to grow until it reaches the ovary. It feeds on the protoplasm contained in the pollen. The vegetative nucleus is inserted into the tube, and the generative nucleus divides into two cells (called anterozoides) as the pollen tube enters the egg cell through the micropyle.
  5. Anterozoides in the Embryo Sac: In the embryo sac, the pollen tube releases the anterozoides. One goes to the oosphere’s core and the other to the side.
  6. Embryo and Albumen Formation: The oosphere and an anterozoide join, forming the egg. The secondary nucleus joins with the other anterozoide to form the secondary egg. The zygote and the accessory zygote are activated. The zygote cells multiply to form the embryo. The albumen cell causes the cells to accumulate substances that nourish the embryo.
  7. Seed and Fruit Formation: After fertilization, the oosphere becomes the seed. The seed is inside the fruit, which comes from the ovary. The fruit is the transformed ovary.

Flower Structure

A flower originates from a flower bud. It is defined as a set of organs (modified leaves) that form the reproductive system of flowering plants.

Parts of a Flower

  • Peduncle: The cylindrical portion of the flower that attaches it to the stem. It contains a vascular bundle that provides support and carries sap to the receptacle and floral cycles. When the flower lacks a peduncle, it is sessile.
  • Receptacle: The widened end of the peduncle where the floral cycles are inserted.
  • Floral Cycles: A set of modified leaves with different forms and functions.

Cycles of a Flower

  1. Calyx: Composed of modified leaves called sepals.
  2. Corolla: Formed by modified leaves called petals.
  3. Androecium: Formed by modified leaves called stamens.
  4. Gynoecium: Formed by modified leaves called carpels.

The calyx and corolla are the protective cycles, while the stamens and gynoecium are the reproductive cycles.

The Calyx

The calyx is the first cycle of the flower, usually green and visible when the flower is in bud stage. The sepals can be separated (dialysepalous) or joined (gamosepalous). Based on its duration, the calyx can be:

  1. Obsolete: Falls off when the corolla opens.
  2. Deciduous: Falls off with the corolla.
  3. Persistent: Survives after the corolla falls, accompanying the fruit.

The Androecium

The androecium is the male cycle of the flower, consisting of stamens. The stamens can be separated or united. The number of stamens varies among flower species.

Pollination

Pollination is the transfer of pollen grains from the anther of a stamen to the stigma of a carpel. It can be natural or artificial.

Artificial Pollination

Humans are involved in carrying pollen from one flower to the stigma of another. It can be direct or indirect.

  • Direct: Pollen falls directly from the anther onto the stigma of a carpel.
  • Indirect: Various actors are involved in carrying the pollen from an anther to a stigma. The most active agents are wind and insects.

Natural Pollination

Nature intervenes in natural pollination. It can be direct or indirect.

  • Direct: Pollen falls from the stamens of a flower located higher than another flower. This occurs in many hermaphrodite flowers. For this self-fertilization (autogamy) to be complete, three factors are required:
    1. Simultaneous maturation of stamens and carpels.
    2. Stamens higher than carpels.
    3. Dehiscence of the anther facing the stigma.
  • Reasons for Self-fertilization Failure:
    1. Dehiscence occurs on the opposite side of the anther from the stigma.
    2. Even if the stamens are higher and dehiscence is towards the stigma, the stamens may ripen first and be pollinated with pollen from another flower.
    3. Stamens are shorter than the carpels, and the anthers do not reach the stigmas.