Double Fertilization:
- Within the embryo sac, there are two female gametes, the egg cell (ovum) and two polar nuclei. These are produced by the megagametophyte during megasporogenesis.
- When the pollen tube reaches the embryo sac, it releases its two sperm cells.
- One sperm cell fuses with the egg cell (ovum), resulting in the formation of a diploid zygote. This zygote will develop into the embryo of the plant.
- The second sperm cell fuses with the two polar nuclei, forming a triploid cell known as the primary endosperm nucleus. This is the beginning of the endosperm development.
POST-FERTILISATION : STRUCTURES AND EVENT
Post-fertilization in flowering plants refers to the events and structures that occur after the successful fertilization of the ovule, leading to the development of seeds and the formation of fruit. This phase is crucial for the plant’s reproduction and dispersal of its offspring. Here are the key structures and events in post-fertilization in flowering plants:
Zygote Development:
- After fertilization, the zygote, which is the result of the fusion of a sperm cell and an egg cell, undergoes multiple rounds of cell division through mitosis.
- The zygote eventually develops into an embryo, which is the young, multicellular plant within the seed.
Endosperm Development:
- In many plant species, the primary endosperm nucleus (formed by the fusion of a sperm cell and two polar nuclei during double fertilization) undergoes multiple rounds of cell division to form the endosperm.
- The endosperm serves as a nutritive tissue, providing essential nutrients (starch, proteins, lipids) to the developing embryo.
- The endosperm’s composition and function vary among different plant species. In some cases, it is absorbed entirely by the developing embryo, while in others, it persists in the mature seed.
Seed Coat Formation:
- The integuments surrounding the ovule, which protected it during development, will develop into the seed coat or testa.
- The seed coat provides protection to the developing embryo and seed.
Fruit Development:
- The ovary, which surrounds and protects the ovules, typically undergoes significant changes after fertilization and forms a fruit.
- The fruit may develop from the entire ovary (simple fruit) or from multiple ovaries (aggregate or multiple fruit).
- The fruit serves as a protective structure for the seeds and plays a crucial role in seed dispersal.
Maturation of Seeds:
- As the embryo continues to develop, it acquires the structures necessary for the future plant’s growth, including the shoot (plumule), root (radicle), and cotyledons (seed leaves).
- The seed accumulates reserves of nutrients, such as starch and proteins, to support the embryo during germination.
Desiccation Tolerance:
- As the seed matures, it often undergoes desiccation (drying), leading to a reduction in water content.
- Many seeds develop desiccation tolerance, allowing them to survive in a dehydrated state until conditions for germination are favorable.
Seed Dispersal:
- Once the seeds are mature, the fruit may facilitate their dispersal through various mechanisms, including wind, animals (endozoochory or exozoochory), water, or mechanical means.
- Seed dispersal is critical for the colonization of new areas and the survival of plant populations.
Seed Dormancy:
- Some seeds exhibit dormancy, a state of reduced metabolic activity and delayed germination, allowing them to survive adverse conditions and germinate when environmental conditions are favorable.
- Dormancy can be broken by specific environmental cues, such as temperature changes, light exposure, or scarification.
APOMIXIS AND POLYEMBRYONY
Apomixis and polyembryony are two unique reproductive strategies observed in certain plants. They involve the formation of embryos without the involvement of traditional sexual reproduction, such as fertilization. Here’s an explanation of both concepts:
Apomixis:
Definition: Apomixis is a form of asexual reproduction in plants where seeds are produced without fertilization. It involves the development of embryos from unfertilized egg cells within the ovule.
Key Characteristics:
- In apomixis, the embryo develops directly from the maternal tissue, such as the nucellus or integuments, without the need for pollination or fertilization.
- The offspring produced through apomixis are genetically identical to the mother plant (clones) because no genetic recombination occurs.
- Apomixis can occur naturally in some plant species or can be induced artificially in others.
Advantages:
- Apomixis allows plants to reproduce rapidly and efficiently, particularly when they are well-adapted to their environment and want to preserve their genetic traits.
- It can be advantageous in environments with limited pollinators or where sexual reproduction may be less reliable.
Examples: Some grasses, dandelions, and citrus fruit trees are known to reproduce through apomixis.
Polyembryony:
Definition: Polyembryony is a phenomenon in which multiple embryos develop within a single seed. These embryos may originate from different sources, such as fertilization and asexual processes.
Key Characteristics:
- In polyembryonic seeds, one embryo typically results from normal sexual fertilization (zygotic embryo), while the others develop asexually from various sources, such as nucellar tissue or the integuments.
- The zygotic embryo is genetically distinct, while the asexual embryos are often genetically identical to the maternal plant or each other.
Advantages:
- Polyembryony can increase the chances of seedling survival and genetic diversity within a plant population.
- It may be advantageous in environments with high competition, where multiple embryos provide a better chance of survival.
Examples: Polyembryony is observed in several plant families, including the citrus family (Rutaceae), where some citrus fruits, like oranges and grapefruits, exhibit polyembryonic seeds.
