Seed Plant Reproduction: Pollination and Fertilization
Spermatophytae Reproduction: From Flower to Seed
In plants with seeds (Spermatophytae), the seed is the characteristic reproductive structure. Let’s examine the key parts of the flower involved in this process:
- Flower Stalk: The stalk that joins the flower to the rest of the plant.
- Receptacle: Where the other floral parts attach.
- Nectaries: Glands that produce a sugary liquid called nectar.
- Perianth (Floral Envelope): The set of sterile leaves that protect the fertile leaves. It consists of two whorls:
- Calyx: The set of green leaves called sepals that protect the flower bud.
- Corolla: Formed by colored leaves called petals.
- Sexual Organs: Protected by the perianth, these include:
- Stamens: The male part of the flower. Each stamen consists of a filament (a stalk) with an anther at its tip. Inside the anther are pollen sacs, which contain pollen grains (carrying the male gametes).
- Gynoecium: The female part of the flower, made up of leaves called carpels. In angiosperms, these are welded to form a closed cavity that houses the ovules. The carpels form a bottle-shaped structure known as the pistil. The enlarged inner part is the ovary, where the ovules (containing the female gametes) are located. The middle part is the style, and it ends in a widening called the stigma.
Pollination: Transfer of Pollen
Pollination is the process by which pollen grains (containing the male gametes or sperm nuclei) are transported from the anther of a flower to the stigma of the carpels (which contain ovules with oospheres).
Types of pollination based on pollen origin:
- Self-Pollination: Occurs when pollen comes from the same flower or from different flowers on the same plant.
- Cross-Pollination: Occurs when pollen comes from flowers on different plants. Cross-pollination allows for greater genetic diversity in the offspring, which is advantageous for the species. Plants have mechanisms to prevent self-pollination, such as preventing contact between pollen and the stigma of the same flower or having male and female organs mature at different times.
Types of pollination based on the transport mechanism:
- Anemogamous Pollination: Pollen transport is done by the wind. Plants have simplified perianths to facilitate access of pollen grains, and the stigmas of the carpels are feathery to capture and hold the pollen.
- Entomogamous Pollination: Transport is carried out by insects, although birds and bats can also carry pollen. This form of transport is more secure and effective. Plants have developed adaptations, such as the production of sweet nectar or pollen itself, which serve as food, and attractive colors and scents.
Fertilization and Seed Formation
The seed is the structure resulting from the development of the ovule after fertilization in seed plants.
Double Fertilization: Once a pollen grain has reached the stigma of a carpel, it develops a pollen tube that grows and moves through the style and ovary wall to reach the ovule. The ovule develops inside the embryo sac, which contains eight haploid nuclei (n) included in seven cells. One of these cells contains two nuclei, called the secondary nuclei. Among these haploid cells is the oosphere (egg cell).
The pollen tube carries down two sperm nuclei, leading to double fertilization:
- One sperm nucleus fuses with the oosphere, forming the zygote, which gives rise to the embryo of the seed.
- The second sperm nucleus fuses with the two secondary nuclei, giving rise to a triploid (3n) tissue called the albumen, which is the seed’s nutrient reservoir.
Seed Structure
The seed consists of the embryo, the albumen, and membranes derived from the tissues surrounding the embryo sac. These membranes protect the embryo until germination occurs. The embryo is essentially a miniature plant with a root, a plumule (shoot), and cotyledons.
- Monocotyledonous Plants: The embryo has a single cotyledon.
- Dicotyledonous Plants: The embryo has two cotyledons. The albumen may be incorporated into the cotyledons, causing them to increase in size. Alternatively, the albumen may remain around the embryo, and the cotyledons have few reserves.