Flowers are the reproductive structures of many plants, responsible for producing seeds. The development of seeds begins within the flower.
Key Floral Structures
For seed development, specific flower parts play distinct roles. The pistil, the female reproductive organ, comprises the stigma, style, and ovary. The stigma is a receptive tip, the style is a stalk connecting it to the ovary, and the ovary is a swollen base containing ovules. Each ovule can become a seed upon fertilization. Stamens are the male reproductive organs, each consisting of an anther, which produces pollen grains, and a filament that supports the anther.
The Process of Pollination
Seed development begins with pollination, the transfer of pollen from the anther to the stigma. This transfer occurs through various means, such as wind, water, or animals. Once a pollen grain lands on a compatible stigma, it absorbs moisture and nutrients, leading to germination.
A tube-like structure, called a pollen tube, then emerges from the pollen grain and grows down through the style. This tube acts as a conduit, navigating through the female tissues to reach an ovule located within the ovary. The growth of the pollen tube ensures that the male gametes, contained within the pollen, are delivered to the female gamete (egg cell) inside the ovule.
Fertilization and Seed Formation
Once the pollen tube reaches an ovule, fertilization takes place. This is the fusion of the male gamete from the pollen with the female gamete (egg cell) inside the ovule. This fusion results in the formation of a zygote, which then develops into an embryo, the rudimentary plant within the seed.
Simultaneously, another male gamete fuses with other central cells within the ovule to form the endosperm. The endosperm serves as a nutritive tissue, providing the necessary food reserves for the developing embryo. The outer layers of the ovule, known as integuments, transform into the protective seed coat. Therefore, after successful fertilization, each ovule develops into a mature seed, containing the embryo, endosperm, and a protective seed coat, all enclosed within the original ovule structure.
The Ovary’s Transformation
After fertilization, while the ovules within the ovary are developing into seeds, the ovary itself undergoes a significant transformation. The ovary wall matures and expands, developing into the fruit. The fruit’s primary purpose is to protect the enclosed seeds and aid in their dispersal, allowing the plant to spread its offspring.
This transformation can result in a wide variety of fruit types, from fleshy structures like apples and tomatoes, where the entire ovary wall ripens, to dry structures such as pea pods, where the ovary wall becomes a protective casing that splits open. In essence, the fruit is the ripened ovary, serving as a protective vessel for the newly formed seeds, ensuring their survival and propagation.