Angiosperms, or flowering plants, are the largest and most diverse group within the plant kingdom. These plants produce flowers and enclose their seeds within a fruit, including forms from grasses to broad-leaved trees and aquatic plants.
Flower Structures for Reproduction
The flower serves as the reproductive organ of angiosperms. Sepals protect the developing bud. Petals attract pollinators.
Within the petals are the male reproductive parts, stamens, each consisting of an anther and a filament. The anther produces pollen. Filament supports the anther. The female reproductive structure, the carpel or pistil, is at the flower’s center. It comprises the stigma (receptive surface for pollen), the style (stalk connecting stigma to ovary), and the ovary (containing ovules housing female gametes).
Pollination
Pollination is the initial step in angiosperm reproduction: transfer of pollen from anther to stigma. This transfer can occur within the same flower or plant (self-pollination), or between different plants of the same species (cross-pollination). Self-pollination limits genetic diversity; cross-pollination enhances it, contributing to adaptability.
Various agents facilitate pollen transfer, including wind and animals. Wind-pollinated plants often produce vast amounts of lightweight pollen, carried over long distances, as seen in grasses. Animals, such as insects, birds, and bats, are significant pollinators, attracted by floral cues like color, scent, and nectar. As these animals visit flowers, pollen adheres to their bodies and is inadvertently transferred to other flowers.
For successful pollination, pollen must be viable, and the stigma receptive. Pollen viability is its ability to germinate and fertilize. The stigma’s receptivity ensures it can capture and hydrate pollen grains. Once pollen lands on a compatible stigma, it begins the next phase of reproduction.
Fertilization and Seed Development
Following successful pollination, the pollen grain germinates on the stigma, absorbing water and nutrients. This germination forms a pollen tube, which grows down through the style towards the ovule in the ovary. The pollen tube navigates stylar tissues, guided by ovule signals. As it grows, the generative cell within the pollen tube divides to form two sperm cells.
Once the pollen tube reaches the ovule, it enters the embryo sac, through an opening called the micropyle, and releases the two sperm cells. Angiosperms exhibit “double fertilization.” One sperm cell fuses with the egg cell, forming a diploid zygote, which develops into the plant embryo. Simultaneously, the second sperm cell fuses with two polar nuclei within the central cell of the embryo sac, creating a triploid primary endosperm nucleus. This nucleus develops into the endosperm, a nutrient-rich tissue providing sustenance for the developing embryo.
After double fertilization, the ovule transforms into a seed. The seed has three main parts: the embryo, the endosperm (food source), and a protective seed coat (from ovule’s integuments).
Fruit Formation and Seed Dispersal
After fertilization, the ovary develops into a fruit, while the ovules inside transform into seeds. The fruit’s primary function is to protect and disperse seeds away from the parent plant. The ovary wall matures into the pericarp, forming the fruit wall, which can be fleshy (e.g., apples) or dry (e.g., nuts).
Seed dispersal helps avoid competition with the parent plant and allows species to colonize new areas. Various methods contribute to this dispersal.
Wind dispersal (anemochory) involves lightweight seeds with wings or feathery bristles, carried by air currents (e.g., dandelions, maples). Water dispersal (hydrochory) is common for plants near water, where seeds float and are carried by currents (e.g., coconuts, water lilies).
Animals play a significant role in zoological dispersal, either by consuming fleshy fruits and depositing seeds, or by carrying seeds that attach to their fur or feathers. Examples include berries eaten by birds or burrs clinging to animal fur. Some plants also employ mechanical or ballistic dispersal, where fruits burst open upon ripening, ejecting their seeds away from the parent plant.
Germination and New Plant Growth
Germination is when a seed sprouts and grows into a new plant. For germination, specific environmental conditions are required: adequate water, appropriate temperature, and sufficient oxygen. Water absorbed by the seed rehydrates cells and activates metabolic processes. Oxygen is necessary for respiration, providing energy for growth. Temperature requirements vary by species, with many needing a specific range or a period of cold before sprouting.
During germination, the embryonic root (radicle) is the first part to emerge from the seed. The radicle grows downwards, anchoring the seedling and absorbing water and nutrients. Soon after, the plumule (embryonic shoot) emerges and grows upwards, developing into the stem and leaves. The emergence of these structures marks the continuation of the angiosperm life cycle, establishing a new seedling.