Seasonal allergies, whirlybird seeds that litter porches and patios, the vibrancy of May flowers, weekly turns around the yard on a lawnmower and piles of autumn leaves--the life cycles of plants are evident every day, throughout the year, whenever people have contact with plants through food, gardening and lawn care or in their natural environment. Like humans and other animals, plants use sexual reproduction to achieve genetic variety in their offspring, and many of their most familiar traits have evolved to achieve reproduction.
Germination and Growth
In early spring, the bleak grays and browns of winter are undergoing transformation as last year's seeds begin to sprout and cast a green haze over the landscape. Within those seeds, tiny plant embryos lie, awaiting the cue to begin growth. Different seeds require different conditions to begin germination, but generally the seed begins by absorbing large quantities of water, which allows the seedling to puncture the tough, protective seed coat. The embryo unfolds into a root, anchoring the plant in the soil, and a shoot that reaches for the light at the surface. From there, the plant's roots develop, allowing it to uptake water and nutrients from the soil, and its leaves flourish and begin the process of photosynthesis, converting sunlight into usable chemical energy.
To humans, flowers provide the colors and scents of spring but, to plants, flowers are vital reproductive structures. Within the flower, you can find the female sex cells--an ovary containing one or more ovules that will become seeds. The dusty yellow filaments arising from the center of the flower are called anthers, and those contain the male sex cells in the form of pollen. Flowers are brightly colored and sweetly scented to attract insects, birds and bats that, while feeding, also help to disperse the pollen grains from plant to plant.
Production of Pollen
Plants produce pollen when the cells found on the anther divide in two. Those cells, in turn, multiply until the pollen grain contains two sperm cells and a tube cell needed for fertilization. The wind or pollinating animals, such as honeybees, carry the pollen. Eventually, it lands on another flower's pistil, the structure emerging from the center of the flower and leading to the flower's ovary. Here, fertilization occurs.
Fertilization begins when the pollen grain develops a pollen tube, which extends into the pistil, eventually reaching the egg cells inside of the ovary. The pollen tube then bursts, releasing the sperm cells into the egg cells. During fertilization, two important processes occur. Remember that the pollen grain contains two sperm cells. One fuses with the egg, producing a zygote that will eventually develop into the embryonic plant. The second sperm cell fuses with other cells found inside the ovary and produces endosperm, a nutrient-rich substance that fills the seed and nourishes the young embryo as it begins to grow.
Seed Production and Dispersal
Each ovule within the ovary is separated by thin walls called integuments. After fertilization occurs, the integuments harden and become the tough seed coat that protects the young plant while it awaits germination. The ovary itself sometimes swells and develops into a fruit.
From dandelion fluff borne on the breeze to the thousands of whirlybirds released by maple trees to the fruits that fill the farmers' market, plants have evolved many ways to disperse their seeds and spread their genetic material to new locations. Some plants rely on water and wind, while others create fruit to entice animals to consume the seeds and disperse them in their droppings. Regardless of the mechanism, as the seed drops to the ground, a tiny plant embryo waits inside for the cue to begin its growth. The cycle is ready to begin again.