Flowering and the life cycle

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The life cycle of a flowering plant begins when the seed germinates. It progresses through the growth of roots, stems, and leaves; formation of flower buds; pollination and fertilization; and seed and fruit development. The life cycle ends with senescence, or old age, and death. Depending on the species, the life cycle of a plant may last one, two, or many years. Plants called annuals carry out their life cycle within one year. Biennial plants live for two years: The first year they produce leaves, and in the second year they produce flowers and fruits and then die. Perennial plants live for more than one year. Some perennials bloom every year, while others, like agave, live for years without flowering and then in a few weeks produce thousands of flowers, fruits, and seeds before dying.

Whatever the life cycle, most plants flower in response to certain cues. A number of factors influence the timing of flowering. The age of the plant is critical—most plants must be at least one or two weeks old before they bloom; presumably they need this time to accumulate the energy reserves required for flowering. The number of hours of darkness is another factor that influences flowering. Many species bloom only when the night is just the right length—a phenomenon called photoperiodism. Poinsettias, for example, flower in winter when the nights are long, while spinach blooms when the nights are short—late spring through late summer. Temperature, light intensity, and moisture also affect the time of flowering. In the desert, for example, heavy rains that follow a long dry period often trigger flowers to bloom.

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