The Stages of Flowers' Blooming

Flowering plants, or angiosperms, bloom in glorious variety, not for the enjoyment of human eyes but to attract the insects and animals necessary to carry pollen from anther to stigma. Alternatively, some plants take advantage of wind as a pollinator. Each plant’s flowers are defined by its genes for success in a specific geographic area and climate, but all flowers follow the same basic stages of development.


Each bud on every angiosperm has the potential to become a branch, a leaf or a flower. Within each plant’s DNA are cues that trigger choices for each bud. Four cues tell the bud how to develop on the plant -- growing enough leaves to start producing food is the first cue. At maturity, cues from the photoperiod, or length of daylight, intensity of light and the balance of day and night time temperature cue a process called induction. Primitive flower parts, called primordia, begin forming within buds. Each plant began its evolution at a specific time in the earth’s climatological evolution -- and in a particular geographic location -- so the combination of induction clues varies with each angiosperm species.


Once the identity of the bud is established, the primordia must begin differentiating cells to form the gynoecium, the sexual organ of the flower where seeds form, which will divide into carpels and stamens. The carpel, or female part, of the flower organizes itself into a set of ovules where seeds will grow and a stigma to receive pollen. Tissues that will become the male part of the flower organize into stamen and anthers. At the same time, outer coverings organize into a combination of bracts, sepals and petals that protect the gynoecium and attract pollinating insects.


During maturation, the flower develops its sexual identity. Sporogenous tissue lines the anthers, setting the stage for meiosis, the process that produces pollen. An embryo sac grows around the ovules, preparing for its growth as an ovary, and the stigma opens at the top of the style. Sepals and petals thicken and begin coloring. As the flower matures, environmental clues -- and the presence of its pollinators -- cue the commencement of meiosis. Sepals and petals open when the flower has fully matured and when pollinators are present. Flowers that depend on bees, for example, are typically diurnal, opening during the day, and those that use night-flying moths are nocturnal bloomers.


Once the flower is completely open, its real work begins. Flushes of color, production of bright pollen and other pollinator-attracting tricks may continue for as little as one or for several days until the flower’s stigma transmits the correct amount of fertile pollen to the ripening eggs in the ovary. Plants that use wind for pollination may close during rainy periods, and those that depend on a specific insect may close when it is not present, only to reopen when it is present in the environment. Once the flower’s work is complete, its petals fall away as the ovary ripens.