Flower power

Ah, spring! What better time to celebrate flower power?

The use of flowers for gifts and decoration has a long history (the ancient Romans and the Chinese were wild about roses), but to a plant our infatuation might seem a little kinky: flowers are plants’ reproductive organs.

Or, to be more precise, they’re the reproductive organs of angiosperms (Greek for “seed in a vessel”). Gymnosperms (Greek for “naked seed”) such as conifers, along with ferns, mosses and algae, do not flower. However, there are more than 250,000 species of angiosperms, compared to only 1,000 species of gymnosperms and a few tens of thousands of species of the others..

Our attraction to flowers is no coincidence: the whole purpose of flowers is to attract animals, which most angiosperms need for pollination.

Flowers are modified leaves consisting of four parts (not necessarily all present in every flower): carpels and stamens (the female and male reproductive structures) and petals and sepals (secondary structures). The stamen produces pollen, which fertilizes the egg cells formed in the ovary of the carpel. Petals are the often colorful and often fragrant part of the flower, while the sepals are the more leaf-like structures that protect the flower bud before it opens.

In some species, the flowers have both male and female parts; other species have distinctly male and female flowers that grow on the same plant (corn, for example); and some species have distinct male and female flowers that grow on different plants. (Cottonwood trees, for example, are either male or female.)

In most flowering plants, the pollen is transferred from stamen to carpel, whether within the same flower or between flowers, by insects. Bees are the most famous pollinators, but the cacao plant, from which we get chocolate, is pollinated by a tiny midge, and others are pollinated by moths and butterflies. Other plants are pollinated by larger animals: hummingbirds, for instance, or bats.

It’s this interaction with animals that has resulted in the incredible diversity of flowers. Long, tubular flowers, for example, grow that way because an insect crawling through them gets well-dusted with pollen. The next flower the insect visits then gets that dusting of pollen delivered to the carpel. The cacao plant has hundreds of tiny flowers growing in a carpet over its trunk, perfect for pollination by the little midges. One plant pollinated by bats has large, sausage-shaped flowers.

Petal colours, also attractive to pollinating animals, are the result of extremely complex compounds of pigments, sugars, acids and other chemicals. Some flowers change color to be more attractive to pollinators at just the moment when they’re most receptive to fertilization. And many flowers have patterns of pigment that absorb only ultraviolet light, which insects can see and we can’t. These patterns are often radiating lines that lead the insect to the nectar (which also exists solely to lure insects).

Scent is another important attractant. Flower scents start as non-fragrant compounds formed in the chloroplasts, the structures in the green part of the plant where photosynthesis takes place. These compounds are transported to the petals, where they’re broken down into alcohols and sugars. Petals have tiny cones on their inner surfaces that expose the alcohols to the air, which oxidizes them into aldehydes, which are as much as 200 times as fragrant as the original alcohols. The scent changes as the flower ages, as these compounds altered by contact with the air.

Some plants are pollinated by the wind, including grasses and cottonwood trees, whose pollen grains are winged so the wind can carry them a long distance. A few plants are self-pollinated, including most of our crops, and thus have less showy flowers. Wind-pollinated plants often have the reproductive parts of their flowers extended on long filaments, to better disperse pollen into the wind and to better catch it.

Flowering happens at different times in different species, but is usually triggered by environmental factors: the length of the day, the length of the night, or the temperature.

Eventually the flower withers, having served its purpose. The flower’s ovules, containing the fertilized egg cells, mature into seeds, each containing an embryo plant and stored food. Sometimes the seed is encased by a fruit, which may provide additional food for the embryo or may be intended to attract animals which, in eating the fruit, also eat the seeds, which eventually emerge somewhere else.

Of course, before the plant’s reproductive organs wither, we humans like to cut them off and give them to each other as a sign of sympathy, friendship or love.

If intelligent spacefaring aliens ever find Earth, we’d better hope they’re not angiosperms–or we’re in a lot of trouble.

Permanent link to this article: https://edwardwillett.com/2004/04/flower-power/

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