It’s a warm, it’s sunny, it’s spring, and the ’60s musical Hair is coming to town. What better time to celebrate flower power?

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

Or, to be more precise, the reproductive organs of the plants classified as angiosperms (angiosperm is Greek for “seed in a vessel”). There are plants, such as conifers (which are gymnosperms: gymnosperm is Greek for “naked seed”), ferns, mosses and algae, that do not flower. However, flowering plants dominate the world’s vegetation. It’s estimated there are more than 250,000 species of angiosperm, compared to only 1,000 species of gymnosperms and a few tens of thousands of species of the others.

The fact that we’re attracted to flowering plants is more than just coincidence: the whole purpose of a flower on a plant is to attract animals, because angiosperms, more than any other kind of plant, depend on animals–and animals depend on them. Most flowering plants need animals for pollination and to disperse their seeds, while from the animals’ side, the need is even greater: angiosperms provide most of their food. Almost every plant humans cultivate is a flowering plant, as is grass, which feeds the livestock we also eat.

Flowers are really 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.

The colour of flowers is all part of this effort to attract pollinating animals, and are the result of extremely complex compounds of pigments, sugars, acids and other chemicals, delicately balanced so that not all flowers even of the same species are necessarily exactly the same colour. Some flowers even change color to be more attractive to their pollinating insects at just the moment when they’re most receptive to fertilization. And to an insect, there’s even more to flowers than meets our limited eyes: 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. (Nectar is another example of the close relationship between angiosperms and animals, because nectar serves no purpose for the flower: it’s just there to lure insects.)

Scent, too, is designed to attract animals to the flower. Flower scents are incredibly complex. They 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 of a flower changes as it ages, as these compounds are more and more altered by contact with the air. Flower scents aren’t always pleasant, either: the flowers on the tree in front of my house (a mountain ash) smells like stale bacon to me. But then, the tree isn’t trying to attract me.

Some plants are pollinated by the wind, including grasses and the aforementioned cottonwood trees, whose pollen grains are winged, allowing them to be carried a long distance. A few plants are self-pollinated, including most of our crops–wheat, rice, barley, oats, peas. etc. (“Animals? We don’ need no steenkin’ animals…”) Self-pollinated and wind-pollinated plants tend to have less-showy flowers, because they don’t need to attract animals. Instead, they’ve adapted in other ways: 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 (the flowering of summer vegetables in your garden, for example, is promoted by a brief exposure to lower temperatures).

Eventually the flower withers, having served its purpose. The flower’s ovules, containing the fertilized egg cells, matures into seeds, each of which includes a tiny embryo plant and stored food to help supply the seedling after it sprouts. Sometimes the seed is surrounded by additional food, producing a fruit. That food may be for the embryo, or, again, it may be intended to attract animals which, in eating the fruit, also eat the seeds, which eventually emerge somewhere else.

And we humans, after having cut off the reproductive organs of the plants and arranged them in colorful vases or given them to each other as a sign of sympathy, friendship or love, also like to grind up tiny embryo plants to make bread.

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

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