Long budding, brief bloom


Redbud: Over thousands of years, the native North American tree has evolved to outwit frost and lure honeybees for pollination.

April 29, 2001|By Heather Dewar | By Heather Dewar,SUN STAFF

You can be lulled by Japanese cherry trees and nursery-bred pear trees into believing that spring has truly arrived. They bloom, but then the temperature sinks back into the 40s.

Better to trust cercis canadensis - the redbud.

Their bare branches hazed with magenta flowers, these North American native trees first blossom in the region's warm corners - on sunny south-facing hillsides, in Eastern Shore forests, amid heat islands of city concrete. In pockets of persistent chill, they were on the verge of bloom, the buds a little fatter each fine day.

Since the last Ice Age, natural selection has calibrated the internal clocks of wild redbuds, which grow in dappled sun along the edges of forests from Michigan to Mexico. Each locale now has a strain that blooms only when the danger of prolonged cold has passed, says Florida botanist Henry Donselman, who studied wild redbuds across the Eastern United States in the 1970s and '80s.

It's important to get the timing right, because each individual flower has been about nine months in the making.

In redbuds (and many other spring-flowering plants), the blossoms opening today were almost fully formed last summer and fall, says botanist Shirley Tucker, an expert in flower development. Tucker, a professor emerita at Louisiana State University, used to comb the campus and nearby woods looking for the earliest stages of redbud blooms: "It took me three years to get the samples I needed. I was always too late. I finally found them when I started looking in early June."

Tucker has been dissecting blooms-to-be for 30 years. She is one of about 400 scientists worldwide specializing in legumes - one of the oldest and most successful plant families, whose 17,000 members include sweet peas and soybeans, a slew of tropical trees, and the redbuds.

She has collected legume flower buds in Brazil, Martinique, Hawaii, New Zealand and Australia. A London-based amateur botanists' network sends her samples from Africa and South America. In some places, she says, collectors preserve their samples in the local rum.

A redbud's preparation for flowering in spring begins the previous June. That's when the next year's buds are at their earliest stage, about an eighth of an inch across, "so tiny you can flick them off with a fingernail." Each one contains the beginnings of about 100 flowers.

Tucker uses needles and forceps to take them apart, peeling away layer upon layer of bracts - waxy, petal-like protective coverings that, in nature, will eventually fall away.

Next she removes the sepals, the outer whorl of green husks that will lie underneath the flower's opened petals. Then the still-unformed petals can be stripped away.

Magnified under an electron microscope, each flower cluster is as intricate as a coral colony on a tropical reef. Tiny hairs float above them like the tendrils of sea creatures, insulating the delicate structures from cold and heat.

The blossom forms from the outside in - first five protective sepals, then five showy petals whose only purpose is to dazzle insects and draw them into the flower.

As summer progresses, providing the plant with the surge of energy needed for the demanding work of forming flowers, the redbud's pollen-carrying stamens begin to form.

First discernible as a ring of five small bumps alternating with the petals, the stamens eventually grow long stalks, and a second ring forms inside the first. Simultaneously, a bulge forms at the center of the bud. This will become the carpal - the floral equivalent of an ovary.

All of this is taking place almost invisibly with the tree in full leaf.

In redbuds and other legumes, the carpal's early stage has a distinctive notch. As the carpal grows taller, that notch becomes the seam that creases its side like the "string" of a string bean. The carpal hollows out and the seeds begin to form inside. Meanwhile, at the ends of the stamens, tiny sacs have developed and are filling with pollen.

These two processes take up so much energy that the flower has none left over for growing, Tucker says. By late summer or early fall it is fully formed, in miniature.

About this time, sensors in the leaves detect cooler temperatures and longer nights. The tree stops producing the growth hormone or hormones - their composition is still a mystery - that control the flowering process. Instead it switches over to manufacturing another chemical that induces winter dormancy.

The blossoms pass the winter underneath a waxy cuticle and more than 20 bracts - the equivalent of an overcoat and "20 layers of sweaters," Tucker says.

About a month before blooming, a combination of higher temperatures and shorter nights triggers a frantic growth spurt.

"There's a tremendous size increase," Tucker says. "The supporting stem elongates so that the flowers are more widely spread. They each develop a little stalk, and the parts within are all elongating. They're enlarging so much that it forces the buds open."

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