WASHINGTON — On a usual hot Monday in July, 2,000 visitors might wander into the U.S. Botanic Garden’s conservatory. Its various glasshouses offer lush plant displays, but the place is no cooler than outside on the Mall, and there is no cafeteria. It has no space capsules, no paintings by Vincent van Gogh, no Hope Diamond.

Monday of this week was different: As many as 20,000 folks lined up to get in. There are tens of thousands of plants growing in this art deco glass palace, but the throng came for one bloom alone: The titan arum, a plant type known as an aroid and distinguished by having the largest unbranched inflorescence on the planet. That’s botany-speak for one helluva flower, with a central column surrounded by a pleated ruff.

It was a freak show of sorts – the thing is just big and otherworldly. When it was brought center stage to the conservatory on July 11, it was 4 feet high. When it opened on Sunday evening, it was 8 feet high, and soon began pulsating heat and a notorious stench that was so nauseating that plant curator Bill McLaughlin said he couldn’t face dinner until about 11 p.m. that night.

The obvious allure of this flower is its bizarre, exotic form taken to extreme size, no doubt.

Subconsciously, it’s about sex and death, and I can’t look at Amorphophallus titanum without thinking of the old British Hammer horror films of the ’60s and ’70s. When will the bat fly out of enveloping spathe, I wonder? When will Christopher Lee be found standing behind it, flashing those fangs?

The flower mimics death in the alchemy of its odor. “I can’t specify which dead animal,” McLaughlin said. (The smell had waned by Monday when the crowds appeared). In its native Sumatra, the titan arum lures carrion beetles to pollinate its many pistils. Beyond the campy stage death, there’s the actual mortality of the flower. It opens for a day or two, ages rapidly and withers. We don’t know when its 90-pound tuber will flower again; perhaps in another eight years.

The botanic garden staff is delighted, of course, that colleague Elliott Norman raised this crowd-puller, but you can’t help feeling that they might wish the crowds to regard some of the other treasures that grow here. The arum was set beneath two palmlike “trees” that probably went unnoticed but are old specimens of a cycad, a plant that predates the arum. Cycads were thriving along with conifers, ferns, and ginkgos about 200 million years ago, when the world was younger and life was raw and eager. If the arum is about death, these plants are about life, or primal life forces that remain.

McLaughlin took me to a steamy glasshouse away from the titan mob scene in the Garden Court to a display called the Garden Primeval. Amid tree ferns and more cycads, he showed me one of the largest ferns that is still with us, a prehistoric creature called Angiopteris evecta. It measured 8 feet across, and its burgeoning stems, its fiddleheads, were actually more the size of celloheads. It is afflicted with mealybugs, but the gardeners bring in predatory ladybugs (also known as ladybirds), which kill the pests. This seems at least as interesting as the arum’s corpse ploy.

In another house, I saw a young girl touching the frondlike leaves of the sensitive plant, named for the way its leaflets clasp shut when disturbed. Pei Pei Chien sat nearby as she watched her 9-year-old daughter, Fiona, play with the plant. Chien, of Frederick, Md., grew up in Taiwan, where the plant was almost a weed. “When I was a child, I liked to touch it and watch it close,” she said. “So I showed her.”

Her daughter said she found the titan arum “big and very interesting” but her interest in plants has already led to other discoveries. “Some plants, if you break a leaf and put it in a pot, it will just grow.”

The titan arum comes and goes, but the even larger plant kingdom, it turns out, is full of more permanent wonders. One of them is its ability to connect one generation to the next.