SEATTLE -- From the fourth floor of a striking rainforest-like conservatory built of glass and white steel in downtown Seattle, Amazon.com boss Jeff Bezos turned toward the top of his newest headquarters building to summon his favourite assistant.

In a proud little show on Monday for the media and dignitaries -- which also doubled as product placement for Amazon's voice assistant -- the world's richest man ordered out loud, symbolically: "Alexa, open The Spheres."

The domed structure is only steps away from the executive office tower where Bezos leads the online retail behemoth. It's part of the company's urban campus near downtown Seattle that is largely made up of unmarked office buildings where more than 40,000 people report to work.

The four-story Spheres structure from the outside looks like three connected glass orbs planted into the ground in a caterpillar shape. Lighting mimics a position near the equator, with 12 hours of shade and sun. During the day, the interior is maintained at 72 degrees with 60 per cent humidity, to emulate a cloud forest ecosystem.

Amazon's Amazonesque rainforest-like conservatory is now home to more than 40,000 plants from 50 countries on five continents. Its centerpiece is a 50-foot (15.2-meter) fig tree. Most plants will flower and some can yield fruit, though visitors must keep their hands off all plant life. About 90 per cent of the plants were grown and tended to in a suburban greenhouse for years in anticipation for their permanent home in The Spheres.

Though masked by nature, the sleek and minimalist "alternative work space" is also designed to make you forget you're at work, in a startup environment that is rumoured to be aggressively demanding.

"The idea is that we connect them with nature. We get them away from their normal desk environment so you don't see any desks or cubicles around," said Ron Gagliardo, Amazon's leading horticulturist.

The corporate office space, however selfie-worthy, is already such a hit that the company had to create a reservation system to contain the flow of traffic for the time being. Employees will have to snag a reservation to get in but it's currently already booked out until April. The building has capacity for about 1,000 people but is more comfortable with about 800 at a time.

Once inside, workers can use nooks with tables and chairs that can serve as a casual meeting space. Coffee breaks can be taken in a cafe and "picnic" area offering an interior reprieve from Seattle's unrelenting rainy season.

Critics have called the Spheres a vanity project, illustrating Seattle's sometimes strained relationship with its largest employer. The company's presence has changed the local economy and raised its cost of living -- but none of that was on show at the unveiling of the Spheres.

Instead, Bezos received glowing praise from the governor, mayor and county officials for his recent commitments to the city's homelessness crisis plus a personal donation of US$33 million to a scholarship foundation that helps immigrant youth in the country illegally.

Bezos was coy when asked by The Associated Press whether the company's much-anticipated second headquarters to be built in an undetermined U.S. city will have another statement building space like the Spheres, saying: "We'll see."