Raven, a regular visitor to China, last year completed the 49-volume Flora of China with his collaborators, Wu Zhengyi and Hong Deyuan.

"The reason there are so many plants in China is that they've survived there better," he says in a National Geographic video.

"If you go back 15 million years in the past, you find much the same kinds of plants in China, Europe and North America, and they've survived in the warm temperate forests that stretch across the middle of China."

That plant wealth that has been a magnet for botanists for centuries and it continues to pull today. Research scientists from the US Department of Agriculture, for example, will come to China this fall - the third such team from the agency in as many years.

"The famous plant collector E.H. Wilson once referred to China as the 'mother of all gardens'," says Glin Varco, director of horticulture at Lan Su garden in Portland, Oregon.

China is home to more than 30,000 species - one-eighth of the world's total. Lan Su provides a microcosm of that treasure trove.