Last Monday I wrote:

I post this mystery courtesy of Sally Teschon for two reasons -- you can see this mystery plant at NY Botanical Gardens, and you can see the gardens' annual train show as well. The buildings -- almost all of them NYC landmarks -- are made from birch bark, acorns,  cinnamon sticks and other natural materials.

But you really should I.D. that plant.

The best answer came from Diane Louie, who wrote:

I believe it is a Milkweed, Gomphocarpus physocarpus, whose common names include "Hairy Balls" and "Family Jewels" -- for obvious reasons, given those seed pods. It is a native of Africa... (Nice work, Diane!)

Sally Teschon provided also provided this: