Does this branch come from a relatively larger shrub or tree? The only I could think of is Cephalanthus occidentalis (buttonbush). I am not sure if the 2nd image with those flowers look like buttonbush.
Can you take a photo of the plant while it's still growing in the ground? Or at least fresh, like in a jar of water; hard to tell from photo if flowers were spherical or not when fresh. And how tall it grows.