Ah, you're talking about the technique called "cloud pruning."

Have never done anything in that style (being a geometric topiary kind of person), but David Joyce in his book "Topiary and the Art of Training Plants" says that a favorite for cloud pruning is the Chinese juniper (Juniperus chinensis), although other junipers and (mainly) coniferous trees and shrubs can be used.

Hope this helps.

Elizabeth

THANK YOU!!! That would be it! I'll look it up immediately.

I'd been investigating other 'cloud' pruned conifers, and I figured they were junipers, but now I have a specific to head toward...