There are so many of these dried cone-like structures on this particular willow tree that I was sure it was some kind of samara or seed pod, despite recalling nothing like it in any description of willow. The truth is more interesting: it's the work of an insect manipulating the growth of the terminal bud to create a safe shelter for its larvae! http://blog.audubonguides.com/2011/01/20/pinecones-on-a-willow-tree/