On a lap around the farm last christmas I came across some odd lumps on the petioles (leaf stems) of a cottonwood tree. I suspected straight away they were galls but had no idea the contents. When I came back with the camera I started to promptly and rudely split some galls to photograph what was inside and found little colonies of what I thought were plant lice but turned out to be aphids. I didn’t think aphids were responsible for galls but these are quite common on poplar trees in the U.S. (turns out poplars and cottonwoods are from the same genus). Another interesting thing to note is there were a lot of dead leaves under the tree that looked like they’d beed severed at the galls. I’m wondering if birds (or some other animal) had opened the galls, eaten the contents and discarded the leaves.