"Why do plants have galls?", a teacher of mine once asked. Before anyone could answer, he said, "So that we can identify them more easily!"
Galls are abnormal plant growths, often specific to individual species or plant families, and are believed to be triggered by insects. Galls forms around insect eggs, and as a result provide them with a custom shelter. Regardless of why they appear, their presence serves as a useful field identification tool.
The above Blueberry Kidney Gall (or Blueberry Stem Gall), is typically found on Highbush Blueberry (Vaccinium corymbosum) and is home to a small gall wasp (Hemadas nubilipennis).
The Goldenrod Ball Gall is triggered by a gall fly (Eurosta solidaginus) and can be found on various Goldenrods (Solidago sp.). While this stem contained a trio of galls, I usually find just a single gall on a given plant.
Oak trees (Quercus sp.) are another common host of galls, one group of which are the Oak Marble Galls (or Oak Bullet Galls) which are caused by various gall wasps and can be found year-round. Stokes Guide to Nature in Winter by Donald Stokes (1979) discusses these galls and several others commonly encountered in winter.