My thanks to Guy Bussières at
Université Laval, in Québec City for directing me to a 2012 paper by Colin et al. (Annals of Botany 110: 995–1005) which describes these bumps as a form of epicormic growth
they called spheroblasts, “Isolated buds progressively engulfed in a small ball of wood generated from the base of the bud on the external side”.
John A. McLaughlin, PhD
Forest Research Pathologist
Ontario Forest Research Institute
1235 Queen St. E.
Sault Ste. Marie, ON
Canada
P6A 2E5
tel: +1 705-946-7419
cell: 705-542-2851