Large black stem galls occur sporadically on trembling aspen (Populus
tremuloides) in western Canada. Although little is known about their c
ause or structure, trees having these galls are less likely than surro
unding aspen to have advanced decay caused by the fungus Phellinus tre
mulae. The anatomy and histochemistry of black galls and associated br
anch galls were studied and compared with normal wood and bark. Light
microscopy showed that the cambium of black galls produces greater num
bers of cells per growth ring and that growth rings are two to three t
imes wider than normal. Vessel elements and fibers are unusually small
and misshapen. Gall xylem has characteristics associated with woundin
g or infection: ray cells filled with phenolic deposits, and vessel el
ements occluded by tyloses and granular material. Frequent radial stra
nds of undifferentiated callus tissue surrounded by necrophylactic per
iderms indicate sites of cambial damage of unknown cause. White areas
within dark-colored gall xylem of some samples were free of most of th
ese abnormalities, suggesting that a persistent agent is required for
continuing tumor growth. Thickened outer bark harbored a variety of sa
prophytic fungi, especially hyphomycetes. Surface and internal morphol
ogy of black galls was also compared with similar stem galls caused by
poplar budgall mites (Aceria parapopuli) and was found to be differen
t. Bacteria, fungi, or mites were not obvious within living tissue, an
d further studies are necessary to determine the etiology of black gal
ls.