BLACK STEM GALLS ON ASPEN - ANATOMY AND HISTOCHEMISTRY

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.