Specimens of Fraxinus nigra Marsh growing in permanently water covered areas adjacent to certain streams in Cheboygan County, Michigan, have a very conspicuously enlarged base, in most cases nearly twice that of the diameter of the trunk above. The swollen base is due, mechanically, to the great increase in the number of cells of the summer wood of each year’s growth, there being about twice as much growth in the swollen part as in the normal part. The individual cells of the swollen part are not abnormal either in size or shape.