This paper presents a comparative anatomical study of callus tissue produced on stem cuttings of balsam poplar (Populus balsamiferaL.) grown in distilled water and in saturated solutions of CaSO4from 6.0 to 11.0 pH. Striking differences in callus morphology, hardness, and cellular structure were observed. The fact that acidity retards and alkalinity accelerates the rate of calcification suggests that calcification or hardening of the middle lamellae could be a function of the rate at which the pectic acid component is neutralized.The emergence and subsequent growth of adventitious roots decreased markedly with increasing alkalinity, until at pH 11.0 most callus masses appeared to be rootless. However, sectioning usually revealed a well-developed root embedded some distance below the surface. Failure of most adventitious roots to emerge in a strongly alkaline calcium solution was attributed to the hard compact nature of the surrounding callus.