When balsam poplar stem cuttings are grown in a saturated solution of CaSO4(pH 11.0) for 10 days and then transferred to distilled water for another 10 days a resulting callus mass shows two distinct kinds of callus tissue. When the procedure is reversed, the position of the two kinds of callus tissue in a single callus mass is correspondingly reversed. The relationship of each kind of callus tissue to fungal infection and to the development of adventitious roots is briefly discussed.