In the fern Osmunda cinnamomea, leaf primordia may be excised and grown in sterile culture before or after their irreversible determination as leaves. It has been demonstrated that P3 primordia (third most recently formed primordia) are not irreversibly determined as leaves at the time of excision for they exhibit a tendency to develop as whole shoots (approximately 67.0% of the time) whether grown singly, or as pairs grown in physiological contact. Results from the present investigation support the hypothesis that a morphogenetic factor(s) is found in older primorida which is transmitted to younger primordia to influence the latter to develop as leaves rather than shoots, for P3's grown in physiological contact (as pairs) with P10, P12, or P13 primordia are expressed as leaves at a level approximately twofold or greater than P3's grown singly (controls). Results from the present investigation do not support the hypothesis that increased bulk of tissue, and therefore increased capacity for nutrient productivity by older and larger primordia, is responsible for imposition of leafness on undetermined P3 primordia, for P14 primordia grown in physiological contact with P3 primordia (as pairs) are observed to increase the percentage of P3's expressing leafness at a level only slightly greater than P3's grown singly, and slightly less than when P3's are grown in physiological contact with other P3's as pairs. The slight enhancement in leafness exhibited by undetermined P3's grown as pairs with other P3's, or with P14's has been shown to be non-significant at the 5% level (P < 0.05) by the standard t test and Kramer's (1956) modification of Duncan's (1955) new multiple range test.