DEVELOPMENTAL POTENTIALITIES OF LEAF PRIMORDIA OF OSMUNDA CINNAMOMEA: THE INFLUENCE OF DETERMINED LEAF PRIMORDIA ON UNDETERMINED LEAF PRIMORDIA

In the fern Osmunda cinnamomea, leaf primorida may be excised and grown in sterile culture before or subsequent to their irreversible determination as leaves. It has been demonstrated that P3 primordia (third-oldest primordia) are not irreversibly determined as leaves at the time of excision for they exhibit a tendency to develop as shoots rather than leaves when cultured aseptically. When grown singly they become shoots 75.0% of the time; when grown in physiological contact as paired units they exhibit a tendency to become shoots at approximately the same level. The hypothesis that the active site of leaf determination is through the direct or indirect control of the shoot apical meristem is questioned and an alternative hypothesis is proposed which suggests that a morphogenetic factor is formed in older primordia which will diffuse to younger primordia to influence younger primordia to develop as leaves rather than shoots. The latter hypothesis is supported by experiments in which P3 primordia grown in physiological contact (as pairs) with P10 or P12 primordia produce more than twice as many leaves as unpaired or paired P3 primordia.