Quantitative Studies of Pinnule Development in the Ferns Adiantum raddianum and Cheilanthes viridis

Abstract

Pinnule development was investigated in two fern species, Adiantum raddianum Presl cv. Decorum and Cheilanthes viridis (Forsk.) Swartz, by using clearings to facilitate the recording of mitotic divisions. Both species were found to possess a marginal meristem. This meristem consists of both a marginal row of large initials and a submarginal meristematic zone. The marginal meristem in these ferns is responsible for establishing the layers of the lamina, providing new cells which by enlargement will expand the pinnule, establishing general pinnule form, initiating the procambial stands, and forming the false indusia. The cells of the submarginal meristem were found to divide parallel to the pinnule margin more frequently if they were to become ground tissue, while dividing perpendicular to the margin more frequently if they were to become procambial. Details of vein dichotomies were also studied. Perimeter expansion was found to be associated with dichotomy of the veins, and venation pattern was found to be correlated with leaf form. The marginal meristem is active from the time of pinnule initiation until the pinnule reaches about 50% of its final length or width. Leaf development in leptosporangiate ferns resembles the traditional concept of development in angiosperms somewhat more than it does the more recent concepts. It is clear, though, that there is not a high degree of convergence in the marginal growth of fern and angiosperm leaves.