Shoot Ontogeny and the Comparative Development of the Heteroblastic Leaf Series in Lygodium japonicum (Thunb.) Sw.

Abstract

Heteroblastic development in the climbing fern L. japonicum (Thunb.) Sw. was investigated. The apical cell and zonate organization found in the adult shoot apical meristem are already present in the embryo, but there is a progressive increase in the size and complexity of the meristem during ontogeny. Dichotomous branching of the shoot and a reorientation from orthotropic to plagiotropic growth occur during the development of the young sporophyte. The 1st primary (juvenile) leaves are small and dichotomously lobed. Later primary leaves are progressively larger and exhibit a transition to anisotomous and, finally, pinnate form. The adult leaves are pinnate, indeterminate climbing organs. Comparative analysis of the development of the heteroblastic leaf series determined that early divergences in the extent of apical growth and localization of pinna initiation account for the differences in final form of the leaves. The climbing (adult) leaves are highly specialized, but in spite of their indeterminate growth and complex structure, they are clearly foliar in nature. Their morphology results from modification of the basic processes of fern leaf development seen in the morphogenesis of the determinate primary leaves.