EARLY HISTOGENESIS OF THE ADULT LEAVES OF DARLINGTONIA CALIFORNICA (SARRACENIACEAE) AND ITS BEARING ON THE NATURE OF EPIASCIDIATE FOLIAR APPENDAGES

Abstract

In order to assess the validity of various interpretations of tubular leaves of angiosperms, a histogenetic study of the ontogeny of adult leaves of Darlingtonia californica was undertaken. The adult leaf of Darlingtonia is characterized by a sheathing leaf base, an elongate ascidium, an overarching hood, and two “fishtail” appendages which arise near the leaf apex. A keellike growth, with two rows of alternate vascular bundles, traverses the tube from base to mouth. Ontogenetic studies show that the primordium arises by a monopodial rather than a sympodial mode of growth as previously reported. After the formation of a small, erect primordium, a restricted adaxial meristem is initiated that expands both adaxially and upwards. This “querzone” serves, in effect, to congenitally combine the two primordial margins during its extension. Growth and maturation of the subjacent portions cause tubular elongation in the leaf. Primordial apical divisions are later replaced by more general intercalary growth with acropetal and centrifugal maturation. The hood and fishtails are established early in ontogeny by adaxial growth of the primordial apex and subsequent activation of juxtaposed localized meristems. Comparative morphology has established that the epiascidiate leaf is a foliar appendage that undergoes certain specific morphogenetic modifications. It has a structural relationship to ensiform appendages of Acacia and Acorus as well as to peltate foliar organs. The early ontogeny of Darlingtonia leaves is considered to be homologous with other epiascidiate foliar organs, including some supposedly primitive carpels.