Comparative Morphology and Early Leaf Histogenesis of Adult and Juvenile Leaves of Darlingtonia californica and Their Bearing on the Concept of Heterophylly

Abstract

The shoot system of Darlingtonia is characterized by both adult and juvenile leaves. The juvenile leaves are tubular but lack both a keel and the highly elaborated fishtail. The adult leaves are initiated at a divergence angle of 135.3⚬ with an equivalent phyllotactic index (EPI) of 2.09. The juvenile leaves arise at 147.6⚬ and show an EPI of 1.89. Both correspond well to a classical 2/5 (2 + 3) phyllotaxis. The apical meristem has plastochronic fluctuations corresponding to a Schmid maximum/minimum of 270 μm/120 μm in the adult portion of the stem and 90 μm/30 μm in the juvenile portion. During ontogeny the upper leaf zone of the juvenile leaf differentiates much faster than the corresponding zone of the adult. Biometric relationships of the various leaf components indicate that although the ratios of growth are allometrically equivalent, the corresponding graph lines are elevated or depressed from one another. This may indicate that the juvenile leaves are not simple arrested forms of adult foliage. Evidence from allometric relationships indicates that the adult and juvenile leaves display different ontogenetic growth patterns at the earliest stages of initiation. The pattern of venation is significantly different between the two leaf types. Adult leaves show a greater elaboration of the pinnate venation pattern at all stages of ontogeny. This work clarifies the relationship of this epiascidiate leaf type to other tubular organs and reveals the exact morphological nature of these remarkable leaves.