The rachis-leaves of Lilaeopsis brasiliensis (Glaziou) J. M. Affolter (Apiaceae)

Abstract

The leaves of Lilaeopsis brasiliensis have the general features of the terete linear rachis-leaves which have been described in other umbellifers. The leaf consists of a septate structure which appears to correspond to the rachis of compound umbellifer leaves, and at some of the septa a single hydathode is inserted on the adaxial midline of the leaf. The septa appear to correspond to the positions of pinna insertion in compound leaves, and the hydathodes to reduced pinnae. Although by these criteria the leaf of L. brasiliensis is a rachis-leaf, it differs from most other umbellifer rachis-leaves in presenting the appearance of a petiole and a dorsiventrally flattened leaf blade. As in other rachis-leaves of umbellifers which have been examined previously, the leaf arises from a leaf primordium which is initially dorsiventral in symmetry. However, the leaf of L. brasiliensis is distinctive in that it retains dorsiventral symmetry throughout development, while it also acquires the median hydathodes and a vascular system characteristic of ‘unifacial’ rachis-leaves. The leaf therefore shows a mixture of features which are not normally associated. It provides an argument for the restriction of the term ‘unifacial’ to a solely descriptive role without developmental implications. Simplification of the structure in the distal region of the blade results in an appearance close to that of a normal dorsiventral leaf, though a small number of inverted vascular bundles remain. This may indicate the occurrence of further evolution of leaf structure.