Air Spaces, Secretory Cavities, and the Relationship between Leucophylleae (Scrophulariaceae) and Myoporaceae

Abstract

The isobilateral leaves of Leucophyllum have numerous, spherical air spaces between the abaxial and adaxial palisade mesophyll layers. These spaces are larger than the secretory cavities of Bontia (Myoporaceae) and have no epithelium. Early in ontogeny of air spaces in L. minus, cells lining the spaces enlarge periclinally to the surface of the space, as do the cells lining the young secretory cavities of Bontia. Dimensionally preferential cell enlargement does not occur in the early development of similar air spaces in L. frutescens var. frutescens and appears to serve no function in the development of the air spaces in L. minus. Dimensionally preferential cell enlargement may be a vestige of a morphogenetic pattern such as that that produces secretory cavities in Bontia. In the context of the several other lines of evidence favoring an alliance of Leucophylleae with Myopor- aceae, we suggest that the air spaces of Leucophyllum are homologous with the secretory cavities of