Foliar Idioblasts in Physostegia virginiana (Lamiaceae)

LERSTEN, NELS R. (Department of Botany, Iowa State University, Ames, IA 50011) AND JOHN D. CURTIS (Biology Department, University of Wisconsin-Stevens Point, Stevens Point, WI 54481). Foliar idioblasts in Physostegia virginiana (Lamiaceae). J. Torrey Bot. Soc. 125:133-137. 1998-A literature survey of internal oil secretion in Lamiaceae revealed that only Brazoria and Physostegia have greatly enlarged foliar mesophyll idioblasts. These have been regarded as specialized oil storage cells that evolved independently of oil cells in other angiosperm families. Resin sections and scanning electron microscope preparations of mature Physostegia virginiana (L.) Benth. leaves showed that each idioblast is an unusually large ellipsoid cell, which is dead at maturity. Idioblasts occur in all mesophyll layers but are most common in the mid-stratum, often abutting a bundle sheath or even occurring as part of it and bulging conspicuously into the mesophyll. Freehand leaf sections treated with Sudan IV showed one small oil drop per idioblast, a similar oil drop in other mesophyll cells, and a larger oil deposit atop each of the many small capitate secretory epidermal trichomes. Oil accumulation is common among leaf cells, therefore we speculate that the idioblasts may primarily serve a defensive function, one as yet unknown, possibly during leaf development.