Unique calcium oxalate "duplex" and "concretion" idioblasts in leaves of tribe Naucleeae (Rubiaceae)

This basic study may help plant biologists better understand the variety and occurrence of crystal forms and their function in plants. Literature records hold four examples of two different crystal types in one cell. One, unillustrated report mentions druses and crystal sand from one species of tribe Naucleeae (Rubiaceae) and "occasional occurrences" in additional unnamed taxa. Here, we surveyed Naucleeae (103 of 179 species, 23 of 24 genera, all seven subtribes) for "duplex idioblast" distribution for systematic significance and describe examples of this rare feature.

Cleared, dehydrated, herbarium leaves were mounted unstained in resin. Slides were examined with polarization optics for crystal types and locations, and representative areas were electronically digitized. Scanning electron microscopy and x-ray analysis verified calcium oxalate composition.

Idioblast configurations occur as crystal sand (CS) only (most common, 92 spp.) to CS plus one embedded druse (55 spp.), to CS plus 2-3 druses (6 spp.), to one druse with scanty surrounding CS (a few spp.), to a "naked" druse (16 spp.). Trends occur in some subtribes. A previously undescribed conspicuous, spheroidal calcium oxalate aggregate "concretion" idioblast occurs in only four species of Mitragyna (Mitragynineae). Idioblasts are most common along vascular bundles and in mesophyll, less so only along vascular bundles, and least common only in mesophyll. Tiny "secondary" crystals are common in ordinary mesophyll cells.

Crystal types appear to be systematic features in Naucleeae. Duplex idioblasts (CS and druses) and aggregate concretions are a demonstration that much is yet to be discovered about crystals.