Two foliar idioblasts of taxonomic significance in Cercidium and Parkinsonia (Leguminosae: Caesalpinioideae)

Leaf anatomy was surveyed for idioblasts in Cercidium (eight to nine species) and Parkinsonia (four to six species), two problematic genera of caesalpinioid legumes variously merged and separated for over a century. Leaflets from 16 accessions (11 dried and five liquid-preserved) of ten species were cleared (protoplasts removed chemically) and stained, supplemented by resin-embedded sections and scanning electron microscopy. Four Parkinsonia species had large, dimorphic, living subepidermal idioblasts (SI)—one form adaxially restricted, the other form abaxial. The fifth species, P. africana, had tiny scalelike leaflets without idioblasts. Idioblast contents are unknown but visibly different from those of adjacent cells. All five Cercidium species lacked SIs, but instead had veinlet idioblasts (VI). These large idioblasts varied in size and abundance, but in C. floridum and C. texanum they were especially large, spheroidal to ovate, thick-walled, and devoid of protoplasm at maturity. VIs occurred mostly as pairs or clusters, with individual cells joined by a large gap resembling a simple perforation plate. Almost all VI units abutted minor veins, but no perforation plate joined the proximal VI to a tracheary element. Cells similar to SIs occur in other Caesalpinieae genera, but VIs are unique to Cercidium. The mutually exclusive distribution of SIs and VIs supports the view that Cercidium and Parkinsonia are different.