LEAF MORPHOLOGY AND TAXONOMIC ANALYSIS OF CISTANTHE-TWEEDYI (NEE LEWISIA-TWEEDYI, PORTULACACEAE)

Cistanthe tweedyi (Portulacaceae) is a rare perennial herb endemic to the Cascade Mountains of northern Washington and adjacent British Columbia. It has heretofore been classified in Lewisia, probably because of the persistent, yet erroneous, characterization of its fruit dehiscence as circumscissile, as in Lewisia, rather than valvate, as in most other Portulacaceae. Cladistic affinity of C. tweedyi to Cistanthe is evidenced by its unequal inflorescence bracts (a synapomorphy of the genus) and by the presence of several characters that show homoplasy in Portulacaceae but do not occur in combination outside of Cistanthe, including wide-petiolate leaves, festooned brochidodromous venation, an apically diminished primary vein, predominantly brachyparacytic and similar stomata, and strophiolate seeds with colliculate sculpture and micropylar hooks. Cistanthe tweedyi lacks sinuous and ribbon-like veins, which are otherwise rarely absent in Cistanthe. Cistanthe tweedyi lacks other characteristics plausibly synapomorphic for all or part of Lewisia, including glandular-dentate sepals and bracts, cladodromous leaf venation, and laterocytic stomata. The chromosome base number of Cistanthe tweedyi, n = 46, is possibly based on n = 11 - a number common in Cistanthe, but rare in Lewisia. Cistanthe tweedyi constitutes a monotypic sect. Strophiolum within Cistanthe. Its precise phylogenetic position and biogeographic origin remain uncertain.