Forchhammeria and Stixis (Brassicales): Stem and Wood Anatomical Diversity, Ecological and Phylogenetic Significance

Abstract

Qualitative and quantitative data are given for wood anatomy of six of the 11 recognized species of Forchhammeria (Mexico, Central America, West Indies), a genus formerly placed in Capparaceae. Though still in Brassicales, the genus has been excluded, along with several other genera, from the major recognized families of that order on the basis of molecular data. Liquid-preserved material of several species permitted detailed histological accounts of the successive cambia and their development in the stems of Forchhammeria. Successive cambia have a curious distribution in Brassicales that may represent homoplasies. Most wood features of the genus do not appear highly xeromorphic, but presence of tracheids as a wood background tissue and abundance of starch and perhaps water storage in ray parenchyma and conjunctive tissue can be cited as mechanisms likely to resist embolism formation. Forchhammeria retains green leaves throughout the dry season. Forchhammeria tamaulipana, known only from Tamaulipas State, Mexico, the single species of a new subgenus, Pauciflora, is newly described. Its embryos have nearly equal cotyledons and germinate epigeously, whereas all remaining species of the genus are pseudomonocotylous and hypogeous. These and other distinctive features of F. tamaulipana may prove significant in providing links to other brassicalean genera. The family name Stixaceae Doweld is now appropriate for Forchhammeria, Neothorelia, Stixis, and Tirania.

Development of Inverse Cambia and Structure of Secondary Xylem in Ipomoea turbinata (Convolvulaceae)

“…Seven of these 19 families have climbing taxa (Carlquist & Hanson 1991). In the Convolvulaceae, cambium (termed 'internal cambium') initiates between the intraxylary phloem and primary xylem (Rajput et al 2008(Rajput et al , 2014Terrazas et al 2011;Carlquist et al 2013). This internal cambium produces secondary phloem, often in large quantities, on its inner side.…”