The xylem and phloem of Brassicaceae (116 and 82 species respectively) and the xylem of Resedaceae (8 species) from arid, subtropical and temperate regions in Western Europe and North America is described and analysed, compared with taxonomic classifications, and assigned to their ecological range. The xylem of different life forms (herbaceous plants, dwarf shrubs and shrubs) of both families consists of libriform fibres and short, narrow vessels that are 20–50 μm in diameter and have alternate vestured pits and simple perforations. The axial parenchyma is paratracheal and, in most species, the ray cells are exclusively upright or square. Very few Brassicaceae species have helical thickening on the vessel walls, and crystals in fibres. The xylem anatomy of Resedaceae is in general very similar to that of the Brassicaceae. Vestured pits occur only in one species of Resedaceae.Brassicaceae show clear ecological trends: annual rings are usually distinct, except in arid and subtropical lowland zones; semi-ring-porosity decreases from the alpine zone to the hill zone at lower altitude. Plants with numerous narrow vessels are mainly found in the alpine zone. Xylem without rays is mainly present in plants growing in the Alps, both at low and high altitudes. The reaction wood of the Brassicaceae consists primarily of thick-walled fibres, whereas that of the Resedaceae contains gelatinous fibres. The frequency of sclereids in Brassicaceae bark is an indicator of ecological differences: sclereids are rare in plants from the alpine zone and frequent in plants from all other ecotones.
Size variation in the vascular cambium and its derivatives in two Alstonia species