Abstract

In Crataegus series Douglasianae vein density and other architectural features of foliage leaves vary in manners suggesting different degrees of adaptation to xeric conditions, notably the risk of xylem cavitation and embolism. We document variation in leaf venation in sexual diploids and allopolyploids in which gametophytic self-incompatibility, pseudogamous gametophytic apomixis, and hybridization have apparently also played roles in determining the extents of the geographic ranges of these taxa.