The Development of Split Axes in Ambrosia dumosa (Gray) Payne (Asteraceae)

Seedling structure and the origin of the vascular cambium in Ambrosia dumosa are typical of other dicotyledonous plants. After a few seasons of growth, fluting appears in the xylem of the main axis because of the production of fewer cells by the vascular cambium in the regions, or sinuses, between flutes; 84% of the flutes can be traced acropetally into diverging branches. Splits arise in the axils of diverging branches of intermediate-sized shrubs and extend laterally between sinuses of flutes. The splits proceed basipetally through the axis, segmenting it into independent root-stem-canopy segments. Between split segments, lignified ray parenchyma expands and divides to form a tissue similar to the bark. The bark is comprised of expanded, slightly suberized phloem parenchyma and bundles of fiber-sclereids that differentiate secondarily from phloem parenchyma. A periderm was never observed.