Laticifer starch grain morphology and laticifer evolution in Euphorbia (Euphorbiaceae)

The micromorphology of starch grains in the nonarticulated laticifer was examined in 200 taxa of succulent and nonsucculent Euphorbia. Rod, spindle, osteoid, discoid and intermediate shapes were found in latex. These unusual starch grain shapes are assumed derivatives from more rounded grains present in other angiospermous cells. Unbranched, rod–shaped grains were present in latex of annual and perennial herbaceous taxa. Spindle, osteoid, and discoid shapes, often branched, occurred in xerophytic, succulent taxa. Leafy taxa in several subgenera possessed rod–shaped grains. Taxa of the primitive subgenus, Esula, possessed the shortest rod–shaped grains derived from rounded grains common in parenchyma. Length of rod–shaped grains increased through herbaceous subgenera and culminated in Poinsettia where grain length may be extremely great. Dwarf or shrubby succulents with thickened roots have osteoid grains. Shrubby or arborescent coralline and cactiform species, specialized taxa, possessed large branched grains. Grain morphology of Euphorbia was compared with that from latex of other genera of the Euphorbiaceae. These in–depth analyses of laticifer plastids demonstrates the applicability of starch grain morphology for interpreting the progressive evolution of the nonarticulated laticifer as a cell type within the genus Euphorbia.