Liana structure function and selection: a comparative study of xylem cylinders of tropical rainforest species in Africa and America A general analysis of xylem cylinders is presented on the basis of intercontinental (Africa-America) comparisons of 448 species (35 families) of tropical forest lianas. Drawings of stem transverse sections are presented for 58 species. From an evolutionary point of view the liana represents a woody plant form at the crossroads of two life-form types: either self-supporting or non-self-supporting. The interconnections between anatomy life-form and dynamics of forest space occupation are highly integrated. The American lianas show a distinct propensity to form a crown in the forest canopy constituted of multiple leafy units (small crowns) which grow apart with an increasing distance between each unit (expansion with a centrifugal tendency). Xylem cylinders associated into polystelic and multiple stems appear to facilitate this behaviour (nearly 80% of American species compared to less than 50% in Africa). Neotropical forests seem to show a comparatively higher range of sites exposed to light than African ones.