Permanent tetrads originated in two major lines : the xylopioids, corres·
ponding to part of Walker's Fusaea subfamily but excluding Fusaea, with
granular structure and a nexine of fused granules ; and the annonolds, corras·
ponding to Walker's Annona subfamily, with columellar structure and a com
plexly foliated nexine. The single grains of /solona are secondarily derived
from tetrads. Sulculate pollen arose in both monosulcate and inaperturate
lines. The genus
Polya/thia, which has unusually diverse pollen, appears to be
polyphyletic. Columellar structure originated in smooth monosulcates with
granular structure, leading to the malmeoids, and in forms with larger,
irregular granules,
in the disulculate Polyalthia group and possibly the anno
noids.