The factors which have governed pollen evolution are inadequately known, although phylogenetic schemes showing the derivation of pollen types, and statements concerning the taxonomic value of pollen characters are common in the literature. Wodehouse (1935) first attempted a comprehensive review of pollen evolution. He emphasized the close relation between size, mode of pollination, and structural differentiation of the exine and discussed functional aspects. Size may be related to chromosome number (polyploids often have larger grains with more apertures), pollination ecology (wind-dispersed grains are small, insect-dispersed ones large or small), size of flowers (Fouquieraceae), or length of style (Glandularia, Verbenaceae).