Leaf and petiole anatomy were investigated for nearly 400 samples representing 105 out of 106 genera of Araceae. The study included four duckweed genera (as Araceae) and two species of Acorus (as Acoraceae). The questions posed involved the identification of anatomical characters most useful in microscopic diagnosis, as well as in elucidating evolutionary trends of specialization. The tissue patterns best relating to these aims include mechanical tissues (sclerenchyma and collenchyma and their relationship to each other), laticifers and other secretory tissues, and raphide crystals and their cell structure. Mesophyll structure and ground tissue patterns of the petiole also exhibit unexpected systematic value. Collenchyma patterns, considered as uninteresting in most families, is very informative in Araceae. It varies from existing as peripheral bands in petioles and midribs in subfamilies Pothoideae and Philodendroideae, to independent strands aligned with vascular bundles in the subfamily Aroideae. Various transitions exist. The anatomical data correlate well with results of the previous cpDNA study of the family by French, Chung and Hur and are the basis for a modified arrangement of genera.

Key words: Acoraceae, anatomy, Araceae, evolution, leaf, petiole