Leaf development in the anisophyllous shoots of Pellionia daveauana (Urticaceae)

The dorsiventral shoot system of Pellionia daveauana (Godefr.) N.E. Br. is characterized by opposite pairs of dimorphic leaves. The small dorsal leaves differ from the large ventral leaves by having a reduced leaf blade, fewer tissue layers in the epidermis and mesophyll, a reduced vascular system and significantly smaller cell size in all tissue layers. The observations reported here document the developmental basis of these morphological and histological differences. Although both dorsal and ventral leaves appear to be initiated simultaneously, the volume of the ventral leaf primordium is greater than that of the dorsal leaf primordium and growth in length occurs over a longer time period. Early plate meristem activity results in the elaboration of the ventral leaf blade, while plate meristem activity is lacking in dorsal leaves. During ventral leaf expansion periclinal divisions in adaxial and abaxial protoderm and ground meristem give rise to multiple epidermis and new mesophyll layers, respectively. Similar periclinal divisions in dorsal leaves occur at an earlier developmental stage and are restricted in extent. Measurements of cell dimensions show that cell enlargement also ceases at an early developmental stage in dorsal leaves. Development of the ventral leaf is characterized by a relatively long period of cell division and enlargement. In contrast, early cessation of cell division and precocious cell maturation result in the distinctive structural features of the dorsal leaf blade.