Small pit-like structures in upper surfaces of dried leaves of Aporosa (Phyllanthaceae) explained by leaf anatomy

Aporosa leaves, when dried, generally have small depressions or pits on the upper surface that probably do not occur when the leaves are still fresh. The presence of these pits form a good identification tool to recognize the genus Aporosa . An anatomical relation is made between the pits and the presence of adaxial epidermal mucilage cells. The mucilage cells are also present in the abaxial epidermal layer, but there they are smaller and do not show as pits. The pits are seemingly depressions caused by epidermal cells partly overlying the slightly sunken top cell of the mucilage cells and leaving a kind of hollow. However, in SEM scans the mucilage cells seem to be elevated, which might be due to the vacuum conditions of the SEM or the preparation of the SEM material. The mucilage cells also occur in Baccaurea , part of the sister group of Aporosa , but there the pits are not visible in dried leaves. The negative correlation between the size of the pits and their number (fewer and larger in Aporosa , more and often smaller in Baccaurea ) probably explains why the pits are visible in dried leaves of many Aporosa species only. There is a slight difference between both genera in the presence of druses; large ones are present in mainly Aporosa species, and larger quantities mainly in Baccaurea species.