Classification and terminology of plant epicuticular waxes

Plant cuticles are covered by waxes with considerable ultrastructural and chemical diversity. Many of them are of great systematic significance. Waxes are an essential structural element of the surface and of fundamental functional and ecological importance for the interaction between plants and their environment. An extensive literature has been published since the introduction of scanning electron microscopy (SEM). Hitherto, the area has lacked a complete classification and terminology necessary as a standard for comparative descriptions. A refined classification and terminology of epicuticular waxes is therefore proposed based on high-resolution SEM analysis of at least 13 000 species, representing all major groups of seed plants. In total 23 wax types are classified. Thin wax films appear to be ubiquitous, while thicker layers or crusts are rare. The most prominent structures are local wax projections, which most probably result from self-assembly of wax molecules. These projections are supposed to be mainly of a crystalline nature and are termed crystalloids here. Among these, platelets and tubules are the most prominent types, while platelets arranged in parallel rows and stomatal wax chimneys are the most striking orientation and aggregation patterns. In addition, a comprehensive overview on the correlation between wax ultrastructure and chemical composition is given.