Classification of trichome types within species of the water fern Salvinia, and ontogeny of the egg-beater trichomes

Species of the water fern Salvinia are well known for their extremely water-repellent floating leaves. The architecture of Salvinia surfaces is of great interest for biomimetic applications, because submersed in water, they retain air films for a long period. Knowledge of these surfaces is also important for pest control, since one species (Salvinia molesta D.S. Mitch.) is a pantropic invasive aquatic weed. The micromorphology of the leaf surfaces of six representative species has been characterized by scanning electron microscopy. Based on their morphology, the trichomes are classified in four types, named after the typical species. Among the species examined, numbers, distribution, and sizes of the trichome types vary significantly. The simplest types, the Cucullata trichomes, are multicellular, uniseriate, and up to 800 mu m high. Groups of two multicellular, uniseriate trichomes are described as the Oblongifolia trichomes. The Natans trichomes are grouped as four multicellular, uniseriate trichomes. The Molesta trichomes are composed of four trichomes, which are connected by the second last apical cells of the trichome. The ontogeny of the Molesta trichome groups was puzzling and resulted in various names being applied to them (Kronchenhaare, egg-beater, or coroniform hairs). Their unique development from four solitary uniseriate trichomes to groups of four connected trichomes is described in detail.