Structure of Ovular and Placental Trichomes of Araceae

Abstract

A systematic survey of the occurrence and anatomy of ovular and placental trichomes was made in 94 genera and more than 220 species of Araceae. Trichomes are present in virtually all examined species and are always associated with locular mucilage. Trichomes are absent from the monotypic genera Gymnostachys and Symplocarpus and from some species of Theriophonum, Typhonium, and Arisaema, which also lack mucilage. Trichome anatomy varies widely throughout the family and within certain genera. However, there are no clear overall systematic trends for cell size, shape, cell number, and branching, although certain taxa show characteristic patterns. Trichomes are typically single-celled, less commonly with two or three cells mixed with unicellular forms. Species, such as Monstera deliciosa and Eminium albertii, and several Amorphophallus species have a predominance of bi- to multicellular hairs. Branched trichomes occur rarely in some species of Amorphophallus and Biarum. Trichome length ranges widely, from papillae extending ca. 30 μm above the epidermis to hairs more than 1 mm long. Among the most distinctive trichomes are those of both Acorus species, which occur as placental hairs and as outgrowths of the integuments, principally the outer integument. In A. gramineus a fringe of elongated hairs extends beyond the micropyle. The location and organization of ovular hairs set Acorus apart from all other Araceae "species" or "taxa" and constitute an additional unique character supporting its removal to a separate family.