Are the trichomes in corollas of Mutisieae (Asteraceae) really twin hairs?

Abstract

The corollas of three species of Onoseris, 13 species of Trichocline and one species of Uechtritzia (Asteraceae, Mutisieae) exhibit trichomes that are morphologically similar to the twin hairs which are traditionally considered exclusive of the cypselas in Asteraceae. Four types of trichomes were found: 1-celled, 2-celled, 3-celled, and 4-celled. The 3-celled and 4-celled corolla hairs are those which most resemble the typical cypsela twin hairs. The ontogeny of the corolla hairs was analysed and coincidences with the ontogeny of the cypsela twin hairs were found. (1) An anticlinal division of the epidermal mother cell originates two hair cells which, in turn, originate the basal cells (sometimes one of them is reduced or similar to the epidermal cells) by oblique or periclinal subdivision. (2) In some cases the basal cell(s) of the 3- or 4-celled corolla hair elongates and reaches the same length as the hair cells. The same kind of trichomes have been demonstrated in cypselas of Mutisieae. (3) The 1- and 2-celled corolla hairs have already been described as variants of the cypsela twin hairs. Based on this evidence we conclude that the corolla hairs of Onoseris, Trichocline and Uechtritzia are twin hairs. It is hypothesized that the 1-4 corolla hairs could be involved in water absorption, as occurs in the cypsela twin hairs.