Epicuticular Wax Ultrastructure
- Barthlott and I. Theisen
- Introduction
Over the past decades, systematically relevant information on structure and composition of the plant cuticle based on SEM studies have been published (survey in Barthlott 1990). The cuticles of the majority of higher plants are covered with epicuticular wax secretions. They often cause a glaucous appearance. Epicuticular "waxes" occur throughout bryophytes, pteridophytes, gymno-sperms, and angiosperms. They are chemically multicomponent mixtures, in which a particular compound or class(es) of compounds prevails, such as lipophilic substances like long-chain fatty acids (true waxes), as well as alkanes, ketones, exters, and cyclic compounds such as pentacyc-lic triterpenes, phytosterols, and flavonoids (Barthlott and Wollenweber 1981; surveys by Baker 1982; Jeffree 1986; Bianchi 1995). These substances occur usually as local projections of crystalline nature (Jeffree et al. 1975) and exhibit a high ultrastractural diversity. The genetic control of their biosynthesis has been studied by Wettstein-Knowles (1979, 1995). The dominating constituent(s) are responsible for the particular structure of the crystalloids. The correlation between micromorphology and chemistry has been proven by recrystallization experiments (Jeffree et al. 1975; e.g., Jeflree 1986; Jetter and Riederer 1994, 1995; Meusel 1997). Hence, the ultrastructure of epicuticular waxes manifests a visualized chemo-taxonomy. Yet in cases of ultrastructural similarity and simultaneously uncertain relationships, it is necessary to analyze the chemical composition of the waxes in order to differentiate between ultrastructural homologies and convergencies.
A first detailed survey of monocotyledonous epicuticular waxes was published in two earlier papers (Barthlott and Frölich 1983; Frölich and Barthlott 1988). The following conclusions are based on the examination of some 400 genera and 750 species of 97 out of 102 monocotyledon families; they are interpreted in comparison with wax data of about 13000 species of angiosperms analyzed in the past two decades. The terminology used for different wax crystalloids is that of Barthlott et al. (1998).
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