Wood Anatomy and Classification of the Myrtales

Abstract

The wood anatomical diversity of the woody Myrtales, comprising Combretaceae, Lythraceae (including Alzatea), Melastomataceae (including Crypteroniaceae), Myrtaceae, Oliniaceae, Onagraceae, Penaeaceae, Punicaceae (can also be included in Lythraceae), Psiloxylaceae, Sonneratiaceae and also Thymelaeaceae, is summarized. All these families share intraxylary phloem and vestured pits, and their other wood anatomical attributes represent parts of continuous, sometimes divergent, often parallel, specialization series. Rhizophoraceae, Lecythidaceae, Elatinaceae, Cordiaceae, Chrysobalanaceae and Dialypetalanthaceae are excluded because they lack this combination of characters. The phylogenetic relationships within Myrtales are discussed, and pictured in a two-dimensional diagram (Figs. 2-8). Most families are mutually closely related. The wider wood anatomical affinities of the order are with Gentianales. Features from vegetative anatomy such as bicollateral bundles and vestured pits in the wood have entered discussions on the delimitation and classification of the order Myrtales for a long time. In this paper the entire evidence from wood anatomy will be applied to these taxonomic aspects. The knowledge of the wood anatomy of truly or putatively Myrtalean families has considerably increased in recent years (see references cited under the summarized wood anatomical descriptions) so that a discussion of its taxonomic implications can be meaningful. Our paper cannot, unfortunately, take wholly herbaceous families such as Haloragaceae and Trapaceae into consideration. For a survey of all families that have, from time to time, been assigned to the order Myrtales, the reader is referred to Dahlgren and Thorne (1984). DELIMITATION OF THE ORDER MYRTALES All families that have been and are currently regarded as indubitable members of the Myrtales are characterized by a combination of two anatomical features: vestures in the bordered pits of the secondary xylem, and bicollateral bundles in the primary stems (and leaves as far as major bundles are concerned), resulting in the presence of intraxylary or internal phloem in woody stems (not to be confused with interxylary or included phloem, which occurs in a restricted number of genera and families of the order only). Several authors have also used one or both of these characters in their delimitation of the order. Vestured pits and intraxylary phloem are rather uncommon in the Dicotyledons, as can be seen in Figure 1, where the distribution of these characters is illustrated using Dahlgren's (1980) diagrammatic classification. The data are derived from Metcalfe and Chalk (1950), completed for recent records of vestured pits by Meylan and Butterfield (1974), Miller (1977), and Baas and Werker (1981). The combined occurrence of vestured pits and intraxylary phloem appears to be very rare. Outside the Myrtales both features are found only in part of the Gentianiflorae (Apocynaceae, Asclepiadaceae, Loganiaceae pro parte) and in the Thymelaeales (Thymelaeaceae only) of Dahlgren's Malviflorae where the two characters are further only of very sporadic occurrence in the Euphorbiaceae, and then not even simultaneously present in the same genera (Bailey, 1933; Metcalfe & Chalk, 1950). Other combined occurrences are limited to Vochysiaceae (Polygalales) and the single genus Centropodium of the Polygonaceae (Polygonales). The sporadic occurrence of the two features outside the Myrtales as understood by us and Dahlgren and Thorne (1984), and their very conI Prof. Dr. C. Kalkman critically read the manuscript. Mr. J. van Os prepared the drawings. In connection with the research project on the wood anatomy of the Myrtales, concluded with this paper, the first author was over the years 1975-1977 financially supported by the Netherlands Organisation for the Advancement of Pure Research (Z.W.O.). 2 Hortus Botanicus, Nonnensteeg 3, 2311 VJ Leiden, The Netherlands. 3Rijksherbarium, P.O. Box 9514, 2300 RA Leiden, The Netherlands. ANN. MISSOURI BOT. GARD. 71: 783-800. 1984. This content downloaded from 157.55.39.111 on Tue, 02 Aug 2016 05:59:05 UTC All use subject to http://about.jstor.org/terms 784 ANNALS OF THE MISSOURI BOTANICAL GARDEN [VOL. 71 CARYOPHYLLIFLORAE 4 PROTE IFPLORAE POLYGONI FLORAEIFOA q L R R A E~~~~~~~~~~~~RIL/ A