Leaf Histology and Its Contribution to Relationships in the Myrtales

An examination of the leaf histology of a wide array of families thought at one time to be included in the Myrtales has led to a restricted circumscription of the order as follows: Lythraceae, Rhynchocalycaceae, Trapaceae, Oliniaceae, Combretaceae, Alzateaceae, Penaeaceae, Melastomataceae, Crypteroniaceae, Psiloxylaceae, Myrtaceae, and Onagraceae. Data used in testing this concept were obtained from lamina transsections cut to include the midrib. Observations on midrib shape, venation configuration, shape and extent of extraxylary fibers, sclereids, dermal and mesophyll features were most discriminating when examining taxa with the most elaborate and/or generalized anatomy. While leaf histological trends of specialization are not independently producible from this data, correlation with information from leaf architecture should eventually make this possible. Seldom recorded features which have proven useful in this systematic context include length/width ratios of palisade cells and the shape of the boundary between the mesophyll and the midrib ground tissue. The sharpness of tissue differentiation also appears often to be a family characteristic. In most respects, plant families comprising the core of the Myrtales form a coherent group and the recognition of the order has not been particularly controversial (Dahlgren & Thorne, 1984). At the same time, many of the included taxa pose problems as to their level of recognition as well as to which higher taxon they should be assigned within the order. In addition, more than 30 other families have been assigned to the Myrtales at some time in the recent past (Dahlgren & Thorne, 1984) demonstrating insufficient knowledge of the boundaries of the order as well as its evolutionary background. A review of the general literature on the circumscription of the order and the evidence used in its definition has been ably dealt with in the other papers from this symposium from the standpoint of general systematic review (Dahlgren & Thorne, 1984; Johnson & Briggs, 1984), wood anatomy (van Vliet & Baas, 1984), leaf architecture (Hickey, 1981), pollen (Patel et al., 1984), and sieve element plastids (Behnke, 1984). I will restrict my comments to the contribution to be made by leaf histology, recognizing that a clear understanding will depend upon a synthesis of all of the above plus data yet to be obtained. Some formidable problems must be faced when one attempts to use features from young vegetative anatomy in a systematic investigation. First is the insufficient number of studies of this sort to provide any reliable trends of specialization. I am convinced, however, that leaf histology contains not only valuable diagnostic characters, but that these characters will eventually demonstrate great utility in evolutionary studies. Much of the value will be based on both an increasing number of systematic studies of leaf histology, as well as character correlation with studies of leaf architecture (cf. Hickey, 1981). Leaf structure studies of both types should continue to accumulate rapidly. Leaves are obviously the most easily obtained of all plant materials both from the field and from herbarium collections. While leaves respond readily to evolutionary pressures toward xeromorphy or other modifications, their endomorphic characters commonly retain the ground plan of the phylad to which they belong (Keating, 1984; Dickison, 1970). In this study a particular problem concerns the interpretation of the data from my sample. The sample appears comprehensive in that 176 species were examined representing 55 genera from 19 families. Actually, the specimens at hand amount to a very small sample of many of the families. Even though attempts were made to choose specimens representing geographic and taxonomic diversity within each family, obviously the family samples most likely do not include the total spectrum of characters to be found in them. The absence of a character from one group does not I This work was partially supported by National Science Foundation grants DEB 77-15571 to the author and by DEB 78-23400 to Peter H. Raven. I am grateful to curators from many institutions for liquid preserved specimens and to P. H. Raven for his assistance in obtaining many specimens. 2 Department of Biological Sciences, Southern Illinois University, Edwardsville, Illinois 62026. ANN. MISSOURI BOT. GARD. 71: 801-823. 1984. This content downloaded from 157.55. 39.178 on Fri, 05 Aug 2016 05:09:46 UTC All use subject to http://about.jstor. org/terms TABLE 1. Selected histological features of leaf transections of families of Myrtales. O