The Bases of Angiosperm Phylogeny: Ultrastructure

Abstract

A classification of sieve-element plastids by their major accumulation of ergastic products (protein or starch) into P-type and S-type, based on the ultrastructural research of some 500 species, provides systematists with a new micromorphological character to be used for a reconsideration of the outline of some of the higher taxa in the Takhtajan system of Magnoliophyta. Plastid types are listed for families and orders of Liliopsida and the first four subclasses of Magnoliopsida. Of these, Magnoliidae and Caryophyllidae are discussed in greater detail. It is demonstrated that sieve-element plastids can contribute relevant data to the rearrangement of at least some of the taxa in question. In addition some remarks are made on possible phylogenetic trends among the different plastid types. Ultrastructure is a rather new field contributing to plant systematics. Despite the comparatively short time which has passed since its introduction, various techniques have already been used very successfully to contribute valuable micromorphological characters to distinctive taxonomic problems. Cole & Behnke (1975) gave a short synopsis of new characters derived from comparative studies with the electron microscope, and a detailed evaluation of their application to plant systematics is being prepared (Behnke & Cole, in preparation). The data presented so far clearly demonstrate that, unlike the situation with lower plants, ultrastructural characters in higher plant systematics are largely confined to results achieved with the scanning electron microscope. With its potentiality to disclose new dimensions of plant surface the scanning electron microscope greatly extends our knowledge of morphological differences. In all of the recent attempts to come to a natural and phylogenetic system of higher plants, and of Magnoliophyta (Angiospermae) in particular, morphological information still ranks very high. Consequently, the scanning electron microscope seems more adequate to systematists and has more readily been accepted by them than other ultrastructural tools such as the transmission electron microscope and the freeze etching device. Thus the former failure of ultrastructural research to contribute reliable characters to magnoliophytan systematics may in part be due to its restriction to the cellular and subcellular level of the transmission electron microscope. There are two major reasons which explain why, for a long period, transmission electron microscopy did not even provide minor characters of taxonomic significance to the classification of Magnoliophyta: (1) within any given tissue, cell organelles are considered to be uniform; and (2), the distribution 1 Participation in the Symposium of the A. I. B. S. meeting, Amherst, Massachusetts, 1973, was made possible during the tenure of a visiting professorship at the Department of Botany, University of Texas at Austin, Texas 78712. 2 The author wishes to express his gratitude to Mrs. L. Pop for technical help during part of the unpublished investigations reported and to Mrs. D. Laupp during preparation of figures and line drawings for this paper. 3 Lehrstuhl fur Zellenlehre, Universitat Heidelberg, Im Neuenheimer Feld 230, D-69 Heidelberg, Germany. ANN. MissouRi BOT. GARD. 62: 647-663. 1975. This content downloaded from 157.55.39.176 on Sat, 09 Apr 2016 06:32:25 UTC All use subject to http://about.jstor.org/terms 648 ANNALS OF THE MISSOURI BOTANICAL GARDEN [VOL. 62 in Magnoliophyta of the relatively few different species that have been investigated with respect to a certain tissue is random. It therefore was an exciting experience when we first became aware of the applicability of ultrastructural data of sieve-element plastids to taxonomic problems (Behnke, 1967, 1969a). Continued research on a gradually expanding number of species and higher taxa supported and substantiated our concept of the new micromorphological character in seed plant systematics. Application and reliability of sieve-element plastid ultrastructure to systematic and phylogenetic questions in Magnoliophyta is presented in this contribution to the symposium. Since the present discussion will in general only descend to the family level, it may be convenient to refer to the detailed listing of species and references given in Behnke (1972) and to an additional list kept up-to-date by the author. The combined lists include ultrastructural data on more than 500 species.