THE BASES OF ANGIOSPERM PHYLOGENY: ULTRASTRUCTURE12 H.-DIETMAR BEHNKE3 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 Magnolio-phyta. 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 phylo-genetic 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 ail 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 SParticipation 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 partof the unpublished investigations reported and to Mrs. D. Laupp during preparation of figuresand line drawings for this paper. SLehrstuhl fir Zellenlehre, Universit�t Heidelberg, Im Neuenheimer Feld 230, D-69Heidelberg, Germany.ANN. MIssoURI BOT. GARD. 62: 647-663. 1975.