A COMPARATIVE STUDY OF THE PRIMARY VASCULAR SYSTEM OF CONIFERS. III. STELAR EVOLUTION IN GYMNOSPERMS

This paper includes a survey of the nature of the primary vascular system in a large number of extinct gymnosperns and progymnosperms. The vascular system of a majority of these plants resembles closely that of living conifers, being characterized, except in the most primitive forms which are protostelic, by a eustele consisting of axial sympodial bundles from which leaf traces diverge. The vascular supply to a leaf originates as a single trace with very few exceptions. It is proposed that the eustele in the gyn nosperms has evolved directly from the protostele by gradual medullation and concurrent separation of the peripheral conducting tissue into longitudinal sympodial bundles from which traces diverge radially. A subsequent modification results in divergence of traces in a tangential plane. The closed vascular system of conifers with opposite and whorled phyllotaxis, in which the vascular supply to a leaf originates as two traces which subsequently fuse, is considered to be derived from the open sympodial system characteristic of most gymnosperms. This hypothesis of stelar evolution is at variance with that of Jeffreywhich suggests that the eustele of seed plants is derived by the lengthening and overlapping of leaf gaps in a siphonostele followed by further reduction in the resultant vascular bundles. This study suggests strongly that the "leaf gap" of conifers and other extant gymnosperms is not homologous with that of siphonostelic ferns and strengthens the validity of the view that Pteropsida is an unnatural group. It supports the position that gymnosperms have evolved from progymnosperms rather than from ferns, and the inclusion of ferns and gymnosperms in separate subdivisioslS or divisions. IT HAS BEEN clearly established that the primary vascular system of a large number of living conifers is composed essentially of longitudinally oriented sympodia, axial bundles from which leaf traces diverge (Namboodiri and Beck, 1968a,b). While these features are found in all the specimens studied, variations are observed in the number and mode of formation of traces supplying a leaf. Thus in the forms with helical phyllotaxis the trace supplying a leaf is single at the point of origin (Namboodiri and Beck, 1968a), whereas in species with opposite leaves two traces originating from two different bundles are found to fuse to form the single strand supplying a leaf, thus giving rise to a reticulate vascular architecture. Some arguments have been presented in part II of the study (Namboodiri and Beck, 1968b) suggesting that the open system of the genera with helical leaf arrangement is primitive and the closed, reticulate system of conifers with opposite phyllotaxis is derived. This implies that the single-trace condition characteristic of the conifers with helical phylloI Received for publication 17 July 1967. This paper is based on part of a Ph.D. thesis by the first author. This study has been supported in part by National Science Foundation grant GB-3038 to C. B. Beck. 2 Formerly known as G. Kadambari Kumari. Present address: Genetics Program, University of North Carolina, Chapel Hill, North Carolina 27514. taxis is the primitive condition, a view that supports the observations of Pant and 'MIehra (1964). It also suggests the possibility that the leaf gap of conifers is not homologous with that of the siphonostelic ferns. These conclusions would be of greater phvlogenetic significance if they could be applied to the gymnosperms as a whole, especially the extinct forms. A knowledge of the primrary vasculature of fossil gymnosperms is necessary in order to draw any valid conclusions about long-range stelar evolution and homology of structures. It is with these goals in mind that the survey reported herein has been made. Such a survey presents several problems. Shoot apices of woody plants, in which the primary vasculature is clearly discernible, are seldom fossilized. Therefore, the main source of information about the primary vascular system is the stem with secondary growth. But when the maturation of the xylem is endarch it is difficult to discern accurately the details of the primary vascular system. Thus genera characterized by mesarch or exarch development of primary xylem provide most of the data for this study. Among living conifers the number of traces that supply a single leaf may not be the same at nodal levels as at subnodal levels. In the Abietineae, for example, the trace is single at the subnodal levels, but double at the nodal level,