Primary vasculature in Chenopodiaceae: A re-interpretation and implications for systematics and evolution

Abstract

The foliar theory of the stem, the application of which has been restricted to the pteridophytes, is now extended to the spermatophytes and the vasculature of the primary stem is viewed as a sympodium of foliar traces. Several new terms have been coined for the description of the primary vasculature; these include terms for vascular reticula and sympodia and two new conventions, the interleaf articulation ratio and interleaf articulation spacing. The patterns of primary vasculature in 20 Chenopodiaceous taxa have been re-described using a synoptic descriptive style. Based on the primary vasculature, within the Chenopodiaceae, two clear-cut groups have been recognized: (1) a Trioid vascular group, with 3-traced leaves and a median composite double trace, or taxa having such an ancestry; and (2) a Monoid vascular group, comprising taxa with 1-traced leaves and a composite double trace, or with this ancestry. The first group comprises the tribes Beteae, Chenopodieae, Atripliceae and probably the Polycnemeae and Corispermeae. The second group comprises the tribes Salsoleae, Suaedeae, Salicornieae and Camphorosmeae. In both the groups, reticulate and open vasculature are found, the latter being the derived state; the open vasculature derived from 3-traced ancestry is found to be different from that derived from 1-traced ancestry. These two groups differ from the subfam. Chenopodioideae (Cyclolobeae) and Salsoloideae (Spirolobeae), the groups identified on the basis of the cycloid/spiroid embryonic types. They match well with the groups identified from the chloroplast DNA analysis and appear to have distinct adaptive strategies; the first with modified/accrescent bracts, and the second with modified/accrescent perianth lobes. The findings from the primary vasculature therefore support the re-circumscription of the two subfamilies Chenopodioideae and Salsoloideae and the transfer of the tribes Camphorosmeae and Salicornieae to Salsoloideae. The processes involved in the diversification of primary vasculature in the Chenopodiaceae have been: changes from spiral to opposite phyllotaxy, reduction in the number of vertical rows of leaves, closed to open vasculature, non-storied to storied reticula, asymmetric to symmetric reticula, change in the loci of vascular articulations from the contributory arms to terminal arm of the composite trace, and wedged-sagittate to entire-acute base of reticula. Development of open vasculature from closed vasculature probably took place several times and followed independent lines. Therefore, many tribes in the family may be paraphyletic. Evolution of distichous sympodia from monostichous sympodia and vice versa, as proposed by earlier authors have been refuted. Strong correspondence of inference emerging from analysis of chloroplast DNA and primary vasculature refutes the probability of a protostelic origin of the eustele. It is argued that an atactostelic condition might have been ancestral, which gave rise to the protostelic condition on the one hand, and on the other to the eustele. The angiosperm leaf is considered as a composite organ formed by the syngenesis of two or more leaves/leaf lobes from different nodes, the details of which remain to be determined. © 2003 The Linnean Society of London.