Secondary Phloem in Early Carboniferous Seed Plants: Anatomical Diversity and Evolutionary Implications

Premise of research. Secondary phloem produced by a bifacial vascular cambium is the distinctive feature of lignophytes, the group that comprises the seed plants and the progymnosperms. Because secondary phloem is rarely well preserved in the fossil record, our knowledge of the evolution of this tissue remains incomplete. Methodology. We illustrate the secondary phloem anatomy of nine seed plants of Mississippian (Early Carboniferous) age that represent to date the oldest representatives of the group in which this tissue is well preserved. In addition, we review the information available on the secondary phloem anatomy of Devonian progymnosperms and Late Carboniferous–Permian seed plants. Pivotal results. Secondary phloem anatomy was already diversified in the Mississippian seed plants, in terms of cell composition and spatial organization of the different cell types. We propose four models of secondary phloem anatomy and organization in lignophytes of Devonian-Carboniferous age. Conclusions. The diversity of secondary phloem anatomy in Mississippian seed plants parallels what is observed for other parts of their vascular system; it is most likely linked to the Mississippian diversification of seed plant growth architectures and habitats. The accumulated fossil evidence also demonstrates that several characters that have been considered as advanced on the basis of the study of extant gymnosperms (e.g., the presence of fibers in the functional phloem and the arrangement in alternate tangential layers of a same cell type) were actually already present in some early representatives of the lignophytes.