Résumé
Phloem is the principal food-conducting tissue, involved in the active transport of photosynthates, hormones, and various metabolites through the body of vascular plants. In the case of the lignophytes (i.e., seed plants and their putative progymnosperm ancestors), the presence of a bifacial vascular cambium allows the formation of new phloem layers of secondary origin through the life of the plant. This production of both secondary xylem and secondary phloem by the vascular cambium is considered one of the key-innovations that occurred among vascular plants during the Devonian. Unlike secondary xylem, secondary phloem is rarely well-preserved in the fossil record. This is due either to the decay of the unlignified cells that constitute most of the phloem or to the loss of bark tissues (decortication) before fossilization. As a result, while some detailed anatomical studies of fossil secondary phloem exist, our knowledge of the evolution of this tissue remains incomplete. In this talk, we will review the secondary phloem anatomy of 11 lignophyte taxa of Devonian and Mississippian age, based on both previously published and novel specimens kept in the collections of Montpellier 2 University. In Tetraxylopteris and Triloboxylon, two genera belonging to the oldest group of lignophytes, the aneurophytalean progymnosperms, the secondary phloem is a complex tissue containing rays, different types of thin-walled cells, and fibers arranged in radial files. Study of large roots and a trunk of Callixylon from the Late Devonian of Morocco show that a comparable organization is found in the inner secondary phloem of archaeoperidalean progymnosperms. Among Mississippian lignophytes assigned to the seed plants, the genus Calamopitys has a secondary phloem composed of alternating bands of parenchyma and sieve cells, with no sclerenchyma. Phloem fibers are present in all the other Mississippian genera studied: Bilignea,Eristophyton, Faironia, Lyginopitys, Stanwoodia, and two new taxa from the Early Mississippian of Australia and Algeria respectively. In both archaeopteridalean progymnosperms and seed plants, different types of phloem cells tend to be produced rhythmically in tangential bands. Changes between the inner and outer phloem include an increase in diameter of axial parenchyma cells, proliferation of axial parenchyma cells, and/or enlargement of rays. This study of secondary phloem anatomy supports a close relationship between Aneurophytales, Archaeopteridales, and seed plants. At the same time, the variability between taxa of characters such as the presence/absence and distribution of secretory cells or the distribution of fibers is noteworthy.