Résumé
The origin of the ovule during the Devonian period represents one of the most significant events in the history of life on Earth that allowed seed plants to colonise new habitats and to successfully reproduce without available free water. Their evolution however remains tantalising, as no convincing transitional character states with free-sporing ancestors have been found. The first evidences of "extreme" heterospory are found in the Givetian with the occurrence of seed-megaspores such as Spermasporites allenii or Granditetraspora zharkovae. Another type of dispersed organ occurs in coeval deposits: the controversial proto-ovule Runcaria heinzelinii. This taxon is composed of a rounded apically modified megasporangium surrounded by two whorls of tissue. The earliest "true" ovules are found in the Upper-Devonian. They were cupulate, presented a dissected integument and a hydraperman type nucellus. Based on variation of these organs, up to 17 species have been described in Famennian deposits where they already show a wide morpho-anatomical disparity. Because they are most often found dispersed, the available information on the corresponding vegetative systems is very scarce. Based on material from Belgium and from the literature, we here document the vegetative systems of almost all types of Devonian seed plants for which ovules have been described. They show a very narrow diversity when compared to fertile parts, as only one morphology was found. It consists in a "zig-zag"-shaped main axis bearing helically arranged dichotomous branching systems. These new results will be compared with the data available for the Lower Carboniferous. Multivariate analyses indicate that the morpho-anatomical disparity of ovules during that time was comparable to that of the Devonian. In contrast, we observe a much higher disparity for the vegetative parts, with the apparition of various growth habits and new characters of the primary and secondary vascular system. This apparent decoupling between the diversification of seed plants reproductive and vegetative systems will be discussed