Premiers arbres. Le modèle Archaeopteris

Earliest trees – the Archaeopteris model

Laboratoire de Paléobotanique, Institut des Sciences de l’Évolution (UMR 5554), Université Montpellier 2, place Bataillon, F-34095 Montpellier Cedex 5

Résumé

Les premiers organismes végétaux autoportants et dépassant 2 m de haut sont apparus il y a un peu plus de 370 millions d’années, à la fin du Dévonien moyen. Trois groupes inventent l’arborescence de façon convergente à cette époque : les Lycopsida, les Cladoxylopsida et les progymnospermes. Tous partagent une reproduction à partir de spores et un mode de ramification terminale, impliquant la division égale ou inégale des apex. Archaeopteris, chez les progymnospermes, regroupe les arbres les plus hauts (40 m de hauteur maximale estimée) et les plus répandus à la surface du globe à la fin du Dévonien. Il se démarque par un certain nombre d’innovations telles que l’hétérosporie, la possession de feuilles et celle de bois assurant la double fonction de support et de conduction. Des analyses récentes indiquent qu’Archaeopteris avait, de plus, inventé un syndrome de ramification latérale comparable à la ramification axillaire des plantes à graines dont il est le plus proche parent connu actuellement. Certaines particularités anatomiques du système vasculaire suggère qu’il pouvait se bouturer ou se marcotter. Cet ensemble de caractères « modernes » pourraient expliquer le succès d’Archaeopteris jusqu’à la limite Dévonien/Carbonifère, son extinction coïncidant avec la radiation des premières plantes à graines.

Abstract

The earliest self-supporting organisms exceeding 2 m in height evolved about 370 million years ago, approximately 100 million years after the rise of the first land plants. Evidence for the tree habit is usually indirect and assessed from the diameter of the available stem fragments. Four systematic groups of Devonian plants evolved the tree habit independantly: the Lycopsida, Cladoxylopsida, and progymnosperms in the Middle Devonian, the Equisetopsida in the Late Devonian. All share a free-sporing life cycle which limits their habitats to wet areas. Their branching pattern involves the strict division of their apices, whether equally or unequally. The progymnosperm genus Archaeopteris was widespread worldwide and evolved the highest trees of the Devonian (maximum height estimated at 40 m). Besides it ecological significance as the dominant component of the earliest forests, Archaeopteris currently represents the closest known relative to the seed plants with which it shares two derived characters, the heterosporous life cycle, and the possession of leaves. Another distinctive feature of Archaeopteris trees is represented by the double function of their wood for both support and conduction. New analyses involving vascular trace analysis in anatomically preserved specimens have demonstrated that Archaeopteris is not the simple tree reconstructed by Beck (1962). In this fate model, Archaeopteris consisted of an erect trunk bearing short-lived, flattened, leaf-like branch systems forming a terminal crown. New evidence indicates that laterally to these appendages of apical origin, a new type of branches, of adventitious origin, evolved which development compares to that of the axillary branches of the seed plants. These branches which were large and long-lived represent major architectural components of the tree. Evidence for vascular structures comparable to those produced on stem cuttings in modern plants suggest that Archaeopteris may have evolved vegetative strategies for propagation. The set of « modern » characters of Archaeopteris may explain its success until the Devonian /Carboniferous boundary when its extinction is correlated to the radiation of the earliest seed plants.