Résumé
The middle Devonian aneurophytalean progymnosperms are generally accepted candidates as ancestors of the earliest seed plants. Their wood is composed of rather broad tracheids with crowded bordered pits on all walls and homocellular, tall and narrow rays. Suggested evolutionary trends include: 1) a decrease in diameter of tracheids (in both arborescent archaeopteridaleans progymnosperms and pityans seed plants), 2) inversely an increase in diameter of tracheids in the other early seed plants; 3) a decrease in pit seriation in archaeopteridalean, protopityans and some early seed plants; 4) a restriction of pitting to the radial wall of tracheids in early seed plants; 5) an increase in mean width of rays in archaeopterialeans and seed plants. Some characters are rare (ray tracheids, xylem parenchyma) or so variable in the ancestral group (ray height) that their polarity is difficult to establish. To summarize, the early lignophytes, known from the Mid Devonian to the Early Carboniferous exhibit successive, apparently independent, trends in their wood structure representing as many developmental adaptations to different habits and environments.