ABSTRACT
In the Pteridophytes and Gymnosperms, the conducting function of the xylem is not carried out by specialized longitudinal cell files, as it is with the vessels of Angiosperms. The tracheids that constitute most of the axial cells of the xylem participate collectively in this function. The present-day Pteridophytes are grouped in orders of widely varying size: only 30 species in the Equisetales, 200 in the Lycopodiales, 800 in the Selaginellales, and around 10,000 in the Filicales. The primary xylem and the phloem differentiate from the procambium. When they are formed, they constitute one or several steles, isolated from the cortex by the two concentric cell layers of the pericycle and the suberized endodermis. The cast of the metaxylem is made of fragile and fine filaments, with circular pits of large diameter, set in regular files.
