All the surviving forms, however, have a completely established double system with the specific characters alluded to, and since there is every reason to believe that the conditions of evolution of the primitive Pteridophyte must have been essentially similar to those of the Bryophytes, the various stages in the evolution of the conducting system of the latter (p. 732) are very useful to compare with the arrangements met with in the former.
Or many protoxylems. When the protoxylem strands are situated at the periphery of the stele, abutting on the pericycle, as in all roots, and many of the more primitive Pteridophyte stems, the stele is said to be exarch.
As the primitive stele of a Pteridophyte is traced upwards from the primary rout into the stem, the phloem becomes continuous round the xylem.
In some Pteridophyte stems the apical eel is wedge-shaped, in others prismatic; in the latter case segment~