Aneurophyte progymnosperms are the oldest lignophytes and common shrubs of Middle to lower Upper Devonian. Aneurophyton germanicum, Triloboxylon arnoldii, T. ashlandicum, Tetraxylopteris schmidtii, Proteokalon petryi, and aneurophyte roots were studied by serial XS, TLS, and RLS of secondary xylem, secondary phloem, and vascular cambium (VC). Aneurophyte VC is truly bifacial and perennial. Fusiform initials (FI), tracheids, phloem fibers, and sieve cells are long (1-2 cm) even in the first growth layers. Wood has multiseriate pitted tracheids; mostly uniseriate and tall rays that can be multiseriate in late wood; some ray tracheids. Phloem has radial rows of fibers, rays, patches of axial parenchyma, sieve cells, and tanniniferous-like cells. Expansion growth adjustments include tangential cell widening and radial anticlinal divisions of FI that add new rows of equal, axial cell pairs. S-curved anticlinal divisions seem not to occur. Apical intrusion overlap adds few cell rows. FI can convert to ray initials or disappear. Xylem rays are commonly 2 (1-5) cell rows apart. Procumbent ray cells span up to 4 tracheids. Phloem rays dilate by tangential enlargement (young) and radial division (older). Most periclinal VC divisions (xylem:phloem are 1:1 in young axes) are tangential (make rectangular cells) but some are oblique (make polygonal cells). Forked to U-shaped FI divide to make rows of distorted tracheids and phloem fibers. Distorted FI maintain and sometimes expand such rows that can span more than one growth layer. Wood and phloem adaxial to laterals have short, but otherwise undeformed axial cells. Forked and distorted FI, tracheids, and phloem fibers, therefore, characterize normal secondary tissues. These may serve as additional mechanisms for expansion compensation by the long FI and axial cells of aneurophytes. Similarly distorted cells are less common in younger progymnosperms and basal gymnosperms and absent in modern seed plants.

Key words: Aneurophyton germanicum (Aneurophytales), basal lignophytes, Devonian, progymnosperms, Tetraxylopteris schmidtii (Aneurophytales), vascular cambium