Secondary phloem anatomy in Callistophyton boyssetii (Renault) Rothwell and histological changes in the outer phloem

Abstract

Specimens with well-preserved phloem tissues of the pteridosperm Callistophyton boyssetii from coal balls in the Herrin no. 6 Coal of Illinois and the Bevier Coal of Kansas are described. The secondary phloem consists of alternating tangential bands of sieve cells and parenchyma, separated by vascular rays. Secretory cells are scattered throughout the secondary phloem, and bundles of presumed primary phloem are evenly distributed in the cortex. Histological differences between the inner (presumably functional) and the outer (presumably nonfunctional) phloem are described: changes in cell shape, thickening of cell walls, and deposition of dark-colored cell contents. In contrast to living plants, both parenchyma cells and conducting elements appear to undergo these changes. Phloem anatomy and developmental changes in the outer phloem in Callistophyton are compared with those in other Carboniferous seed ferns and extant gymnosperms.