Phloem anatomy of the Carboniferous pteridosperm Medullosa and evolutionary trends in gymnosperm phloem

This is the published version.Secondary phloem anatomy is detailed for four species of Medullosa from various coal ball localities in North America, ranging from Lower-Middle to Upper Pennsylvanian. The zone of secondary phloem can be up to 3.7 mm wide and consists of alternating, tangential bands of sieve cells, phloem fibers, and axial parenchyma separated by parenchymatous rays. Fibers are up to 4.2 mm long, thick walled, and in bands up to five cells wide radially and two to three cells tangentially. Phloem parenchyma cells are generally much smaller in diameter than either the sieve elements or the fibers and occur in bands as wide as 12 cell layers. Sieve cells are up to 4.2 mm long, with circular-oval sieve areas on the radial walls. The sieve areas are relatively uncrowded and contain a number of dark spots interpreted as callose deposits. Some deposits completely cover individual sieve pores, while others appear to line the sieve pores. Phloem anatomy in Medullosa is compared with that in other genera of Paleozoic seed fems and extant gymnosperms