Sporangia are lateral and reniform to globose and the axis has an exarch protostele . Zosterophylls are thought to be either the ancestors or a sister group to the lycopsids, which branch off separately from all of the other vascular plants. Look at examples of Sawdonia in the lab for a general idea about the morphology of zosterophylls. Note the spiny enations along the surface of some axes. The Trimerophytina have axes that can be equally or unequally dichotomous and bear ellipsoid sporangia terminally on branches.
Axes can be either smooth or spiny, and the internal stelar strurcture is a mesarch protostele.
Trimerophytes are thought to be the main stock from which all other vascular plant groups evolved including the ferns, horsetails and progymnosperms. This last group are the ancestors of the seed plants, which include all of the gymnosperms, and, ultimately, the angiosperms. Note the variation in branching structure, size, and presence or absence of enations. Now look at P . Look also at the microscope slides of Psilophyton axes.