There is a great deal of terminology that describes frond morphology. The main axis of the leaf is called the rachis , or pinna rachis . The ultimate, or terminal leafy segments are called pinnules . They unfurl upon development. Fronds are produced by stems that may have protosteles, siphonosteles, dictyosteles , or more complex vascular systems.
Many forms are initially protostelic, becoming siphonostelic upon the initiation of fronds.
In addition to possessing a pith, siphonosteles produce interruptions, called leaf gaps , in the stele in association with leaf trace departure. Ferns are typically characterized by mesarch xylem maturation, but this is also quite variable. Ferns bear either eusporangia or leptosporangia on their fronds either abaxially, marginally or on a separate fertile spike. In this laboratory we will concentrate on the groups that were present during the Paleozoic. This includes the first four groups and the Paleozoic Filicales.