Pteridophyta

Pteridophyta -- from the Greek pteron (feather) and phyton (plant), reflecting the feather-like, pinnately divided leaves of many of its members -- were, evolutionarily, the first plants to become truly vascular and truly terrestrial.

Why they matter evolutionarily. Before pteridophytes, no plant group possessed a true internal conducting system together with a true root, stem and leaves. Pteridophytes were the first successful land plants to combine all of these features, even though their vascular tissue is still primitive compared with that of seed plants: the xylem consists only of tracheids, and the phloem consists only of sieve cells, with no accompanying companion cells or vessels. Because pteridophytes lack cambium, they show no secondary growth -- their stems do not thicken with age the way a woody gymnosperm or dicot stem does.

Habitat. Most pteridophytes -- ferns and their relatives -- favour moist, shady habitats, but the group also includes aquatic members (such as Azolla and Marsilea), xerophytic members adapted to dry conditions (such as Equisetum), and epiphytic members that grow on other plants (such as Lycopodium).

Leaves. Pteridophyte leaves fall into two broad size classes. Microphylls are small, simple leaves with a single unbranched vein, as seen in Selaginella. Macrophylls are large, often pinnately compound leaves with a branched vascular supply, as seen in ferns such as Nephrolepis or Dryopteris -- these compound fern leaves are usually called fronds.

The dominant, independent sporophyte. Pteridophytes show a clearly heteromorphic (differently-formed) alternation of generations. The sporophyte is diploid, autotrophic, structurally complex, and fully independent -- it is what we recognise as 'the fern plant' (or, more generally, 'the pteridophyte plant'), with a rhizome or aerial stem bearing true adventitious roots (its short-lived primary root is soon replaced) and the leaves described above. On the underside of fertile fronds, clusters of spore-producing sporangia, called sori, are visible as small dark or brownish dots. …