Pterophyta  Filicales

This is a large and familiar taxon. We are only studying the Filicales. However, there are several other classes that we are not considering.

The Filicales is the largest and most wide-spread taxon of ferns, and  thus the best representatives this Division. They occupy a wide range of habitats. They are commonly terrestrial or epiphytic in the Tropics. Terrestrial forms predominate in the Temperate zone.

The Filicales contains many large families including the Polypodiaceae, Osmundaceae, & Cyatheaceae plus several others. There are approximately 300 genera & 9,000 species. These are most abundant in the humid tropics but they are also plentiful in temperate regions, including temperate rainforests like those found on the Olympic peninsula (Washington) and New Zealand.  A few species are aquatic or semi-aquatic while a few  species can survive in arduous environments.

Ferns are relatively small plants and they do not generally dominate ecosystems, However,  they can be very important in certain environments. Tree ferns may be among the dominant species is certain habitats. This is  true for the Hawaiian Islands. Ferns have greater ecological significance in Hawaii than elsewhere due to the absence of competitors which would normally displace them as sites mature. In some cases ferns are the only significant ground cover in Hawaiian forests and open slopes. They are consequently important for soil stabilization and erosion control. They should be considered for the initial revegetation of disturbed sites.

The class that we are studying is characterized by pinnate leaf organization. This is evident in their venation (vein pattern) and the overall appearance of their lamina (blade).  However, there are ferns which have entire, lanceolate leaves. Some have Palmate organization.

Another diagnostic leaf characteristic is Circinate Vernation. Immature leaves are tightly coiled and tightly packed at the shoot apex. The leaves begin to uncoil when favorable conditions occur. They resemble the tops of violins and have been called "fiddle heads". They also resemble the shepherd's crook and have been so named. The fiddle heads mature from the base towards the tip (acropetal maturation). Consequently, the lower sections of the petiole and blade mature first. The leaflets of compound leaves also display circinate vernation.

Ferns have Megaphylls (large leaves). However, size is not the chief characteristic of megaphylls.  A Megaphyll is a leaf which contains more than one vein and its leaf trace is associated with a leaf gap in the stele. A leaf gap occurs when the departure of a leaf trace from the stele results in the development of parenchyma rather than vascular tissues in the stele, just above the leaf trace. This gives the stele a dissected appearance like the dictyostele.

Some ferns have very simple leaf organization with little mesophyll differentiation. Others have a complex anatomy which rivals the structural specialization of angiosperm leaves.

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