Pterophyta (Ferns)    

The Pterophyta is a familiar plant taxon, especially in Hawaii. They have a prominent position in our terrestrial ecosystems both as ground cover species and canopy components in certain environments. They are an order of magnitude larger than the plants we have reviewed so far. However, even Tree Ferns are relatively small plants compared to woody Angiosperms and Gymnosperms.

We will concentrate on the Filicales which contains most extant species. They are most abundant in the tropics but some species can be found in the arctic alpine zone.

Ferns typically have subterranean stems called Rhizomes. Some species have Stolons. Most species are terrestrial but some are epiphytic and a few are aquatic.Some species produce significant aerial stems and resemble small trees. Hence, their designation as "Tree Ferns".

Roots are adventitous and arise from the stem. The stems of tree ferns have a thick coat formed by adventitous roots which are initiated near the apex of the stem and grow down the outside of the stem. This has some obvious disadvantages. The roots must travel a long distance through the atmosphere before reaching the soil. Consequently, water loss can readily occur over this distance. They are also subjected to any biotic or abiotic events that occur over this expanse.

The most significant vegetative adaptation seen in these plants is the Megaphyll (Big Leaf). However, size is not the distinguishing trait for Megaphylls. A Megaphyll has more than one vein in its blade and the Leaf Trace is associated with a Leaf Gap in the Stem.

A Leaf Gap occurs when part of the vascular tissue in the stem is diverted towards the leaf. This is called a Leaf Trace. The integrity of the stele is restored above the Leaf Gap. When separate vascular bundles constitute the Stele, they may fork to produce Stem Traces which unite above the Leaf Gap to reform the Vascular Bundle. Parenchyma cells replace the Leaf Trace above the level of its divergence. Consequently, if you make a series of cross sections of a stem, it will appear that a gap occurs in the Stele near the level where the Leaf Trace diverged. The Diagrams below attempt to depict ways in which this could occur.

I will try to show you how leaf gaps occur in the table below.

The predominate organizational pattern for fern leaves is Pinnate.This means that a structure has one central axis which produces lateral structures that are opposite each other along the central axis. A bird feather is a good example of a structure that has pinnate organization. Pinnate leaves can be Simple, Compound and Bicompound. The central axis of compound leaves is called the Rachis at the level of Leaflet production. The leafless portion that joins the stem is the Petiole.

The presence of Megaphylls and their associated Leaf Gaps in the Stele result in the occurrence of complex stelar organization in some ferns. Leafy stems have dissected steles while Rhizomes (produce few or no leaves) have relatively undissected steles.

A Siphonostele has a Hollow Cylinder of Vascular Tissue which encloses a central Pith. The Cortex is the Ground Tissue between the Vascular Cylinder and the Epidermis.

Leaf Venation (vein pattern) can be Dichotomous or Reticulate.

Fern Leaves have a unique manner of development. The immature leaves are tightly coiled into a structure called a "fiddlehead". During maturation, the leaf uncoils from the bottom to its top (acropetal). This is called Circinate Vernation
(Circ = Circle; Verna = Spring).

Leaf Anatomy can be simple or can be as complex as that seen with flowering plants. The take home message is that fern leaves are highly sophisticated in their design and represent a major advance over the Microphyll!

Fern Roots   have the same basic anatomy as other roots. They do exhibit Lateral Branching which is a major advance over Dichotomous Branching.