Bot 201 - Pterophyta - Filicales  Lab

This is a large and familiar taxon. We are only studying the Filicidae. However, there are several others that we are not considering. The Filicidae is the largest and most wide-spread taxon and thus best represents this Division. Ferns do not generally dominate ecosystems but they can be important in certain environments. Tree ferns may be among the dominant species is certain areas. This is especially true for the Hawaiian islands. Ferns   have greater ecological significance in Hawaii than elsewhere due to lack 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.

The class that we are studying is characterized by pinnate leaf organization. This is evident in their venation and the overall appearance of their lamina. Another characteristic  trait 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 on the right. Some ferns have very simple leaf organization with little mesophyll differentiation. Others have a complex anatomy which rivals the structural specialization of angiosperm leaves.

The concept of a leaf gap is a little challenging because you need to think in three dimensions. I am including some drawings by Salvador Webi who is my favorite surrealist artist.

A Leaf Gap is present when a Leaf Trace diverges from the stele without its immediate replacement by vascular tissue. I have tried to illustrate this below. The stele is a Siphonostele. Note that the Stele  (Dark Green) forms a complete hollow cylinder below the node. The cylinder is broken into two crescent-shaped columns near the Node because some of the vascular tissue appears to have diverged from the stele. The divergent vascular tissue is the Leaf Trace. The leaf trace enters the base of the Petiole and forms the midrib of the leaf. The gap in the stele contains parenchyma cells rather than vascular tissues. This area is the Leaf Gap.

The  two drawings below attempt to show the anatomy of leaf gaps from a 3-D Perspective.

The presence of many overlapping leaf gaps produces a highly dissected stele, like a Dictyostele  or a  Eustele.

The stems are usually subterranean. These can be a Caudex (erect) or a Rhizome (horizontal). Some species have Stolons (above ground & horizontal) or Erect Aerial Stems. Tree ferns are an example of the latter.

Branching is sporadic and may be dichotomous or lateral. There is little internodal elongation in most species, however there are viney types which are exceptions. Careful examination of  tree fern stem will show that it is composed of leaf bases which are contiguous because there is No Internodal Elongation. This is an obvious limitation in an environment that contains angiosperms. The latter generally have lots of internodal elongation, and can quickly form a dominant canopy over slow-growing plants. The leaves are tightly packed close the the shoot apical meristem which is an Apical Cell and its most recent derivatives.

The Filicales are Homosporous. The spores are produced by sporangia located on the abaxial (lower) surface of the leaf. The sporangia have a unique developmental ontogeny (developmental sequence). They are Leptosporangiate rather than Eusporangiate. Leptosporangia develop from a single epidermal cell rather than a group of cells. The single cell produces an apical cell. After a period of apical growth, Sporogenous tissue differentiates and is surrounded by two - three layers of nonsporogenous cells. The outer layer has an Annulus which contracts as the sporangium matures such that the wall ruptures at a weak spot (lip), thus releasing the spores. The annulus is sensitive to humidity and flails about like a catapult. This helps to disperse the spores.  Sporangia may cover the entire abaxial surface or large areas thereof. Sporangia  may be limited to the leaf margin or veins, or may occur in small clusters known as Sori (heap or mound). The sporangia may be protected by a curled leaf margin or by an Indusium (underwear). The indusium is a structure produced by the abaxial epidermis in association with sporangia. It may be a simple flap of tissue or can be complex like an umbrella. Indusia shield the developing sporangia. They are useful for identification.

The Gametophytes are usually photosynthetic and require light. They are typically bisexual but the gametangia are separated in time, and by position. The Antheridia form first and are located near the posterior of the gametophyte. The Archegonia are produced later an are close to the apex of the gametophyte.

This is not the first case in which the Antheridia matured first on Gametophytes. What if any significance does this suggest regarding sexual reproduction?

Embryos have a unique development which is called Prone. The first division of the zygote is longitudinal rather than transverse. The embryonic  shoot-root axis develops from lateral cells in the young sporophyte rather than  from an Epibasal or Hypobasal cell. There is no dormancy and the sporophytes grow directly from the gametophyte.

Leptosporangiate development and prone embryo development only occur in these plants. Consequently, the Filicidae comprise a divergent evolutionary line which does not lead to the Angiosperms (Flowering Plants).

These are beautiful organisms and have many horticultural uses.

We will analyze Laua'e (Phymatosorus scolopendrium)  & a Sword Fern (Nephrolepis sp.) Kupukupu in detail.

You will also need to look at other specimens for individual features.

Examine the Stolon or Rhizome or intact plants and note the origin of leaves, branches and roots.

Locate the position of the shoot apical meristem.

Does internodal elongation occur?

How would you describe the branching?

Is there evidence of Circinate Vernation?  (Of course there is !!!!)

Are Scales present?

Examine the Leaves for the presence of Sporangia.

What term describes their clustered organization?

Can you find an Indusium?

Remove Sporangia with a dissecting needle and mount them in water on a microscope slide. Locate the Annulus and the Spores.

Mount some on a dry slide,  place this under a dissecting scope or low power of your compound microscopes and breathe gently on them. What do you observe?

Study cross sections of the leaves, stems and roots prepared by us.

Note the key anatomical features.

Epidermal adaptations
Specialized Ground Tissues
Organization of the Stele

Examine other live plants for specified adaptations.

Examine these to gain insight regarding the organization of tissues in leaves,  stems and roots.

Study Leaves of Cyrtomium with Sori.

Note the Anatomy of the Leaf and the Sori.

Is an Indusium present?

Can you observe Palisade and Spongy Mesophyll?

Why is this a Megaphyll?

Observe DEMO of Cyrtomium Sporangia

Examine Cross sections of other Fern Leaves &  note the

Levels of complexity observed in the Mesophyll

Variety of Indusia

Examine slides of Polypodium Stem or Rhizome

Note the organization of the Vascular Bundles &

Identify the Type of Stele

Examine other Stems & Rhizomes to see the various types of vascular organization.

How does vascular complexity in Ferns differ from Psilotum & Lycopodium.

How does the complexity of fern leaves compare with leaves from Psilotum and Lycopodium?

Do you think these two are interrelated?

You should be able to Recognize Steles in a matching format.

Examine the DEMOs of

Pteris Apical Cell &

Pteris Shoot Apices (These are cut at various angles and are a little challenging)

Examine slides of Pteris Roots

What type of Stele occurs?

Where do branch roots originate?

Examine DEMO of Pteris Xylem (Longitudinal Section)

Note the Tracheids and the way in which they connect.

Examine Gametophytes, locate Gametangia

Observe Living Gametophytes

Examine commercial slides with Antheridia

Examine DEMOs of Archegonia

Examine DEMOs which show Sporophytes attached to Gametophytes. Connect the origin of the sporophyte with the gametophyte.

Which is the Dominant generation?