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A Brief Introduction to Ferns

Introduction

Ferns have been with us for more than 300 million years and in that time the diversification of their form has been phenomenal. Ferns grow in many different habitats around the world. The ferns were at their height during the Carboniferous Period (the age of ferns) as they were the dominant part of the vegetation at that time. During this era some fern like groups actually evolved seeds (the seed ferns) making up perhaps half of the fern like foliage in Carboniferous forests and much later giving rise to the flowering plants. Most of the ferns of the Carboniferous became extinct but some later evolved into our modern ferns. There are thousands of species in the world today.

Structure of Ferns

Ferns and fern-allies are more complicated in structure than most people would suspect. Their structures, though similar in some ways to those of flowering plants are different enough to warrant a distinctive terminology.

Fronds

The frond is the part of the fern that we see as we wander through the woods it is the "leaf" of a fern. It is divided into two main parts, the stipe (leaf stalk or petiole) and the blade (the leafy expanded portion of the frond).

The blade may be undivided to finely cut, each degree of division having a specific term. "Pinnate" blades are divided into leaflets (pinnae), with each leaflet narrowly attached to the central stem (called the rachis in this leafy part of the frond). Blades more divided are designated as bipinnate or even tripinnate with some divided four or five times. The ultimate division are called pinnules. Another type of division is one where the green leafy tissue isn't completely separated from the rachis but rather it spreads along the rachis, instead this degree of division is called "pinnatifid".

Fronds vary greatly in size, from tree ferns with 12 foot fronds to the mosquito ferns with fronds only 1/16 of an inch long.

Rhizomes

Rhizomes would be comparable to "stems" in the flowering plants. Fronds arise from the rhizome. In many cases the rhizome can be inconspicuous or even entirely underground. Rhizomes of tree ferns on the other hand may be 60cm in diameter and up to 12 m tall. In some epiphytic ferns (ferns that grow in trees) and in terrestrial creeping ferns the rhizome roams widely and is quite visible. The fronds that arise from this "stem" arise from the upper side or occur in one or more rows laterally on each side.

The rhizome contains the conducting tissues (xylem and phloem) and the strengthening tissues (sclerenchyma fibers). The conducting tissue, known as the vascular bundle, carries the water, minerals, and nutrients throughout the plant.

Roots

Roots are formed from the rhizomes or sometimes from the Stipe. The roots usually do not divide once they grow from the rhizome. Tree fern roots grow down from the crown and help thicken and strengthen the trunk. The roots anchor the plant to the ground and absorb water and minerals.

Sporangiia

The Sporangia are the reproductive structures of the ferns and fern allies. They are miniature sacks or capsules that produce the dustlike spores that are the "seeds" by which ferns are propagated. Several sporangia grouped together are called a Sorus. The arrangement of sporangia varies greatly in ferns. Most ferns that we would see as we walk through the forest would have their sporangia on the underside of the frond, arranged in an organized pattern usually associated with veins in the pinnule (leaf). Many times (but not always) the ferns provide a protective covering for the Sorus called an Indusium.

Spores

The "seeds" of the ferns and fern allies are called Spores. Normally they are formed in groups of four. Spores contain oil droplets and sometimes chlorophyll in addition to their nucleus. Ferns drop millions, often times billions of spores during their lifetime but very few ever land in a spot suitable for growth.

Life Cycle of Ferns

The life cycle of the ferns may seem complicated but it has worked quite successfully for millions of years. Though spores come from fronds of ferns, the fronds do not come directly from the spores. Spores from the parent fall to the ground and with an enormous amount of luck (millions perish for every success) they will find suitable moisture and light. The tiny single-celled organism starts to grow by cell division. Soon orderly arrangements of cells form little green heart shaped plants or Prothallia (gametophytes). These plants go unnoticed by most people as they are only 1/2 inch or less across and lie flat on the ground. This is an independent plant with its own simple "root" system (rhizoids) to provide it with nutrients and water.

The Prothallium then grows Antheridia or male organs and Archegonia or female organs on its underside. The Antheridium produces spermatazoids (or antherozoids) which will swim via a droplet of water to the egg produced by the Archegonium. The fertilized egg then begins to grow the Sporophyte, the plant that we know as a fern.

Although this is the primary form of propagation there are several other ways that ferns procreate. Apogamy, the production of a sporophyte without fertilization, usually occurs when a gametophyte is grown under dry conditions. Some ferns adapted to life in desert regions use this form of reproduction more frequently than others. The other major form of propagation is vegetatively via their rhizomes, in an infertile hybrid (there are quite a few among the ferns) this would be their only form of propagation..

Life Cycle of the Fern Allies (briefly)

Fern allies procreate in a similar manner to the ferns. The Lycopodiums have a very similar life cycle, the main difference is that their prothallia normally grow underground gaining nutrients with the help of mycorrhizal fungus. Equisetum (the horsetails) have two different spores which grow into separate male and female plants. The spermatazoids must swim from the Atheridium of one Prothallium to the Archegonium of another Prothallium. Selaginella and Isoetes, two other fern allies also have differentiated types of spores. There is the female Megaspore, that is much larger then the male Microspore. The Megaspores are able to render more assistance in the growth of the young sporophyte when it arises.

History of Ferns and Fern Allies

Ferns and their allies were the dominant form of vascular plant until the Mesozoic Era (the age of the Dinosaurs) when seed bearing plants came into prominence. There is fossil evidence of ferns in the Devonian Era (345-395 million years ago) and they evolved from the first vascular plants that had evolved in the Silurian Era (395-435 million years ago).

To make the transition from water to land in the Silurian, plants developed a way to obtain the water and nutrients that they needed for growth. They also needed to develop and epidermis that could retard the loss of water to the air and stomata in the epidermis to "breath" with. By the Devonian several different groups of plants had developed all of the necessary characteristics. Five classes of fern allies had arisen by this time the Psilotopsida , Trimerophytopsida, Zosterophyllopsida, Lycopdiopsida, and Equisetopsida. From these early plants the ferns and fern allies as we know them today arose. The figure below shows how the evolutionary lines developed, with some groups lacking any modern descendants omitted.

The living fern-allies can be divided into four classes:

Psilotopsida ..have sporangia borne singly to the tips of main stems. The only living member, perhaps derived from this group is Psilotum (whisk ferns) it is probably the most primitive vascular plant still in existence. It may be directly related to the first vascular plants to venture onto land.

Lycopodiopsida...members of this class have a single sporangium borne in the axil of a scale-like or leaf-like sporophyll. This class is represented by the Selaginellia (Spikemosses), Lycopodium (clubmosses), and the Isoetes (Quillworts).

Equisetopsida...is represented today by only one genus, Equisetum (Horsetails).

Polypodiopsida (Pteropsida)...The true ferns are by far the most numerous of all of the fern-allies. There are nine sub-classes, about 250-300 genera and over 12,000 different species alive today.


Fern Clade


This is the first cladistic analysis of extant ferns based on Morphological characteristics and rbcl Sequences. The group of plants shown represents all of the major fern groups and one seed plant Cycas. The work in its entirety is published in the "Fern Journal 85(4) (1995) and was compiled by Kathleen Pryer, Alan Smith, Judith Skog.

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