What is a fern?
There are ferns in most New Zealanders’ backyards and local environments. Ferns are green flowerless plants with divided leaves that tend to grow in damp, shady areas. The developing leaves of most ferns uncoil from a koru.
Ferns are ancient plants
Ferns are an ancient group of plants. From the fossil record, scientists consider that land plants emerged from the water around 475 million years ago. By about 400 million years ago, vascular plants had separated from non-vascular plants, and soon after this, ferns separated off. By about 350 million years ago, some of the major families of ferns are seen in the fossil record. This makes ferns older than most land animals – some invertebrate animals were on land by this time – and far older than dinosaurs!
Ferns come in a variety of shapes and sizes, from the very small, like the kidney fern, to the 20 metre tall tree fern. Most ferns share the same basic structure.
The structure of a fern
Ferns have 3 major parts – the rhizome, the fronds and the reproductive structures called sporangia. The characteristics of each of these 3 parts of the fern plant are used for classification and identification.
The rhizome is the stem of the fern plant. It comes in 3 basic forms:
- An erect rhizome, which is a solid mass that gives rise to a tuft of fronds. You can see this type of rhizome on a king fern or a crown fern.
- A laterally growing rhizome that creeps along or under the ground. It may even climb up a tree. Hound’s tongue and thread ferns are examples of a fern with a creeping rhizome.
- A vertical rhizome. This can grow into a short or a tall trunk. The trunk of the ponga (silver fern) is a vertical rhizome.
The fronds are the leaves of the fern. There is usually a stalk (the stipe) with a flat blade (the lamina), often divided into segments. The frond may be simple and undivided or it may be divided into a number of divisions (called pinnae). New fronds are produced from the rhizome. They are tightly coiled into a spiral (called a fiddlehead or koru), and these slowly uncoil as they mature. Fronds have a dual function. They are there for photosynthesis but they are also there for reproduction.
The spores grow inside casings called sporangia. These are found on the underside of fronds. Not every frond has sporangia underneath it. Fronds that have sporangia are called fertile fronds. In the vast majority of ferns, the sporangia are found in clusters (called sori). These are the brown, black or orange patches that you see on the underside of fronds. When the sporangia break open, they release the spores.
Ferns are unique
The leafy fern plants we see in the bush that produce spores are sporophytes. When the spores are released by the sporangia, if they land in a hospitable environment, they can grow into a tiny plant – the gametophyte. This inconspicuous, short-lived plant has 2 sets of reproductive organs – the antheridia (male) and the archegonia (female). In suitably moist conditions, fertilisation takes place either on the same gametophyte or an adjacent one. Fertilisation gives rise to a new sporophyte.
No other land plant has these 2 separate independent living stages. This is a unique characteristic of ferns.
Nature of Science
Clear communication is essential in science. As scientists look more closely at an organism, the vocabulary they use becomes more precise. Hence, the specific labels used when referring to fern structure and reproduction.