Fern structure

Ferns come in a variety of shapes and sizes. The fern body consists of 3 major parts – the rhizome, the fronds and the sporangia. This interactive explores the diversity of form in New Zealand ferns. 

Transcript

Frond
The leaf of a fern. These are called fronds to distinguish them from the leaves of flowering plants. Leaves in flowering plants are purely concerned with photosynthesis whereas fern fronds have both a photosynthetic function and a reproductive function. Each fern plant may have from one to many fronds. They may be as small as 1 cm (filmy ferns) or as large as 5 m (mamaku/black tree fern).
Image acknowledgement: Museum of New Zealand Te Papa Tongarewa

Lamina
The lamina is the flat, green leafy blade of the frond. The lamina includes the rachis (midrib or stalk) and the pinnae (leaflets).
Image acknowledgement: Museum of New Zealand Te Papa Tongarewa

Stipe
The section of stalk between the rhizome and the lamina. The stipe varies in texture, firmness, thickness, length and colour and sometimes has a groove on the upper surface.
Image acknowledgement: University of Waikato

Pinnae
The basic divisions of the frond. Pinnae or leaflets may be arranged alternately or in opposite pairs along the midrib. If the frond is once divided, then it is 'pinnate'. More commonly fronds are divided into secondary pinnae and are 'bipinnate', or into tertiary pinnae and are 'tripinnate'.
Image acknowledgement: Steve Attwood

Rhizome
The stem of the fern plant. This may creep along or under the ground or even up a tree (a creeping rhizome), it may grow into a short or tall trunk (a vertical rhizome), or it may be a solid mass that gives rise to a tuft of fronds (an erect rhizome). Very often, the rhizome grows underground. The rhizome produces roots and new fronds.
Image acknowledgements: University of Waikato, Museum of New Zealand Te Papa Tongarewa

Rachis
The central midrib or stalk of the lamina. It is a continuation of the stipe. Often the rachis differs from the stipe, both in colour and covering of hairs and scales.
Image acknowledgement: University of Waikato

Koru
A young fern frond produced from the rhizome. These are characteristically tightly coiled for protection and may be further protected with hairs or scales. Also known as fiddleheads, they uncoil slowly as they mature.
Image acknowledgement: Museum of New Zealand Te Papa Tongarewa

Sporangium
The reproductive structures on the underside of the frond. Each sporangium is a capsule that contains spores. They are usually aggregated into clusters called sori. The position and arrangement of the sporangia are very important for the identification of ferns. Fronds that have sporangia on their underside are fertile, and those that don't are sterile.
Image acknowledgement: Museum of New Zealand Te Papa Tongarewa

Spore
A single cell. Spores are produced in capsules called sporangia. Most ferns produce 64 spores in each sporangium. Sporangia are aggregated into clusters called sori. When mature, the spores are released from the sporangia. Once released, the spores germinate readily on contact with damp soil.
Image acknowledgement: Australasian Pollen and Spore Atlas, Creative Commons external link 

Sorus
Each sorus is a cluster of sporangia. The shape and position of the sori are important for identification of ferns. They provide the main features for identifying the different genera. Sori can be round, oval, oblong or considerably elongated. They may occur on the edge of the pinna or away from the edge. With a 10x hand lens, you’ll be able to see that the sori are composed of numerous, small, round bodies that are the sporangia.
Image acknowledgement: Museum of New Zealand Te Papa Tongarewa.

Frond dissection
Fronds can be simple and undivided like the leather leaf fern, or pinnate (once divided) like the thread fern. More commonly, fronds are bipinnate (silver fern) or tripinnate (hen and chickens fern).
Image acknowledgements: University of Waikato, Steve Attwood, Public domain and Museum of New Zealand Te Papa Tongarewa

Sori position
The sori may be located on the edge of the pinna or away from the pinna margin.
Image acknowledgement: Museum of New Zealand Te Papa Tongarewa

Indusium
A flap of tissue that protects the sori in some ferns. This can take a variety of forms. When spores are mature and ready for release, the indusia usually shrivel or bend backwards to expose the sporangia. Occasionally, if the indusia completely cover the sporangia. they may tear irregularly.
Image acknowledgement: Museum of New Zealand Te Papa Tongarewa

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