Fern fronds are the leaves of ferns. They are a major organ of a fern and range significantly between species. Some species of tree ferns have fronds that grow as large as 5 m long while other species are limited to growing frond only 1 cm in length. They have vascular tissue with leaf blades and a stalk, which runs from the base of the frond to the tip. Their primary roles are in photosynthesis and reproduction as well as providing many other uses for ferns.
Anatomy of fern fronds
The anatomy of a fern frond is relatively simple. They have a stalk which connects the base of the frond to the rest of the tree and runs along the entire length of the frond to the tip. The stalk can be separated into two parts: the stipe, which is the section of the stalk before any leafy part of the frond, and the rachis, the section with leafy tissue. One of the leafy structures is known as a pinna or pinnae (plural) and the collection of all the pinnae, so the entire leafy portion of the frond, is known as the lamina. Each pinna is often divided into a number of smaller leaflet structures which are called pinnules.
Many fronds will also bear the reproductive structures of the fern on the underside (abaxial surface) of the frond. The reproductive structures are known as spores and are kept within sporangia which are housed within a sorus (plural sori). The sori, which can often be seen as lots of little spots on the underside of fronds, are often protected by thin layer of tissue known as the indusium.
Division of fern fronds
The lamina of a frond is often divided into many pinnae along the length of the rachis. The pinnae can also be divided into many pinnules. If lamina is divided only into pinnae but the pinnae are not divided into pinnules then a frond is known as being pinnately divided. If a frond does have the second division into pinnules, it is bipinnately divided. Alternatively, some fronds are divided palmately, with a number of pinnae growing from the same point on the rachis. Whether a fern has pinnate, bipinnate or palmate fronds is useful in the classification of many ferns.
Many ferns have a high degree of difference between their fronds. For example the ferns of the genus Blechnum have significantly different reproductive and non-reproductive (vegetative) fronds. The reproductive fronds with sori often look brown and dead compared to the lush green vegetative fronds. This high level of variation between the fronds is known as dimorphism and is common to a number of genera of ferns.
Last edited: 23 May 2014