Ferns

In the plant kingdom we find a major division of vascular, spore bearing plants, called the Pteridophyta (ferns & fern allies). 

Although  the Eastern Highlands (+/- 30,000 km²) form only a small part of Zimbabwe (390,759 km²),  they contain about 83% of all Pteridophyta taxa; about 67% of these occur in the Vumba (+/- 200 km² ).

1. CLASSIFICATION

In evolutionary terms the Pteridophyta are put in between the Bryophyta (mosses and liverworts) and the Spermatophyta (seed-bearing plants). The Pteridophyta differ from the Bryophyta by the development of a vascular system , they contain elongated cells in the stalks for the transport of water and nutrient solutions. They differ from seed-bearing plants by the absence of flowers and by the production of spores.



Ferns and Fern Allies do not have flowers. They reproduce through spores, which are produced in sporangia. In true ferns these are situated in groups, called sori. These are usually located on the underside of the fronds. In the fern allies they are born on specialized, small leaves called sporophylls. In some, these sporophylls are crowded into cone-like structures called strobili.


With about 12000 species divided over more then 230 genera, the pteridophytes are a significant group in the plant kingdom. Classification of such a big group is not easy and different systems are in use. In a simplified system, the pteridophyta are further subdivided into five classes. The Polypodiopsida and Marattiopsida have large leaves known as megaphylls. The fern allies (divided into Psilotopsida, Lycopodiopsida and Equisetopsida) on the other hand have minute scale-like structures called microphylls or grass-like, linear leaves, as in the genus Isoetes.


2. STRUCTURE

2. 1  Vegetative structure

A/ Rhizome

The stem  or rhizome is the part of a fern from which the fronds (leaves) and roots grow.

The primary roots (growing directly from the embryo) take care of the initial anchoring and uptake of water & nutrients for the young fern. They are replaced by secondary adventitious roots responsible for anchoring and further uptake of water and nutrients.  The secondary roots grow continuously with the stem.

rhizome of pleopeltis excavata
Rhizome of Pleopeltis excavata



The rhizomes can be very variable: from short & thick to long creeping & wiry, from differently branched to unbranched. It can be hidden underground as is often the case with terrestrial ferns, or it may lay clearly visible on top of the soil or trees as it often does with epiphytic or lithophytic ferns.

The rhizomes carry the growing point of the fern. If the rhizome is creeping, the growing point sits at the end. The fronds arise at different space intervals along the rhizome. Erect rhizomes have their growing point in the centre at the apex, the fronds grow in one tuft.



caudex of cyathea dregei
Caudex of Cyathea dregei



If an underground rhizome emerges above the ground and forms a woody trunk – like stem, we call this a caudex . Treeferns are well known examples of ferns that posses a caudex. To give extra support to the trunk, adventitious roots keep growing from the crown. The Southern African region knows 5 species of tree ferns, 3 are present in the Vumba (Cyathea dregei, C. thomsonii, C. manniana). Apart from tree ferns there are other larger fern species that also develop a caudex with age (Blotiella spp., Marattia fraxinea)



B/ Fronds

These are the leaves of a fern, consisting out of 2 parts: the stipe and the lamina.
The part between the rhizome and the first leaflet is called stipe, it is analogous to the petiole of a leaf.
The lamina is the blade of the fern, it is usually green and responsible for the photosynthesis. The lamina consist of a midrib (rachis) and of pinnae.
pinnae is the first division of the front, this can be further divided into pinnules.

Fronds are very variable. They can be  minute to extremely large. If the frond is undivided it is called simple, once divide pinnate, twice divided bi-pinnate, and so on to quadripinnate.  If the divisions are not complete they are called pinnatifid.

An easy way to recognize a fern is the rolled up state of the young leaves (fiddleheads or croziers), a condition better known as circinate vernation.

Fronds can be sterile or fertile. In monomorphic ferns the sterile & fertile fronds have the same shape, in dimorphic species  the sori are located on a differently shaped fertile frond. The genera AnemiaOsmunda, Blechnum and Schizaea are well known examples of dimorphic ferns.