About 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 kmē) form only a small part of Zimbabwe (390,759 kmē), they contain about 83% of all Pteridophyta taxa; about 67% of these occur in the Vumba (+/- 200 kmē ).
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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.
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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 .
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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. In a simplified system, the pteridophyta are further subdivided into four classes. The true ferns or Filicopsida have large leaves known as megaphylls. The fern allies (divided into Psilotopsida, Lycopsida and Equisetopsida) on the other hand have minute scale-like structures called microphylls or grass-like, linear leaves, as in the genus Isoetes.
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The Vumba has about 150 species of ferns, for more information about these species see list.
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.
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.
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 (Cyathea dregei, C. thomsonii, C. manniana) are present in the Vumba. 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.
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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. A pinnae is the first division of the front, this can be further divided into pinnules.
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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. |
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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 Anemia, Osmunda and Schizaea are well known examples of dimorphic ferns.
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A/ Vegetative reproduction
In some ferns the rachis produces a vegetative bud
or gemmae, these ferns are called
proliferous. When this bud falls
on the ground a new plant
develops, as in Tectaria gemmifera. Sometimes however the new plant will
already start to grow while still being attached to the rachis. Through the
weight of this small plant, the frond will bend towards the ground. When it
makes contact with the soil, the small new fern can start to root.
Thelypteris madagascariensis, Asplenium sandersonii are good examples of
this.
B/ Sexual reproduction
The spore bearing leave or sporophyte carries the sporangia that will burst open once the spores are matured. These mature spores do not straight away grow into a new fern. If the spores germinate they form a gametophyte, a hardly noticeable tiny heart shaped plant of about 1/2 inch wide. This gametophyte carries the male (antheridia) and female (archegonia) organs. Sperm is produced in the antheridia, an egg develops in the archegonia. The sperm has long flagella which makes it possible for it to move to the egg. This however can only happen if a film of water is present on the surface of the gametophyte; a good moist environment is thus essential. When the sperm reaches the egg, fertilization takes place and a new fern (sporophyte) develops.
3. ECOLOGY
3.1 Endemism
The spore of ferns are distributed by air and can travel over long distances. For this reason many species are widespread and occupy similar habitats in different countries, thus bringing the number of endemics down. It is also important to notice that being widespread does not necessarily mean being common! A certain species may grow in several places all over the world, but it's numbers in one particularly place may be very low.
3.2 Habitats
Pteridophyta grow in a wide range of habitats, from semi-deserts to wet forests. The biggest variety however can be found in places that have plenty moisture and shade such as the evergreen forests in the Vumba. Selaginella kraussiana turns the forest floor into a huge carpet, while others such as Tectaria gemmifera, Polystachia zambesiaca, Asplenium anisophylum, Thelypteris dentatum.. give the under storey a thousand shades of green . Along stream banks and seepage zones one can find big clumps of Marattia fraxinea or Diplazium zanzibaricum. Cyathea manniana and C. thompsonii also favor this environment. Boulders and trees are covered in epiphytic species such as Asplenium sandersonii, A. mannii, Hymenophyllum spp., Trichomanes spp., Loxogramma lanceolata while in the higher forks of a tree one can see Huperzia dacrydioides or Vittaria isoetifolia.
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In the woodlands the variety in ferns is less and totally different species are found. Pellaea calomelanos, P. dura, P. pectiniformis are found in rock crevices where they receive more shelter. Among grasses on the vegetation islands Cheilanthes virides var. glauca and C. leachii are well established, while Selaginella dregei might even extend its growth area to the bare rocks. This small moss like fern is well adapted to overcome dry spells. The plant dehydrates in the dry period and revives completely with the first rains.
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Scrublands with their poor soils and high exposure are normally poor in fern species. Elaphoglossum acrostichoides and Mohria lepigera can find moisture & shelter in between or below the rocks in ericoid scrubland. Pteridium aquilinum is common in secondary bracken. In grasland some ferns are found at the bases of boulders, in streambanks or at the edges of vleis. Here they will receive some shelter & moisture .
3.3 Altitude
Pteridophyta grow in a wide range altitudes, from sea-level up to 3000m. In Southern Africa ferns seem to have a large tolerance to altitude changes. The main reason behind this is a fairly low temperature lapse rate with increasing altitude. Indigenous forest, giving shade & shelter from wind also still occur at high elevations. At these heights we also have high rainfall.
In most instances the climatic & morphological factors will outweigh the influence of the nature of the soil. Given sufficient moisture and good morphological conditions (such as shelter, springs,...) some species of Pteridophyta may occur in almost any type of soil.
Pteridophyta occur in all rainfall ranges. Most species (89,23%) need a rainfall between 400 and 1200mm per annum. Species occurring in the mist belt and high mountain ranges (18,08%) live in areas with an annual rainfall of more then 1200mm. Furthermore adaptations to highly seasonal rainfall are plentiful. Some ferns show a seasonal pattern where the lamina shrinks in dry conditions and unfolds again when it is moist. Water loss can be reduced through scales or hairs. Both rhizome and spore can tolerate long periods of drought. Some species produce runners in dry years.
Although none of the ferns likes extreme temperatures, temperature conditions seem to be less important than the availability of moisture. There seems to be a wide temperature tolerance but 60 % of the Southern African ferns prefer a mean annual temperature of 15 - 17,5 degrees Celsius
Shelter The vast majority of ferns like shade. Dappled shade is generally ideal, although some will tolerate deep shade and others need more light for their growth. There is a preference for south facing slopes, north and west facing slopes receive more sun (higher light intensity) especially during the afternoon. Ferns like to be sheltered from extreme hot or cold winds. The most important function of the wind lies in the distribution of the spores. The spores of Pteritophytes are minute and can be transported over long distances across land and sea.