The following points highlight the nineteen essential features of ferns.
1. Ferns are seedless vascular plants of humid tropics and temperate areas. Some ferns live under sub-arctic conditions as well.
2. They constitute the largest living group of primitive vascular plants with over 10,000 species. Ferns once dominated the earth in carboniferous period (about 300 million years ago).
3. Plant body is a sporophyte. It is differentiated into true stem, leaves and roots.
4. The stem is underground rhizome in most of the ferns. Some primitive ferns have above-ground stem with tree-like habit. They are called tree ferns, e.g., Cyathaea, Celeotium. A tree fern is like a small palm. It may reach a maximum height of 20 m.
5. Roots are adventitious.
6. Leaves are large and are called fronds. They are often graceful. Leaves may be simple or pinnately compound. In a pinnately compound leaf, the lamina is divided laterally into leaflets.
It may be uni-pinnate (once pinnate, leaflets borne directly on rachis which is continuation of petiole), bi-pinnate (twice pinnate, leaflets borne on primary branches of rachis) or tri-pinnate (thrice pinnate, leaflets borne on tertiary axes or secondary branches of rachis).
7. Leaves possess apical meristem or growing point which, however, stops activity after reaching maturity.
8. Young leaves show circinate ptyxis when they are coiled in the form of a watch spring. This coiling protects the growing point which comes to lie in the centre.
9. Leaves or their leaflets show open furcate venation in which veins branch dichotomously without forming interconnections.
10. Younger parts of stem, young leaves, petiole and rachis of mature leave possess hairs or scales called ramenta. Ramenta protect them from mechanical injury and desiccation.
11. Sporangia occur on the leaves in clusters called sori (singular- sorus). The fertile leaves are known as sporophylls. A sorus is covered by a flap-like outgrowth from its surface (true indusium, e.g. Dryopteris) or turned margin of the sporophyll (false indusium, e.g. Adiantum).
12. Vegetative reproduction occurs by fragmentation of rhizome, adventitious buds (e.g., Dryopteris), bulbils, etc. Walking fern, Adiantum caudatum, produces a new plant whenever its leaf tip comes in contact with soil.
13. The gametophyte is thalloid and called pro-thallus. Pro-thallus is the term used for free living gametophyte having sex organs. It develops a young sporophytic plant body from inside the female sex organ.
14. Male sex organs or antheridia are small hemispherical structures with generally a 3- celled jacket enclosing 32—48 sperm mother cells.
15. Female sex organs or archegonia are flask-shaped structures that are partially embedded in the pro-thallus. The tubular neck of the archegonium has 4-rows of cells.
16. Sperms are generally multi-ciliate or multi-flagellate.
17. An embryo stage is present.
18. The life cycle has a regular alternation of a dominant sporophyte and an inconspicuous gametophyte. This is called heterologous or heteromorphic alternation of generations.
19. Ophioglossum reticulatum (Adders tongue fern) has maximum number of chromosomes (2n = 1262).