Brief account and life cycle of Osmunda
Osmunda:-
1. Systematics:-
Division:- Pterophyta
Class:- Protoleptosporangiopsida
Order:- Osmundales
Family:- Osmundaceae
Genus:- Osmunda
2. Habit and Habitat:-
> Osmunda, the royal fern is a common member of the green house.
> It is commonly cultivated in gardens for its beautiful, attractive, lush green foliage.
> Smith (1955) mentions about 12-14 species as belonging to this genus.
> Panigrahi and Dixit (1969) have reported four species of Osmunda from India. These are:
i. O.cinnamomea
ii. O.claytoniana
iii. O.japonica
iv. O.regalis
> O.claytoniana is found growing in Kedarnath at an altitude of about 10,000 ft.
> Osmunda is most widely distributed being found in the temperate and tropical regions of the southern and northern hemispheres.
> The various species of Osmunda usually grow in cool, moist and shaded places such as the banks of streams, lakes, ponds, and in the forests.
3. Study of sporophyte:-
> All the species of Osmunda are moderately sized; a few like O. cinnamomea may reach a height of 2-3 metres.
> Rhizome:-
- The plant body consists of an upright rhizome which is mostly subterranean.
- The rhizome is studded with a number of persistent, sclerotic leaf bases which are arranged in a rough spiral.
- The rhizome is generally un-branched but occasionally a dichotomy may result.
- From the lower surface of the rhizome are produced a number a brownish, adventitious roots usually two roots arise at the base of each leaf.
> Leaves:-
- The rhizome bears a crown of leaves.
- The leaves are arranged spirally at the apex of the rhizome. They are uni-pinnately or bi-pinnately compound (O. regalis) and vary in length from 0.30 to 3 metres.
- Each leaf has a stout rachis with a wing like expansion (stipule) at the base covered with glandular hairs.
- The leaves are deciduous and every year a fresh crown of leaves is produced. But leaf bases are persistent and form a thick armour around the rhizome offering mechanical support.
- The lamina has a leathery texture and is covered with brown hairs.
- The venation is of the open dichotomous type.
- The leaves exhibit circinate vernation characteristic of true ferns.
- The leaves may be monomorphic (O.regalis and O.claytoniana) i.e. there is no distinction into sterile and fertile leaves, or dimorphic (O. cinnamomea and O.japanica) i.e. fertile and sterile leaves are different.
- In dimorphic species, fertile leaves appear before the sterile ones when a new crown of leaves is to be unfolded.
- The chlorophyllous lamina is absent in the fertile leaves and they bear clusters of sporangia.
- In monomorphic leaves at least the pinnae are dimorphic. The fertile and sterile pinnae are variously distributed. They may be at the base (O.vahellii, lip (O.regalis) or in the middle (O.claytoniana and O.japanica).
4. Reproduction:-
> The sporophyte reproduces by the formation of spores.
> The sporangia are aggregated on certain pinnae only.
> These fertile pinnae which have almost lost their foliar nature are commonly called tassels.
> The sporangial aggregation has not reached the soral organisation.
> All the sporangia of a cluster mature simultaneously (simple).
> The mature sporangium is pyrifoirm in shape, reddish brown in colour and has a stout stalk.
> The jacket is one layered thick and has a shield or plate like annulus.
> Perpendicular to the annulus and overarching the sporangium is the stomium.
> The annulus is not very well developed. Hence it is often referred to as a rudimentary annulus.
> Within the sporangium are the spores ranging in number from 256-512.
> The sporangium dehisces vertically along the stomium.
> The spores are wind dispersed.
> The spores are minute in size (0.054-0.072 mm.) and have a three layered wall viz., endospore, perispore and an outermost exospore.
> Spore germination:-
- It is very rapid.
- At the initiation of germination the spore swells and the exospore ruptures, exposing the spore contents in the form of a conical mass.
- The first division (transverse) results in forming a smaller rhizoidal cell (this develops into the first rhizoid) and a prothallial cell.
- The spore germination is said to be of the bipolar type because the rhizoidal cell and the prothallial cell grow in exactly opposite directions.
> Prothallus:-
- The pro-thallus is cordate but somewhat elongated.
- It is dorsiventrally flattered and fleshy with a dark green colour.
- It grows on the surface of the soil. There is a mid rib represented by a groove on the dorsal surface.
- From the ventral surface of the pro-thallus are produced a number of rhizoids.
- The prothalli are capable of indefinite growth and under favourable conditions may reach a size of 2.3 cm.
- The pro-thallus is wholly parenchymatous.
- The margin of the pro-thallus is thin, but towards the centre it may be 16-18 cells in thickness.
- Nutrition is autotrophic.
- The gametophytes are monoecious, but underdeveloped ones tend to produce only male sex organs.
- Archegonia occur near the mid rib on the ventral surface and antheridia occur towards the fringes of the pro-thallus.
> Antheridium:-
- A mature antheridium projects above the surface of the pro-thallus.
- The wall consists of curved chlorophyllous cells.
- One of the jacket cells at the apex becomes a triangular opercular cell to facilitate the liberation of the antherozoids.
- About a 100 or more antherozoids are produced in each antheridium.
- They are coiled and multi-ciliate.
> Archegonium:-
- The mature archegonium is massive and has a straight neck of 6-8 cells in height.
- Axial row consists of an egg cell, a small venter canal cell and a bi-nucleate neck canal cell.
> Fertilization is similar to that in other pteridophytes.
> Embryo development:-
- First division of the zygote is vertical.
- The next division is also vertical but at right angles to the previous plane, thus forming a quadrant.
- The division in the quadrant is transverse resulting in an octant.
- Further divisions are not regular.
- the upper half forms the cotyledon, stem and root, while the lower half gives rise to a large foot whose superficial cells are haustorial.
5. Phylogeny:-
> Phylogeny:- The history of the evolution of a species or group, especially in reference to lines of descent and relationships among broad groups of organisms.
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