Osmunda: Morphology, Characteristics, Reproductions, Uses

Systematic position (Source: USDA Plants Database)

Kingdom- Plantae

Subkingdom- Tracheobionta (Vascular plants)

Division- Pteridophyta (Ferns)

Class- Filicopsida

Order- Polypodiales

Family- Osmundaceae

Genus– Osmunda

Table of Contents

Habit and Habitat of Osmunda

Osmunda is a widely distributed genus found in both Tropical and Temperate regions. It is represented by about 14 species. They prefer to grow in moist and shady areas and are much restricted in distribution.

Morphology of Osmunda

Usually, Osmunda are medium-sized ferns but species like O. cinnamomea achieve a height of two to three meters.

Rhizome

The rhizomes are wholly subterranean and upright in position. The rhizomes are short-sized, hard, and stumpy structures and are sparingly branched. The branching is dichotomous. Numerous endogenously developed adventitious roots arise from the rhizome near the leaf base and fix it to the substratum. Usually, two roots originate from the leaf base, and due to the appressed leaf base; they are flattened at the points of their origin.

Root

The roots are hard, rough, and tough, black or dark brown. They are profusely branched and bear brown hair just below their tips.

Leaves

The leaves are circinately coiled when young. An unfolded leaf may be unipinnate as in O. claytoniana or bipinnate as in O. regalis. The leaf base is conspicuously flattened to form stipule-like expansion or wings covered with minute glandular hair.

The leaflets of the lamina are usually leathery in texture and may be entirely or variously incised. The unipinnate leaves are imparipinnate. The young and circinately coiled leaves are covered with un-branched multicellular hairs which fall off as the leaf matures. The leaves are arranged in close spirals and form a basket-shaped crown at the summit of the rhizome.

The leaves may be monomorphic or dimorphic. In the species having dimorphic leaves, there are sterile and fertile leaves. The fertile leaves do not possess green lamina. They only possess sporangia. The monomorphic species possess two types of pinnules, out of which some of them are sterile and some are fertile.

Anatomy of Osmunda

The anatomy of the rhizome reveals the following:

Cortex

It is differentiated into outer cortex and inner cortex. The outer cortex is made up of several layers of thick-walled cells. There is no distinct epidermis. The inner layer is made up of a few layers of thin-walled cells. Some leaf trace bundles are also present. Each of these leaf trace bundles is C-shaped.

Endodermis

Next to the cortex is the endodermis. It possesses distinct Casparian strips on its radial walls. It is absent at places where there is a branch gap.

Pericycle

Next to the endodermis lies the pericycle. It is made up of parenchymatous cells.

Stele

The stele consists of a varying number of C-shaped or horse-shoe-shaped xylem bundles surrounded by a continuous ring of phloem elements. The outer cells of the phloem constitute the protophloem. The phloem is composed of distinct sieve tubes. The phloem is thicker at places opposite to leaf gaps and is known as medullary rays. Next to the phloem lies the xylem which is of variable shapes and sizes.

Pith

It lies next to the stele. It may be entirely parenchymatous or sclerenchymatous.

The anatomy of leaf petiole reveals the following:

The transverse section of the leaf petiole reveals a distinct layer of epidermis. Next to the epidermis are a few layers of thick sclerenchymatous hypodermis. The hypodermis encircles ground tissue. The ground tissue is composed of thin-walled cells in which is embedded in a single petiolar bundle. The bundle consists of a crescentic or horse-shoe-shaped vascular bundle. Endodermis is indistinct.

The anatomy of the leaflet reveals the following:

The cross-section of the leaflet reveals the presence of two epidermal layers. The stomata are present in the lower epidermis. In between the two epidermis lies the undifferentiated mesophyll region. The cells of the mesophyll enclose intercellular spaces of variable dimensions. These cells contain chloroplasts. They are compactly arranged below the upper epidermis and become loosely arranged towards the lower epidermis. There is a single vascular bundle in the midrib.

The anatomy of the root reveals the following:

The roots arise endogenously from the rhizome. There is a distinct outer layer or the epidermis in the young roots but is later replaced by the outermost layer of the cortex which is called exodermis. Next to the exodermis lies the hypodermis which is made up of thick-walled cells. The cortex is made up of parenchymatous cells. Next to the cortex lies the endodermis that surrounds the central stele. The stele is usually a diarch or triarch. Pith is absent.

Reproduction in Osmunda

Osmunda reproduces by spores. The plant body is sporophyte and is homosporous. The spores are produced within sporangia that are not arranged in definite sori. In Osmunda regalis, the fertile pinnules are restricted to the distal region of the leaf whereas in some cases, the leaves developed late in the season are entirely fertile. The fertile pinnae are very much reduced and have no sterile lamina and the sporangia appear to develop on the veins arising from the midrib. Clusters of sporangia borne on these slender and thinned-out leaflets give the appearance of lateral tassels. In O. claytoniana the fertile pinnae are restricted to the middle of the leaf and the sporangia are borne on the margins of the pinnules. The fertile pinnae lack leaf blades and are slender, with the result that pinnules also appear as short slender stalks with sporangia arising in tassels or clusters.

Sporangia of Osmunda

A mature sporangium of O. regalis and most other species of Osmunda is a massive, short-stalked, and pyriform structure. The stalk is many-celled thick and varies in length. The sporangial capsule has a single layer of sterile jacket cells. Next to it lies the tapetal layer surrounding a mass of spore mother cells. The tapetal cells do not disorganize to form plasmodial fluid.

Development– The developing sporangia appear as small protuberances on the fertile and bladeless leaflets or their segments. In each protuberance, a single sporangial initial becomes distinct by its comparatively large size, distinct nucleus, and dense cytoplasm.

This sporangial initially divides transversely into an outer smaller cell and an inner larger cell. The former is called the primary jacket cell and the latter primary archesporial cell. In eusporangiate ferns, the archesporial cell is like an inverted pyramid with a truncated lower end, and in leptosporangiate ferns, the archesporial cell has a pointed end.

The archesporial cell cuts off three peripheral cells enclosing a central primary sporogenous cell. The peripheral cells are called primary tapetal cells. They divide anticlinally and periclinally to form layers of the tapetum. The primary sporogenous cell divides repeatedly to form spore mother cells.

The spore mother cell undergoes meiosis to form spores.

The stalk of sporangium is derived entirely or partially from the surrounding cells.

Gametophytic Generation in Osmunda

Spores– The spore is almost spherical and possesses a distinct tri-radiate mark. The spore wall consists of two layers- exine and intine. A spore protoplast has a distinct centrally located nucleus surrounded by cytoplasm which contains numerous chloroplasts.

Germination– Under adequate conditions of moisture the exine ruptures at the tri-radiate mark. The intine grows out in the form of a small conical papilla.

The protruding spore protoplast divides by a transverse wall into a small primary rhizoidal cell and a large primary prothalial cell. The former comes out in the form of conical protuberance and the latter is still enclosed rupturing exine. The rhizoidal cell contains a few chloroplasts which later on, as it pushes its way down into the soil as a first rhizoid, disappear.

The larger prothalial cell contains numerous chloroplasts which move toward the periphery of the cytoplasm. The prothalial cell grows exactly opposite to the first rhizoid.

There are some variations in the pattern of the laying down of the walls in the prothalial cell. In O. claytoniana the prothalial cell divides by a few transverse walls to form a filament of green cells. The terminal cell of the filament divides by two intersecting oblique walls to include a triangular apical cell with two cutting faces. The lower cell divides by longitudinal walls. The apical cell cuts off the segment towards its left and right in an alternate manner. The segments cut off by the apical cell divide each by a transverse wall to form outer and inner cells. The inner cell divides by horizontal walls. The outer cells divide rapidly and overgrow the apical cell thus giving the young prothallus a heart-shaped appearance. The apical cell remains distinct and functional for a long period or it may be replaced by a row of few marginal cells. These divide in such a manner as to cut off segments laterally as well as dorsal and ventral sides. This type of growth leads to the formation of elongated heart-shaped prothallus with a distinct cushion below the notch. The cushion projects prominently on the ventral side. The prothallus may continue to grow by means of this marginal meristem for a considerable length of time. The prothallus continues to grow by means of this marginal meristem for a considerable length of time.

Sometimes the spores develop into irregularly branched filamentous prothalli.

The development of prothalli varies slightly within the species.

Structure of the prothallus– The mature prothallus is usually elongated, un-branched, dorsiventral, green, and a cordate or heart-shaped structure. The midrib portion is usually thick and cushioned below the apical notch. The rhizoids arise from the ventral side and are unicellular but later become septate. They are dark brown or light brown. The prothalli developed under normal conditions are monoecious, protandrous, flat and ribbon-shaped, and un-branched structures with a smooth or slightly wavy margin. Under certain conditions, the prothalli develop into filaments or irregularly branched structures. Such prothalli are unisexual and bear only antheridia which is terminal or marginal in position.

Sex organs– The monoecious prothalli are protandrous and bear antheridia along the margins or on the ventral surface along the wings. The archegonia appear late and develop either along the margins of the thick midrib or scattered along its surface.

Antheridia of Osmunda

The antheridia are of emergent or projecting type. They are large and globular structures and vary in position from terminal or marginal or even to the ventral side of the prothallus. The antheridia may or may not have a distinct stalk. The body of antheridium has a single-layered wall made up of many curved cells that contain chloroplasts. A triangular cell at the apex of the antheridium is the opercular cell which is thrown off during dehiscence thus permitting the spermatozoids to come out and get dispersed.

The spermatozoids are minute, coiled, and multi-ciliate structures having two coils.

Development– The antheridium develops from a single cell which is known as an antheridial initial. The antheridial initial is cut off from a large superficial cell of the prothallus.

In terminal antheridia, the terminal cell of a branch becomes enlarged and divides by an oblique wall into an outer antheridial initial and a basal cell. The antheridial initial divides by two intersecting oblique walls to cut off a tetrahedral cell surrounded on either side by two flattened cells.

The tetrahedral cell has its broad end facing outwards and the narrow end facing inwards. It cuts off segments along its three lower sides. These segments form the stalk of the antheridium. The length and thickness of antheridium develop upon the number of segments cut off by the apical cell.

After the apical cell stops cutting segments, it divides by a curved periclinal wall towards its outer broad face thus distinguishing an outer curved primary jacket cell and an inner primary androgonial cell.

The primary jacket cell divides further to form the antheridial jacket. The antheridial jacket consists of variously curved chlorenchymatous cells. The triangular cell or the opercular cell is developed by the division of the last formed jacket cells. The opercular cell is either apical or lateral in position.

The primary androgonial cell divides to form androcyte mother cells which divide repeatedly to form androcytes or spermatocytes. Each androcyte metamorphoses into a coiled and multiflagellate spermatozoid.

Archegonium of Osmunda

The archegonia are always produced on the ventral surface and have necks that project in a horizontal direction from the cushion. The neck encloses a single bi-nucleate neck canal cell. The venter is embedded in the prothallus tissue and is not surrounded by its wall. It contains a single ventral canal cell and a large egg cell.

Development– The archegonium develops from a single superficial cell lying on the ventral side of the prothallus which is known as archegonial initial. This archegonial initial undergoes periclinal division and cuts off the primary cover cell and an inner large mother cell or the central cell. The primary cover cell divides by two intersecting walls to form four diagonally arranged neck initials. The lower basal cell takes no further part in the development of the archegonium. The neck initially divides transversely to form neck cells. The central cell divides into the upper neck canal cell and primary ventral cell. The former forms a single binucleate neck canal cell and the latter divides into an upper smaller ventral canal cell and a lower large egg cell.

Before fertilization, the neck canal cells and the ventral canal cells disorganize and become mucilaginous. The mucilage absorbs water, swells up, and forces the apical tier of cells apart, thus making an open passage for the sperms to enter. The mucilage drop oozes out and attracts the spermatozoids.

Embryogeny– As a result of fertilization, a diploid zygote is formed. It is surrounded by dense cytoplasm, which secretes a wall, thus establishing a diploid cell called the zygote or oospore.

The zygote divides by a wall parallel to the long axis of the archegonium into equal cells. The two cells thus formed divide each by wall at right angles to the first wall to form four cells. This is the quadrant stage. The epibasal quadrants give rise to the leaf and stem and the two hypobasal ones give rise to the foot and the root. The anterior epibasal quadrant forms the leaf and the posterior develops into a stem.

The anterior hypobasal quadrant gives rise to the root and the posterior to the foot. The quadrants divide transversely to form an octant stage. Subsequent divisions are not very regular. The leaf, stem, and root develop from that half of the octant which is nearer to the archegonial neck. The foot develops from the other half.

The root apical initial appears endogenously which gives rise to the primary root. The leaf initially makes its appearance as a three-sided apical cell in the anterior epibasal octant. The young leaf appears as protuberances on one side. The stem apex appears on one side of the leaf apex in the posterior epibasal half and grows into the underground stem after the primary leaf and root have been established.

The primary leaf has a broad lamina with irregular margins. The venation is furcate, possessing stomata on both sides.

Economic Importance of Osmunda

Osmunda regalis is widely used in ornamental gardening due to its attractive appearance. Osmunda regalis has also been used as a remedy for various ailments, including treating wounds, and digestive issues, and as a diuretic.
Osmunda cinnamomea is another species used as an ornamental plants. Its unique cinnamon-colored fronds make it a favorite in shaded garden areas and as a ground cover in temperate regions.
The fibrous roots and stems of Osmunda, known as Osmunda fiber, have been used in horticulture as a component of potting mixes, particularly for epiphytic orchids.
Osmunda claytoniana plays a role in soil stabilization and maintaining moisture levels in forest ecosystems, making it important for ecological balance and forestry practices.
Osmunda japonica, known as “zenmai” in Japan, is harvested and used as a traditional vegetable. They are often used in salads, soups, and other dishes.
The rhizome of Osmunda lancea has been used in traditional Chinese medicines which are believed to have anti-inflammatory properties and are used in treatments for fever, hemorrhages, and diarrhea.

References

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