In this article we will discuss about:- 1. Occurrence and Distribution of Osmunda 2. Sporophyte of of Osmunda 3. Gametophyte 4. Phylogeny.
Occurrence and Distribution of Osmunda:
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 O.cinnamomea, O.claytoniana, O.japonica and 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.
Sporophyte of Osmunda:
Morphology of the Plant:
All the species of Osmunda are moderately sized; a few like O. cinnamomea may reach a height of 2-3 metres. 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. 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).
A transverse section shows the usual three regions – epidermis, cortex and stele (Fig. 138). The outline of the section is irregular and discontinuous due to the presence of leaf bases and roots. In a section taken in the middle region of the rhizome, the cortical region shows leaf traces or roots.
The cortex has two well-marked zones. The outer zone is mainly sclerotic and extensive, while the inner zone which is narrow, is composed of thin walled parenchyma. Numerous ‘C’ shaped leaf traces are found in both the cortical zones. These arise obliquely from the main vascular cylinder.
Each leaf trace has a well defined endodermis which is lined internally by one or two layered pericycle. The xylem mass is horse shoe shaped with the concave side, pointing towards the main stele. The xylem has scalariform tracheid’s constituting metaxylem; the protoxylem being situated in the depressions.
Internal to the cortex is the endodermis which is very prominent and continuous. Only when a ramular gap (branch gap) is present, the continuity of the endodermis is disturbed. Faull (1909) has reported the occurrence of an internal endodermis in O.cinnamomea.
In the young stem (sporeling), the stele is protostelic. Subsequently i.e. after the development of few leaves, it becomes medullated. The stele in the adult plant is an cctophloic solenostele. The pith occupies a large portion in the centre. It may be wholly parenchymatous (O.claytoniana) or mixed.
In O.claytoniana the mixed pith has sclerenchyma intermingling with parenchyma, whereas in O.regalis pith may have tracheids and parenchyma. Outlining the pith are the meshes of xylem forming a reticulate cylinder.
Sharma and Purohit (1985) have reported that the trachcids in the central stele of rhizome in Osmunda regulis are frequently anastomosed and branched. In a transverse section, xylem appears like a circle of horse shoe shaped or oval structures separated by parenchymatous rays (Fig. 138b).
These rays are nothing but leaf gaps. The number of xylem strands varies from 15 (O.regalis) to 40 (O.claytoniana) The protoxylcm lies in the sinus formed by the horse-shoe shaped xylem mass. Surrounding the xylem is a sheath formed of parenchyma.
External to this is the phloem. In O.cinnamomea an internal phloem has been reported at the region of branching. Phloem consists of both proto and metaphlocm. External to the phloem is the pericycle, 2-4 layers in thickness.
The petiole has a large concentric vasculature which is crescent shaped. The two arms of the crescent are incurved. The opening is towards the adaxial side. Xylem consists of a number of protoxylem groups embedded in a continuous mass of metaxylem. The protoxylem groups are arranged on the concave face alternating in position with groups of mucilage cells. Phloem completely surrounds the xylem.
The endoderims is not very conspicuous. The vasculature is embedded in a mass of parenchymatous tissue. Some of the outer layers are thick walled. The pinna anatomically shows bifacial arrangement. The mesophyll is differentiated, but palisade parenchyma is not distinct. Bounding the mesophyll on either side is the epidermis. The lamina is hypostomatic.
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).
Development and Structure of the Sporangium:
A detailed account of the sporangial development is given by Bower (1899, 1899). The development is intermediate between the eusporangiate and leptosporangiate types. Each sporangium is traceable to a single sporangial initial. However, the neighbouring cells also contribute to the stalk of the sporangium either wholly or in part.
The sporangial initial divides perclinally to form an outer primary wall cell and inner primary archesporial cell (Fig. 139b). The shape of the primary archesporial cell is variable. It may be truncated at the base (Fig.139b) as in eusporangiate forms or pointed at the base (Fig.139c) as in leptosporangiate forms.
The primary archesporial cell functioning like an apical cell cuts off a peripheral cell from each of its cutting faces (Fig. 139c) including that next to the primary wall cell. According to Bower these are the tapetal cells. Later investigations however have shown that they are not tapetal in nature since they do not play any part in the nutrition of the spore mother cells.
The peripheral cells contribute to the wall which appears to be two layered thick. After cutting off of the peripheral cells, the central cell now functions as the primary sporogenous cell and divides repeatedly to form a mass of sporogenous cells which contribute to the tapetum as in O.claytoniana (Fig. 139e).
At later stages the tapetum forms a periplasmodium in which the spore mother cells bathe. The spore mother cells divide meiotically and produce haploid spores. All the spores are of the same type.
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 (Fig.139h). 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 (Fig.139i). The spores are wind dispersed.
Gametophyte of Osmunda:
Structure and Germination of the Spores:
The spores are minute in size (0.054-0.072 mm.) and have a three layered wall viz., endospore, perispore and an outermost exospore. The exospore is ornamented with spines or protuberances. Spores are cholorophyllous.
Spore germination is very rapid. Spores are viable only for a very short period. 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. Further divisions in the prothallial cell are variable. Early in the development, the pro-thallus may be filamentous or plate like. In either case an apical cell is established. The derivatives of this cell soon form a spathulate pro-thallus, which on further growth becomes cordate.
At this stage the single apical cell is replaced by a series of marginal cells that cut off segments on three sides. By the activity of this marginal meristem, the pro-thallus becomes broader.
Structure of a Mature Pro-thallus:
The pro-thallus is cordate but somewhat elongated (Fig. 140). 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.
Development and Structure of the Antheridium:
Any superficial cell can function as an antheridial initial. It first protrudes out and is cut off from the neighbouring cells by an oblique wall (Fig.141b). The small cell thus formed will be the antheridial initial. This divides by two successive diagonal divisions to form a tetrahedral cell.
This cell functioning like an apical cell with three cutting faces, Cuts off derivatives which contribute to the stalk of the antheridium. After some time the tetrahedral cell divides periclinally to form an outer jacket initial and an inner primary androgonial cell. Further development is as seen in other pteridophytes.
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 (Fig. 141.i).
Development and Structure of the Archegonium:
The development is similar to what is seen in Ophioglossum. 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.
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 (Fig. 142a). It is not certain as to what is the relation of these quadrants to the basic parts of the embryo, The division in the quadrant is transverse resulting in an octant (Fig.142b).
Further divisions are not regular. According to Cross (1931), in O. cinnamomea, the upper half (next to archegonial neck) forms the cotyledon, stem and root, while the lower half gives rise to a large foot whose superficial cells are haustorial.
The root is endogenous in origin. The embryo is more or less spherical and prone (stem apex parallel to the surface of the pro-thallus). Apical cells of the stem and leaf initial arise very close to each other. The leaf first grows laterally and then vertically.
All the species have n-22 chromosomes.
Apogamy and Apospory:
Brown (1920) and Sarbadhikari (1939) have reported apogamy and apospory in O. regalis and O. javanica.
Phylogeny of Osmunda:
Osmundaceae are intermediate between eu and leptosporangiate ferns. This is exhibited in both their morphological and reproductive features.
The sporophyte has many characters of true ferns, like circinate vernation, megaphyllous leaf etc. But the soral organization typical to leptosporangiate ferns has not been achieved yet.
The presence of abundant sclerenchyma in the stem is in strong contrast with the ‘soft’ eusporangiate forms.
In the sporangial development only the initial is differentiated (Leptosporangiate) but surrounding cells also contribute to the stalk (eusporangiate). This intermediate nature is seen in many of the sporangial characters such as conical (lepto) and cubical (eu) initials, number of spores is more than in leptosporangiate forms but certainly less than in eusporangiate forms etc.
The gametophytes are large, fleshy and long lived resembling those of Angiopteris. Antheridia are projecting so also the archegonia, both resembling those of leptosporangiates.
The first division of the zygote is vertical, a leptosporangiate character, but this is not transverse to the plane of the pro-thallus as is seen in typical leptosporangiate forms.
Early stages of embryogeny do not follow either eu of lelptosporangiate forms. One half of the embryo forming the foot and the other half giving rise to the remaining parts is typical of eusporangiate forms. But the embryo is prone as in leptosporangiate (erect in eusporangiates) forms.
Osmunda seems to be a transitional type between eu and leptosporangiate forms. But is not exactly intermediate. Though it may be convenient to keep Osmunda separately in a sub class Osmundidae, it has to be conceded that it (Osmunda) leans more towards the leptosporangiate forms than towards the eusporangiate ones. Smith (1955) actually classifies Osmunda under leptosporangiatae.