In this article we will discuss about the structure and reproduction in Ophioglossum.
Structure of Ophioglossum:
The sporophyte is differentiated into a subterranean rhizome with roots and spirally arranged leaves bearing sporangiferous spike (Fig. 7.92A).
The stem is represented by a short, erect, subterranean rhizome. But in epiphytic species, it is horizontal and dorsiventral structure. The upper surface of the rhizome produces numerous leaves in an irregular spiral. The rhizome is anchored to the soil by numerous fairly stout adventitious roots growing from the under- surface of rhizome (Fig. 7.92A, C).
A T.S. of the rhizome is somewhat irregular in outline due to the presence of persistent leaf bases and roots (Fig. 7.93A). Internally, it is differentiated into an outer epidermis, middle cortex and inner stele. The outermost layer is epidermis which is single layered, suberised and devoid of any stomata.
The cortex is composed of undifferentiated (homogenous) thin-walled parenchymatous cells and is characterised by the absence of sclerenchyma.
The stele shows variations within the same species as well as in different species. The young basal part is protostetic (no pith), whereas the upper mature part is siphonostelic (pith present). The siphonostele may be solenostelic due to one leaf gap but more often it is dictyostelic by several overlapping leaf gaps.
In the dictyostelic condition, the siphonestele is broken into many small or large crescent-shaped meristeles. The meristeles are endarch, collateral and without any endodermis around them (Fig. 7.93B).
Majority of the temperate species produce only one leaf each year, but several tropical species are known to produce up to five leaves in a year. Each leaf has a thin stipule at its base and is differentiated into petiole and a lamina (Fig. 7.92A).
Unlike other ferns, the lamina does not show circinate vernation (ptyxis). The venation is reticulate without any distinct midrib. The veinlets end blindly in the mesophyll which is a very distinct characteristic of Ophioglossum. The lamina is simple with entire margin and linear to ovate in shape.
A T.S. of the petiole shows an epidermis, ground tissue and a number of vascular bundles (Fig. 7.94A).
A single leaf trace arises at the base of the petiole which divides into two strands and enters the leaf. In some species (O. palmatum) each leaf gap is associated with two leaf traces. The epidermis is composed of single layer moderately thick-walled cells.
Stomata are absent in the epidermis. The ground tissue is made up of thin- walled spongy parenchymatous cells. The basal part of the petiole shows a single median vascular bundle, but there are many bundles in the upper part of the petiole.
The vascular bundles are collateral and arranged in C-shaped arc with xylem facing the adaxial side (Fig. 7.94A). Bundle sheath is absent. Some of the bundles enter the lamina and others supply the fertile spike.
A T.S. of the lamina shows an upper and lower epidermis (Fig. 7.94B). Both the epidermis are single-layered and interrupted by the presence of stomata (amphistomatic). The mesophyll is not differentiated into spongy and pallisade parenchyma. Large numbers of intercellular space occur between these cells. Vascular bundles are collatera] and endarch and surrounded by a distinct bundle sheath.
Roots are often branched, heavily infested by mycorrhizal fungi. Roots are perennial and devoid of root hairs. The T.S. of root shows a single- layered epidermis, a massive cortex and a broad central stele (Fig. 7.95). The outer cortex contain endophytic mycorrhiza. Stele may be monarch or diarch or triarch or tetrarch or even pentarch.
Reproduction in Ophioglossum:
Majority of Ophioglossum species reproduces by means of spores. Spores are of same size and shape i.e., Ophioglossum is homosporous pteridophyte. However, some species — e.g., O. pendulum, O. vulgatum, O. reticulatum, O. nudicaule and O. aitchisonii — reproduce vegetatively by means of adventitious buds formed on roots.
The spores are present in sporangia. The sporangia are produced in two rows on a fertile segment that arises at the junction of the petiole and the sterile lamina (Fig. 7.92A).
This sporangia-bearing fertile segment is known as sporangiferous spike. Generally, a single spike is associated with a leaf but in O. palmatum several spikes are present on a single leaf (Fig. 7.92C).
Structure of the Sporangiferous Spike:
The spike is a simple, somewhat cylindrical and stalked structure (Fig. 7.96A). It bears two rows of embedded sporangia on either side, except at the apical region. The length of the spike and the number of sporangia in each spike varies according to the species. A number of vascular strands run longitudinal along the axis and from these strands many lateral branches develop which lead to the sporangia (Fig. 7.96B).
The fertile spike has been considered as an aerial complex by many scientists. A convincing evidence for this interpretation has been provided by the vascular traces. According to U. Sen (1968), the single leaf trace at the base of each aerial complex splits into three before entering the base of the stalk (Fig. 7.97A).
Then the three traces again split up into 12 traces at levels where the stalk divides into fertile and sterile lobes. Out of the 12 traces, 9 enter the sterile lobe (lamina) and do not anastomose, ‘whereas the rest 3 pass on to the base of the fertile spike where they divide and anastomose in the axial region (Fig. 7.97B).
From this axial system, small traces extend horizontally into the sterile tissue in-between the sporangia. Then the tips of each of these traces bend sharply towards the respective sporangia and frequently branch into two or more strands (Fig. 7.97C).
Only one of these ultimate strands enters the sporangial wall, the others end blindly. Thus, the aerial complex is represented by a condensed dichotomous branch system where fertile spike and lamina are the two limbs of the dichotomy.
Structure of the Sporangium:
The development of a sporangium in Ophioglossum is of eusporangiate type. At the very young stage, the primordium of spike differentiates on the lateral side into two vertical strips of cells in the epidermal layer.
Each strip is known as sporangiogenic band (Fig. 7.96C); the hypodermal cells of each band now differentiate into alternate groups of fertile (archesporial) and sterile cells (Fig. 7.96D). The archesporial cells mature into sporogenous cells, thus representing a future sporangium.
Each mature sporangium is spherical or oval in shape and remains embedded in the tissue of the fertile spike.
The size ranges between 0.5 to 3 mm in diameter. It remains surrounded by multi- layered wall of which the innermost wall layer functions as tapetum (Fig. 7.96E). The sporangial cavity is filled with many spore mother cells. Usually, all the spore mother cells are functional and eventually develop into spote tetrads. The number of spores per sporangium varies from 1500 to 15,000.
Dehiscence of the Sporangium:
The sporangia have no specialised dehiscence mechanism. The annulus, a characteristic structure of other fern sporangia are absent in Ophioglossum. The dehiscence takes place by means of a transverse slit or simply by drying out and shrinking of the sterile tissue within the spike.
Ophioglossum is homosporous and the haploid spores are the mother cells of the gametophytic generation. The spores are small, round, with an outer sculptured exine and inner thin intine which germinate shortly after dispersal.
Development of Gametophyte:
The spores, on germination, produce a subterranean gametophyte (Fig. 7.98A, B). Ophiglossales is the only order under ferns showing subterranean gametophytes. The spore absorbs water and enlarges considerably.
The first division is transverse to form a lower and an upper cell. The lower cell divides vertically resulting into a 3-celled stage. Further development only proceeds if it gets infected with mycorrhizal fungi.
The gametophytes are non-green and contain an endophytic fungus that enters gametophytic cells soon after germination and is necessary for sustained growth (Fig. 7.98A, B). The mature gametophytes may be irregularly cylindrical to conical and unbranched (e.g., O. nudicaule, O. vulgatum) or branched (e.g., O. palmatum, O. pendulum).
There is considerable size variation in the gametophytes. The gametophyte of O. crotalophoroides is globose or approximately hemispherical. The size ranges from 2 mm to 6 cm in length and less than 1 mm to 3 mm in diameter.
Sex Organs of Ophioglossum:
The gametophytic prothallus of Ophioglossum is homothallic (monoecious) i.e., the prothallus bears both male (antheridia) and female (archegonia) sex organs. The antheridia and archegonia — in most species — are scattered and intermingled over the entire surface of the gametophyte.
The antheridia are embedded in the tissue of the prothallus. The antheridium develops from a single superficial cell of the derivatives of apical meristem called antheridial initial (Fig. 7.99A-F). The antheridial initial divides periclinally (transverse) to form an outer jacket initial and an inner primary androgonial cell.
The derivatives of jacket initial form jacket with a triangular opercular cell which, on disintegration, forms an opening. The primary androgonial cell (spermatogenous initial) divides repeatedly to form androcyte mother cells.
An androcyte mother cell divides to form two androcytes, each of which ultimately metamorphoses into an antherozoid or sperm. Each antherozoid has a large basal vesicle and the terminal part bears numerous flagella (Fig. 7.99G).
The archegonium is also initiated in the derivates of apical meristem (Fig. 7.100A-E). One of the superficial cells functions as archegonial initial which, on periclinal division, forms an outer primary cover cell and an inner cell. The primary cover cell, by two vertical divisions at right angles to each other, forms quadrant of neck initials.
Further anticlinal divisions of neck initials form the neck of the archegonium. The neck barely protrudes out of the thallus. The inner cell, on the other hand, divides periclinally (transverse) to form a basal cell and a central cell.
The central cell divides transversely into a primary canal cell and a primary venter cell. The primary canal cell directly functions as neck canal cell. The primary venter cell, however, divides transversely and forms a ventral canal cell and a large egg.
At maturity, the ventral canal cell, the neck canal cell and the neck cells at the top are well- disorganised, they thus form an open passage for the antherozoids to come towards the egg (Fig. 7.100F).
As usual the zygote is the mother cell of the next sporophytic generation. The first division of the zygote is transverse i.e., perpendicular to the long axis of the archegonium, giving rise to an epibasai and a hypobasal cell (Fig. 7.101 A, B). The embryo is exoscopic in polarity i.e., the apical cell projects towards the neck of the archegonium.
Both the cells are embryonic. A cell division (vertical) of the apical and the basal cells results in the formation of four cells — the quadrant stage of embryogeny (Fig. 7.101C). Subsequent cell divisions are irregular and indefinite.
There is no suspensor in the embryo and it takes a long time for further differentiation of organs. The root is the first and the prominent organ which arises near the middle of the embryo (from derivates of epibasai cell) and enlarges rapidly. The first leaf and future shoot apex derive from the epibasai region of the embryo and the foot derives from the hypobasal region (Fig. 7.101 D-F).
Chromosome Number of Ophioglossum:
The chromosome number of Ophioglossum is generally high, although there is no uniformity in the basic number. O. pusitanicum has n = 125-130, while in O. vulgatum n = 250-260. The highest chromosome number has been recorded in O. reticulatum where n = 631 +10 fragments.
Systematic Position of Ophioglossum:
In modern classifications, Ophioglossum has been placed under the order Ophioglossales, a small group of eusporangiate ferns consisting of three living genera (Ophioglossum, Botrychium and Helminthostachys). They do not have any early fossil record. The plant is unique in having sporangia borne on a fertile spike that projects from the leaf near the junction of the lamina and the petiole.
Some botanists considered the Ophioglossum (Order Ophioglossales) to be different from other ferns by the presence of peculiar characteristics, such as:
(i) The peculiar fertile spike projects from the leaf near the junction of the lamina and the petiole,
(ii) Presence of collateral vascular bundle in the stem,
(iii) Endophytic fungus in the root,
(iv) Absence of root hairs,
(v) Non-circinnate vernation of leaves even in the embryonic stage,
(vi) Absence of sclerenchyma in any part of the plant body, and
(vii) Subterranean gametophyte with an associated fungus and scattered sex organs (reminds of the Lycopsida).
However, Sporne (1975) placed Ophioglossum in the order Ophioglossales under the group Eusporangiatae along with Marattiales. He emphasised that these two orders are quite different from other ferns and have reached the same stage of evolution, rather than their close relation.
They show some similar characteristics like:
(1) Basically erect axis,
(2) Stipules at the base of the petioles,
(3) Absence of sclerenchyma,
(4) Sporadic endodermis,
(5) Massive sporangial wall, with stomata,
(6) The sporangia showing a tendency to fusion,
(7) Large spore output,
(8) Prothallus long lived,
(9) Massive antheridium.
Though, Marattiales is distantly related because of the following characteristics:
(i) Circinate vernation, and
(ii) Superficial sori.
According to Bower (1936) the order Ophioglossales is an imperfectly modernised relic of Palaeozoic flora showing a blind evolutionary series from Primofilicales of the Carboniferous. However, Ophioglossum is the most primitive genus among Ophioglossales, having resemblance to the ancestors of the ferns.
But Bower considered Ophioglossum as an advanced member over the other two genera. He argued that the simplicity in Ophioglossum is due to its derived condition.
The Ophioglossales is certainly a primitive group. However, the evolutionary history of the Ophioglossales has been found to be sketchy due to the non-availability of proper fossil record. Though Ophioglossum represents a primitive stock of Pteropsida, it is specialised to a high degree.
Morphological Nature of the Sporangiferous Spike:
The morphological nature of the fertile spike of Ophioglossum is a matter of controversy.
Following views have been put forward by various workers to explain the morphological nature of the fertile spike of Ophioglossum:
(a) Derived from Septate Sporangium:
Bower (1896, 1908) suggested that it is a septate sporangium growing on the adaxial face of the sporophyll. He opined that the two marginal rows of sporangia originating from a common mass of mother cells have become partitioned to form two rows of sporangia. But later (1911, 1926) he himself rejected this idea as it is not supported by the course of vascular supply from the petiole to the sporangia.
(b) Derived from Pinna-Like Structure:
Roeper (1859) considered that the spike represents two basal pinnae of a compound leaf which have fused laterally to form the spike, the remaining pinnae fused to form a sterile blade. Later Holle (1875), Chrysler, (1910) and Bower supported this hypothesis.
Goebel (1915) also considered the fertile spike as pinna-like structure, but, it is derived from a single pinna.
Khandelwal (1986) investigated 12 species of Ophioglossum and concluded that fertile spike has derived from the modified pinnae of a compound leaf.
(c) Derived from Dichotomous Shoot:
On the basis of Telome theory, Zimmermann (1930) explained the spike as a modified dichotomy of a shoot, where one limb of the dichotomy became vegetative (photosynthetic) and the other fertile.
According to the present considerations, the fertile spike is an aerial complex which represents a condensed dichotomous branch system.
The stalk (petiole) of the aerial complex has been considered as a stem-like organ and renamed as Phyllomophore by Nozu (1950) and Nishida (1957). The Phyllomophore is a structure intermediate between a stem and a leaf. One limb of the dichotomy becomes the fertile spike and the other sterile leaf.