Classification of Ophioglosside | Plants

In this article we will discuss about the classification of ophioglosside:- 1. Genus Ophioglossum of Ophioglosside 2. Sporophyte of Ophioglosside 3. Gametophytic Generation 4. New Sporophyte 5. Genus Botrychium 6. Genus Helminthostachys.

Genus Ophioglossum of Ophioglosside:

Ophioglossum (sometimes called ‘Adder’s Tongue’) is taken up first as it is the commonest, although it is possibly more advanced (mainly by reduction) than the other two genera. Clausen (1938) recognised only 28 species but Reimers in Engler’s Syllabus (1954) recognises 45.

The species may be divided into the sections Euophio- glossum, Rhizoglossum (O. bergianum), Cheiroglossa (O. palmatum) and Ophioderma (O. pendulum and two other species). The species are of world-wide distri­bution in temperate as well as tropical zones.

Most of them grow on humus soil and are perennial although the aerial shoots may be annual. A few tropical species are epiphytes. There are about a dozen Indian species of which Ophioglossum vulgatum (Fig. 601A), O. reticulatum and O. pedunculosum are very common.

Other Indian species are – O. costatum (= fibrosum), O. gramineum (a very small species from Maharashtra), O. nudicaule, O. capense, O. ailchinsoni and O. lusitanicum. O. pendulum, a large, pendulous, epiphytic species (Fig, 601G) often grown in greenhouses, is known from South India and Ceylon.

Sporophyte of Ophioglosside:

The sporophyte (Fig. 601 A) has a short, vertical, subterranean rhizome. Seve­ral leaves develop spirally from the top (in Euramerican species only one leaf grows in a year because of slow growth in the cold climate). Numerous fairly stout adventitious roots grow from the base of the rhizome and these are increased by one root growing from the base of every leaf.

Some of the roots grow horizontally like runners and young plants grow from buds on their upper surface. Each leaf has a thin stipule at its base and is differentiated into a petiole and a lamina which does not show circinateptyxis. The venation is closed reticulate with some finer veins ending blindly within the meshes.

This is an advanced character. The lamina in most species is entire and linear to ovate. But, in 0. palmatum (Fig. 601B), growing on rotten wood, it is palmately dissected. In O. pendulum (Fig. 601C), an epiphyte with dorsiventral rhizome, the lamina is pendulous, narrow and di­chotomously dissected truly like Adder’s tongue.

The sporangiferous spike develops later at the junction of the petiole and the lamina and is, again, un-branched and single in most species including O. pendulum. But, in O. palmatum, several of these grow from the same region and there may even be several from the same spot.

The rhizome apex shows a single inconspicuous pyramidal apical growing cell with three cutting faces. The part of the rhizome may show a protostele surrounded by an endodermis but the stem is generally siphonostelic. A. t.s. of the mature rhizomatous stem (Fig. 602) shows a broad homogeneous cortex which may sometimes show an outer periderm.

The endodermis is not defined in the upper part of the rhi­zome. The siphonostele inside may be solenostelic due to one leaf gap but more often it is dictyostelic by several leaf gaps showing small or big crescent shaped meristeles. The meristeles are endarch and collateral.

The protoxylem lines the entire inner face of the xylem mass. The metaxylem has irregularly shaped reticulate tracheides. The phloem lies on the outer side. In O. vulgatum a small amount of secondary thickening has been observed. The central pith is homogeneous but in O. pendulum small strands of xylem are found mixed with it.

The roots are endogenous. They (Fig. 603A) have no root hair. There is a my­corrhizal phycomycetous fungus in the cortex. The endodermis is distinct. The stele is monarch to tetrarch and the phloem patches alternate with protoxylem patches. The xylem is exarch.

The leaf trace is not usually branched but in O. palmatum and O. pendulum two traces depart at each gap. In all cases the traces branch further before they run into the leaf base. The petiole (Fig. 603B), thus, shows a number of vascular bundles arranged in a semicircle. The leaf t.s. (Fig. 603C) shows stomata on both the epi­dermises, a uniform spongy mesophyll with air spaces and a number of vascular bundles.

The fertile spike (Fig. 604A) meristem grows with an apical cell having three or four cutting faces originating from a superficial cell of a young leaf. Bower stated that, as the spike begins to project, two continuous sporangiogenic bands (Fig. 604B) are noticed on the two sides.

Later, the band is differentiated into alternate blocks of archesporial and srteile cells (Fig. 604C) but U. Sen (1968) has noticed that in all cases examined by him the sporangiogenic band is not continuous but the sporogenous patches are separated by sterile patches from the beginning (Fig. 605A).

Each archesporium supported by contiguous cell at forms a sporangium (Fig. 604D) with a multi-layered jacket, a weak tapetum and the sporogenous tissue inside. The tapetum ultimately breaks down into a plasmodial mass with persistent nuclei which invades the space between the isolated spore mother cells.

The sporangia become somewhat dissociated from one another when mature and ultimately open by transverse clefts after the spores are mature (Fig. 604E). There is no annulus.

The nature of the petiolar and the foliar part of the leaf and, more specially, that of the sporangiferous spike has proved to be a problem. Sometimes, to avoid the problem, the whole structure is called an aerial complex. As shown by U. Sen (1968), a single leaf trace runs for the base of the stalk of each aerial complex but this trace splits into three before actually entering the base of the stalk (Fig. 605B).

The 3 traces, again, form 12 traces or bundles by repeated divisions at the level where the stalk divides into fertile and sterile lobes. Of these 12 bundles, 9 pass on to the sterile lobe (leaf) where they do not anastomose while 3- pass to the fertile lobe (sporangiferous spike) (Fig. 605C). The 3 traces in the base of the sporangiferous spike divide and anas­tomose here and there in its axial region.

Smaller vascular strands extend horizontally from this axial system into the sterile tissue in between the sporangia where their tips are sharply recurved towards the respective sporangia below and frequently branch into 2 or more strands (Fig. 604E & 605D). In each case one of these ultimate strands supplies the sporangial wall while others end blindly.

The morphological nature of the fertile spike of the Ophioglossales has been a matter of much discussion. Instead of going into the hirtorical details of these argu­ments, the salient features may be stated.

Roeper (1826) interpreted it as a pair of fused leaves but, later, changed his opinion. Bower (1896) suggested that it is a sep­tate sporangium growing on the adaxial face of the leaf bringing it near to Lycopsida but himself discarded this idea as untenable in 1911. Caiftpbell (1890) also considered it as a homologue of the Lycopsid sporangium but, later (1911) considered it as similar to the sporangium of Anthoceros.

The idea that the fertile spike is formed by the fusion of two fertile pinnae remained popular for a long time but now has given way to a more acceptable view. Chrysler also has changed his opinion.

Zimmermann (1930) explained the spike on the basis of his telome theory as being derived from a dichotomous shoot (as in the Psilophytales or the Coenopteridales) in which the fertile spike and the vegetative leaf represent different arms of the dichotomy.

Ontogenetic studies show that the spike often develops from the mesophyll cells on the adaxial side whereas a pair of modified pinnae could arise only from the margins. The pinna-like nature is, therefore, not supported.

The present view is that the aerial complex represents a condensed di­chotomous branch system (Zimmerman 1930, Bower 1935, Chrysler 1945, U. Sen 1968). Nozu (1950) and Nishida (1957) consider the stalk of the leaf (or the atrial complex) to be a stem-like organ which they name as phyllomophore.

Thus, it is reasonable to conclude that the stalk or petiole of the complex (phyllomophore) is a structure intermediate between a stem and a leaf. It dichotomises to form the fertile spike by one arm and the sterile leaf by the other. It, therefore, differs from the Lycopsida and the Filicopsida in general.

The chromosome number of Ophioglossum is very high. The n number has been found to be from 125-130 in O. lusitanicum to more than 630 in O. reticulatum. In O. vulgatum it has been found to be 250-260.

Gametophytic Generation of Ophioglosside:

The spores occur in large numbers (1500 to 15000) and each of them is endowed with an exine and an in tine. The exine is pitted, moderately thick, often sculptured but colourless. The spore, on germination, produces a subterranean gametophyte. Ophioglossales is the only group of known Filicopsida with a subterranean gameto­phyte and this again reminds the Lycopsida.

The gametophyte does not grow beyond the rudimentary stage if it does not get infected with a mycorrhizal fungus at an early few-celled stage. The mature gametophyte (Fig. 606A & B) is irregularly cylindrical to conical and branched or unbranched. O. vulgatum prothallus is not branched at all looking like a piece of root while O. pendulum freely branches in different directions.

The growing apex grows by a single apical cell with 3 or 4 cutting faces and this growth is indefinite. The prothallus may be long living (often 20 years) or annual. It is brown with little differentiation of tissue except that the axial cells may be a ‘little elongated. The endophytic fungus is located in the inner tissue of the lower part only. There is no rhizoid and chlorophyll may develop only if some part gets exposed to light.

Antheridia and archegonia are formed on the surface indiscriminately inter­mingled and scattered. They arise from superficial cells. The antheridium (Fig. 606C) develops as in other Eusporangiates with a jacket initial and a spermatogenous initial and the final triangular opercula cell in the single-layered jacket disintegrates to form an opening.

The androcytes or sperm mother cells form two sperms each, each of which re­ceives a blepharoplast as in Isoetes. Ultimately, a coiled multi-ciliate sperm (Fig. 606D) is formed and there are thousands of these in an antheridium. All antheridia are em­bedded. The archegonia (Fig. 606E) also develop as in other cases.

The primary canal cell rarely divides to form two neck canal cells— usually there is only one of them. The ventral canal cell disintegrates as soon as it is formed. The neck barely protrudes out of the thallus.

New Sporophyte of Ophioglosside:

The zygote divides by a transverse wall giving rise to an epibasal and a hypobasal cell. The growth of the embryo is very slow taking a year or several years to develop. There no suspensor in the embryo and it takes a long time for the different organs to diffe­rentiate. An embryo of O. vulgatum a few months old (Fig. 607A) shows the foot and the root in the hypobasal region. The epibasal tissue is still undifferentiated.

The root is the first and the prominent organ to pierce the calyptra and at this stage the embryo seems all root with a short foot and an undifferentiated epi­basal region (Fig. 607B). The epibasal region then gives rise to the stem apex and the cotyledon when the new spo­rophyte becomes established.

In some other species, the growth of the epi­basal region may be at the same rate as the hypobasal. The first root gets in­fected by the endophytic fungus at a very early stage.

Genus Botrychium of Ophioglosside:

Botrychium (‘Moonwart’ or ‘Grape fern’), with 35 cosmopoli­tan species from temperate regions, has pinnately branched leaves and the fertile spike also is similarly branched. Secondary growth is clear in the stem and a cambial cylinder is present. The endarch xylem shows boidered pitted trac­heides. The leaf traces in Botrychium are complex.

The genus is well represented on the Himalayas and the South Indian as well as the Ceylonese hills. The commonest species is B. virginiacum (Fig. 608A & B). Other Indian species are B. lunaria and B. daucifolium.

The genus is divided into two sections— EuBotrychium and Sceptridium. The first section shows an Embryo as in Ophioglossum. The second section (B. obliquum, B. dis- sectum, etc.) shows an embryo in which the epibasal cell forms a suspensor (Fig. 608C) and the hypo­basal cell forms an embryo in which the foot is not apparent.

There is a suggestion that Sceptridium should be placed in a separate genus because of the nature of its embryogeny.

Genus Helminthostachys of Ophioglosside:

Helminthostachys is a monotypic genus found only in Indoraalesia and the Poly­nesian Islands including New Zealand. The simple species is Helminthostachys zeylanica (Fig. 609A).

It is well represented in the Lower Bengal plains and all over such places in India. The rhizome is dorsiventral and the leaves are very large and palmately compound showing pronounced dichotomy. The fertile spike is compound with clus­ters of sporangia on the rachis (Fig. 609B). The xylem is mesarch in this genus, bor­dered pitted tracheides are present but no secondary growth has been detected.

The embryo (Fig. 609G) shows a suspensor and the normal foot. The genus is considered the most primitive in the family.