Describe briefly the systematic position and salient features of morphology and anatomy of Ophioglossum.

Ans. Systematic Position: Pteridophyta

Division – Pterophyta

Class – Eusporangiopsida

Order – Ophioglossales

Family – Ophioglossaceae

Genus – Ophioglossum.

Distribution and Habitat: According to Clasuen there are 8 species in this genus. They are world-wide in distribution. Many species are found in temperate regions, but this is least thrived in the tropics.

About a dozen species are found in our country. The species areOphioglossum vulgatum, O. pendulum, O. nudicaule, O. costatum, O. lusitanicum and O. aitchisoni.

Most of the species of Ophioglossum are terrestrial plants and commonly grow in humus soil. Two species are epiphytic, e.g., O. pendulum and O.palmatum. These species grow on trunks and branches of trees. This genus has been divided into three sub-genera- 1. Euophioglossum; 2. Ophioderma and 3. Cheiroglossa.

The Sporophyte: The sporophyte of most species of Ophioglossum possesses a short, erect, subterranean rhizome. A few species (e.g., O. pendulum O. intermedium) have a markedly dorsiventral stem. The branching of the sporophyte is generally due to a formation of axillary buds in the axils of leaves. Petry has also described the branching of rhizome as dichotomous.

Root: The rhizome bears numerous adventitious roots. These roots are commonly thick, smooth and devoid of hairs. As a rule the roots are borne singly below the leaf or leaves of the current season and the leaf scars of previous seasons. The roots are mycorrhizal. They arise endogenously.

Leaves: Most of the species of Ophioglossum produce but one leaf each year. Some tropical species may produce four or five leaves in a season. The leaves are usually borne in an irregular spiral on the stem. Where growth is seasonal, the development of the leaf is very slow. In such cases the embryonic leaf at a stem apex develops slowly for three years and then grows above ground and unfolds the fourth year.

A fully expanded leaf is sharply differentiated into blade and petiole. The blade of the leaf in all but one species (O. palmatum) is entire and narrowly linear to broadly oval in outline. At the juncture of the blade and petiole, in all but one species (O. palmatum) there is a single fertile spike. In O. palmatum there are several small fertile spikes in the region where blade and petiole adjoin. In O. palmatum the blade is palmately incised into a number of narrow segments. In O. pendulum the leaf may attain a length of 1.5 metres or more and its petiole merges gradually into a strap-shaped lamina. The venation of the blade is always reticulate. There is no distinct midrib, but according to the species the small veinlets may or may not present. These veinlets end blindly within the larger meshes. However, the veins near the central axis of the blade are more conspicuous than the remaining veins. The venation of leaf is erect  and not circinate.

Apical Growth: A single inconspicuous pyramid-like apical cell with three cutting faces is found to be situated at the apex of the stem. This apical cell cuts off derivatives which divide and redivide forming an embryonic tissue which becomes differentiated into stele, cortex and epidermis on its maturity.

1. Rhizome: A transverse section of the rhizome reveals an irregular outline due to the attachment of adventitious roots and leaf bases. The cortex remains bounded by a single-layered epidermis. The cortex consists of large thin-walled parenchymatous cells having abundant intercellular spaces among them. Maheshwari and Singh reported the formation of a periderm layer in old rhizomes; but there is no functional cambium.

The lowermost portion of the rhizome may have an endodermis delimiting stele and cortex (Bower). In upper portion of the rhizome there is no endodermis. Here the innermost cells of the cortex and the outermost cells of the steles are so similar in appearance that it becomes difficult to differentiate where one region leaves off and the other begins. Lang has reported an internal and external endodermis in young plants of the Ceylonese species of Ophioglossum.

Generally the rhizome possesses an ectophloic siphonostele throughout its entire length. This siphonostele may have but one leaf gap when cut at any particular level. More often two or more leaf gaps may overlap forming a dictyostelic condition. Most of the species have a single leaf trace departing at each gap and traversing the cortex without branching. A few species, e.g. O. palmatum and O. pendulum have two traces departing at each gap. There is a division of each trace into several strands before it runs into the leaf base.

The xylem of each meristele is endarch in arrangement. The protoxylem remains situated on the inner face of the xylem mass. The reticulate tracheids of irregular shape form the metaxylem. Boodle (1899) has reported a small amount of secondary thickening in the stem of O. vulgatum. The protoxylem is composed of spiral and scalariform elements. In O. palmatum, the protoxylem is ill defined. The relation of the xylem to the phloem is collateral, that is the phloem lies external to the xylem. Commonly the phloem consists of a layer four or five cells in thickness. However, in O. pendulum the phloem consists of a single layer of cells separated from the xylem by a layer of parenchyma three to five cells in thickness. The pith internal to the xylem is composed of thin walled large parenchymatous cells. According to Petry, sometimes (e.g., in O. pendulum) the tracheids occur in the pith region.

2. Leaf: The epidermis of the petiole is single layered. The interior of a petiole is composed of a spongy parenchyma that may or may not possess conspicuous cavities in the central region. Just beneath epidermis, the parenchymatous cells contain numerous discoid chloroplasts.

The number of vascular bundles is one or two at the base. The trace, or the pair of traces, departing to a leaf become divided into strands which lie parallel to one another throughout the length of the petiole. The bundles may be arranged in a crescent or C-shaped outline. The bundles are collateral. The xylem is endarch in development. The phloem is found toward outer surface.

The internal structure of the sterile leaf lamina resembles with that of petiole. In the species with erect leaves, stomata are present in equal numbers on both faces of the blade. In O. reticulatum where the leaf blade is more or less horizontal, the upper epidermis contains more stomata. There is no distinct hypodermis. The bulk of lamina is composed of thin walled green cells with frequent intercellular spaces. The structure of bundle is similar to that of petiole. Several veinlets are found to be present in the leaf lamina. All cells of mesophyll tissue contain chloroplasts.

3. Root A transverse section of the root reveals a single-layered epidermis, a broad cortical region and a central stele. In young root the epidermis remains intact and the root hairs are lacking. In the older roots the epidermal cells are thickened and suberized. According to Maheshwari and Singh (1934), in O. fibrosum the epidermal layer is soon replaced by an exodermis developed from the outer cortex.

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