Life Cycle of Ophioglossum | Pteridophyta | Botany

In this article we will discuss about:- 1. Origin of Ophioglossum 2. Morphology of the Spike 3. The Gametophyte 4. The New Sporophyte.

Origin of Ophioglossum:

The genus comprises of 28 species. The common Indian species are O. gramineum, O. vulgatum, O. nudicaule, O. reticulatum, etc.

The plant is characterized usually by the presence of a short, erect, rhizome, which rarely undergoes a dichoto­mous branching. Many adventitious roots are borne on the underground stem. In the majority of species there is only one leaf produced by the plant every year.

Each leaf is clearly differentiable into a petiole and a lamina, the venation being of the reticulate type. The leaves may be simple or lobed and sometimes forked in a dichotomous manner; the phyllotaxy is spiral. The circinate type of ptyxis is unknown in this case.  

Internally, the stem is siphonostelic as a rule, but occasionally the basal region of the stem may be protostelic with the upper part siphonostelic. The xylem is endarch with phloem lying ex­ternal to xylem. The pith is generally homogenous and composed of comparatively large, thin-walled, parenchyma cells, but in some cases (e.g., O. pendulum) small scattered patches of xylem may be found in the pith. In O. vulgatum a small amount of secondary growth has been noted.

 The cortex of the root is a massive structure whose innermost layer gives rise to a typical endodermis. The outer region of the cortex is made up of angular parenchyma cells without any inter­cellular space and harbours a phycomycetous mycorrhizal fungus. The inner region is composed of more or less rounded cells with prominent intercellular spaces. The stele may be monarch to tetrarch with radially arranged xylem and phloem; xylem is exarch.

All the species of Ophioglossum, excepting O. palmatum, bear a single, linear, simple or slightly irregularly branched fertile spike at the junction of petiole and lamina. This spike resembles the tongue of an adder. There is a good deal of controversy regarding the morphological nature of the fertile spike in Ophioglossum.

The sporangia, however, are 0.5-3.0 mm. in diameter, sub-spherical and sunken within the leaf-tissue; these are also regarded by some to be fused forming a synangium along the margin of the leaf. There are two marginal rows of sporangia which are eusporangiate in their mode of development.

With reduction division and formation of spores, the gametophytic or haploid generation begins. At maturity, the sporangia turn yellowish or brownish in colour and dehisce by means of a ‘lateral’ slit, liberat­ing the spores in multitude. The spores are originally tetrahedral but later on become sub-spherical; the wall is thick and sculptured.

Morphology of the Spike:

Speculation on the morphological nature of the fertile spike of Ophioglossaceae dates at least from the time of Roeper (1826, 1859), who suggested that the spike in Botrychium has been derived from two basal pinnae of the sterile frond, and is secondarily erected upright, as in case of Anemia. It was later shown by Holle (1875) that Roeper’s theory might be applied to Ophioglossum.

Bower (1896), however, consi­dered the spike of Ophioglossum as a septate sporangium, but this hypothesis was rejected by himself in 1911. Chrysler (1910) supported Roeper’s view. He found that the fertile spike of Botrychium has the same number of vascular supply as would naturally go to a pair of pinnae.

He further pointed out that in the abnormal specimens, where there is a pair of fertile spikes instead of one, each spike has the same number of vascular supply as would be present in a single pinna. The pinna-nature of the fertile spike is more evident in Botrychium lanuginosum, where the spike is situated in the middle of the lamina, and in the position of the third to the sixth leaflets from the base.

However, the strongest evidence against the pinna-nature of the fertile spikes is found in Ophioglossum palmatum. In this species, unlike the pinnae, at least some of the spikes are non-marginal in position.  

Recently the frond has been re-interrupted by Chrysler (1945), Nishida (1957), Sen (1961) and others in the light of branching of its vascular tissue. The repeated dichoto­mous branchings of the vascular tissue are considered as being indicative of a once-free dicho­tomous branch system charac­teristic of Coenopteridales, and the fronds of Ophioglossales are thus considered as intermediate forms, connecting the archaic radial types with the modern plane fronds.  

Some species of Ophio­glossum (O. vulgatum) can re­produce vegetatively by means of root buds.

The Gametophyte of Ophioglossum:

A spore on germination, sooner or later, gives rise to a prothallus, which in its early stage of development becomes infected with an endophytic mycorrhizal fungus. In case of entirely subterranean prothalli, this fungus is responsible for the supply of food to the young gametophyte.

In some cases, however, the prothalli possess small, sub-aerial green lobes. A mature prothallus of Ophioglossum is conical or irregularly cylindrical or stellate, a few mm. long, unbranched or copiously branched and perennial in habit. The sex organs are scattered generally near the growing, tip of the prothallus and are formed in an irregular sequence, but in acropetal succession.  

An antheridium develops from a superficial cell (antheridium initial cell) of the prothallus, which divides periclinally into an outer and an inner cell. This inner cell gives rise to the antheridium proper. The antherozoids are coiled and multiflagellate.

An archegonium is also developed from a single superficial cell of the gametophyte. Here also, the first division of the archegonial initial cell is periclinal. The neck of the archegonium is slightly projecting beyond the surface of the prothallus.

The ventral canal cell is extremely transitory in nature. By disintegration of the canal cell or canal cells an opening is formed in the neck of the archegonium, through which an antherozoid glides in and fuses with the egg cell, thereby causing fertilization.

The fertilized egg soon covers itself by a wall and forms the oospore. With fertilization and formation of oospore, the sporophytic or diploid genera­tion begins.  

The New Sporophyte of Ophioglossum:

The new sporophyte develops from the oospore in course of a year, or the development may take place through a number of years. In all the species investigated so far, the first divi­sion of the oos­pore is approxi­mately transverse, producing an up­per cell, the epibasal cell, and a lower one, the hypobasal cell.

It cannot be said with certainty whether the latter one gives rise to the foot only, or to the foot as well as the primary root. The epibasal cell develops into the stem and the first leaf.