ADVERTISEMENTS:
Occurrence and Distribution:
This is a cosmopolitan fern being distributed in almost all geographical regions. Pteris however, prefers tropical and sub-tropical climates. Plants usually grow in well drained places or in the crevices of rocks. They are very common along the slopes of hills and can be seen even at 1200 metres above sea level.
There are about 250-280 species reported for the genus. Some of the common Indian species are P. quadriaurita, P.critica, P.vittata, P.pellucida, P.wallichiana, P.stenophylla, P.biaurita, etc. Tanaka and Chin-ming (1982) have isolated Pterosin and Astragalin from Pteridium acquilinum sub spp Wightianum.
Sporophyte:
Morphology of the Plant:
ADVERTISEMENTS:
The plant body has a rhizomatous stem that produces roots and leaves. The rhizome may be creeping (P.grandiflora) or compact and erect as in P.cretica and P.longifolia. The rhizome may or may not show branching and is covered with scales. Roots arise at the base of the leaf or all over the rhizome. The growing point of the rhizome is covered with ramenta.
The leaves arise from the upper surface of the rhizome and have a long rachis. They are unipinnately compound, decompound or multi-pinnately compound. The dissections of the pinnae are not as deep as in Pteridium.
Venation is of open dichotomous type. The pinnae are small near the base, large towards the middle and once again small towards the apex as in P. vittata. The leaves show circinate vernation that is typical to true ferns.
Very often pinnae are Coriaceous. The leaves bear reproductive structures-son along the ventral margin of pinnae. The sorus is continuous along the margin but avoiding the apices of the segments and usually in the sinuses between them.
Internal Structure:
Rhizome:
The rhizome in a transection exhibits quite a good amount of diversity particularly in the stelar organisation. It is solenostelic (P. grandiflora, P.vittata), or dictyostelic (Fig.157). In P. biaurita the rhizome shows a mixed protostele in the lower region. It becomes siphonostelic a little up, and near the apex it shows a polycyclic dictyostelic organisation.
The epidermis is single layered and it encloses a broad cortex. There is a hypodermal sclerenchyma. The bulk of the cortex however, is made up of parenchyma. The stele consists of a number of meristeles. Usually there are two rings of meristeles (Fig. 157). The inner ring consists of 2 or 3 large meristeles whereas the outer ring consists of a number of small meristeles.
Each meristele has a band or plate like xylem mass. Sometimes it is angular. One or two protoxylem groups are embedded in the metaxylem (mesarch). Surrounding the xylem is the phloem. Each meristele may have its own endodermis (This however should not be mistaken for the polysteles of Sehginella). Here the breaking of the vascular strand is due to the leaf gaps.
Leaf:
The rachis is traversed by a single leaf trace which is variable in shape. ‘C’ shaped leaf traces are found in P. vittata. In P. biaurita the leaf trace is ‘U’ shaped while entering the leaf base, but further up it becomes ‘V’ shaped.
The xylem strand appears hooked. As usual xylem is surrounded by phloem, pericycle and endodermis. The cortical region has an inner parenchymatous zone and an outer sclerotic zone. Epidermis is a single layered with a deposition the cuticle over it. Ramenta arise from some of the epidermal cells.
ADVERTISEMENTS:
The lamina appears bifacial. It may or may not have a differentiated mesophyll. Leaves are generally hypostomatic. The midrib has a concentric vascular bundle (P. vittata) with a distinct endodermis (bundle sheath).
Root:
Anatomically the root shows an outer piliferous layer and a cortex divided into an outer parenchymatous zone and an inner sclerotic zone. The stele is protostelic and has an exarch, diarch xylem.
Reproduction:
ADVERTISEMENTS:
This is brought about by spores.
Spore producing organs:
The sorus is called a Coenosorus as it is a continuous one the coenosorus is borne on marginal connecting veins and is protected by the reflexed margin (false inducium) of the pinnae. A true inducium is totally absent.
A section of the pinna passing through the sorus (Fig. 158) clearly shows the false inducium surrounding the sporangia in different developmental conditions. Sorus is of the mixed type; sterile hairs being present in between the sporangia.
Development and Structure of the Sporangium:
The development is of the leptosporangiate type. A single superficial cell arises from the receptacle (the fertile tissue) and functions as the sporangial initial. This divides transversely to form an upper cell and a lower cell. The lower cell does not contribute to the sporangium. The upper cell by intersecting oblique walls (Fig. 159) gets differentiated into an apical cell with three cutting faces.
This cell cuts off segments along its three faces which give rise to the stalk of the sporangium. The stalk has three rows of cells. After sometime the apical cell divides periclinally to form an outer jacket initial and an inner archesporial cell. The jacket initial contributes to the jacket of the sporangium which is one celled thick.
ADVERTISEMENTS:
The jacket at the base of the sporangium is derived from the segments cut off by the original apical cell. The archesporial cell cuts off one cell each on all its four sides to form four primary tapetal cells surrounding a sporogenous cell.
The primary tapetal cells divide once periclinally and several times anticlinally to form a double layered tapetum (Fig.159). Meanwhile the sporogenous cell divides to form about twelve spore mother cells. The tapetal cells disorganize and provide nutrition to the spore mother cells. The spore mother cells divide meiotically and produce haploid spores. All spores are of the same type. A mature sporangium has a stalk which terminates in a capscule.
The wall of the capscule has three different types of cells. They are:
(1) An obliquely vertical annulus which incompletely overarches the sporangium,
(2) a thin walled, radially arranged stomium and
(3) large parenchyma cells with undulated walls.
ADVERTISEMENTS:
The sporangium splits transversely along the stomium due to the shrinking of the annular cells. The spores are wind disseminated.
Gametophyte:
Structure and Germination of Spores:
The spores are roughly triangular and have a distinct triradiate mark. Spore wall is two layered. The outer wall is variously sculptured. At the beginning of germination, the outer wall reputures and the contents protrude out in the form of a germ tube.
A transverse division in the germ tube forms the first rhizoid and the first porthallial cell. The prothallial cell forms a small filament first. The terminal cell of the filament forms an apical cell with two cutting faces. Further development and structure of the mature pro-thallus are similar to those of Adiantum.
Reproduction:
The prothalli are monoecious, Antheridia are found among the rhizoids and archegonia are found towards the apical notch. Prothalli are protandrous.
Development and Structure of the Antheridium:
One of the superficial cells on the pro-thallus towards the ventral surface projects out a little and functions as an antheridial initial. This undergoes a transverse division to form two superposed cells. (Fig.161) Due to the increase of turgor pressure in the upper cell, the cross wall bulges down and ultimately touches the wall of the lower cell. The upper cell now divides to form a dome cell and the primary androgonial cell.
At this stage the young antheridium consists of a dome cell, a primary androgonial cell and a ring cell (lower cell looking like a ring). (Fig. 161) The dome cell divides transversely to form a cap cell and the second ring cell. The cap cell may are may not divide to form two cover cells. Meanwhile the primary androgonial cell divides to form 20-25 androcytes.
A mature antheridium has two ring cells, one or two cover cells enclosing a mass of antherozoids which are coiled and multi-ciliate. At maturity the antheridium absorbs water and swells, the cover cell opens out releasing the antherozoids.
Development and Structure of the Archegonium:
The development is essentially (Fig. 162) similar to what is seen in Ophioglossum.
A nearly mature archegonium has a short, projecting, curved neck directed towards the antheridia. Neck is 5-6 celled in height.
Fertilisation is as usual to leptosporangiate ferns.
Embryogeny:
The first division of the zygote is vertical (Fig.163a). .second division is transverse resulting in the formation of a quadrant. Due to further divisions a 32 celled embryo results. At this stage a proper differentiation of the embroyonal organs takes place. Embryogeny is endoscopic; however, no suspensor is formed.
Hypobasl half of the embryo forms stem apex and foot, and the epibasal half forms the cotyledon and root. The embryo is prone. The root originates quite early in the development. It pierces the pro-thallus and enters the soil. Later the first leaf emerges.
Apogamy (P.cretica) and Apospory (P.aquilina) have been reported.
Phylogeny:
The classification of homosporous leptosporangiate mixtae ferns is a ticklish problem. Even those who club all these ferns in a single family agree that it is purely an artificial one. Holtum (1939) believes that the close relatives of Pteris are Pteridium and Dennastaedtia.
According to Eames (1964) the pteroids (to which Pteris belongs) can be traced back to schizaeaceae. He believes that pteroids evolved from schizaeaceae via, Dicksoniaceae. The Davalloid ferns evolved parallely with pteroids. The two have a common stock in Dicksoniaceae.
