Whisk Fern:-Psilotum Nudum, Its Classification, Systematics, Sporophyte, Anatomy, Reproduction And Development
The Psilotum nudum (Sym P. triquetrum) and P. flaccidum (Syn.P. complanatam) are the two well defined but polymorphic species of genus Psilotum. This genus is frequently found in tropical and sub-tropical regions of both eastern and western hemispheres. Psilotum nudum is the commoner species of the two. P. flaccidum is a rare species and is found in the tropical islands. Both the species are usually epiphytic in habit and grow upon tree ferns. These species may also be terrestrial and grow in humus or in the crevices of the rocks.
Classification of Psilotum And Characteristic Features of Order Psilotales
- Division – Psilophyta
- Class – Psilotopsida
- Order – Psilotales
Characteristic Features: The Psilotales have a rootless, dichotomously branched sporophyte that generally differentiated into a rhizome and an aerial shoot. The function of water absorption is by large rhizoids borne on the rhizome. The aerial shoots may be leafless or it may bear scale-like or small leaf-like appendages. The sporangia are borne in diads or triads and directly upon stem or at the base of leaves. The radially organized cylindrical gametophytes are subterranean and without chlorophyll. The antheridia remain partially embedded in the and project above the general surface of the prothallus.
Classification: The class Psilotopsida has a single order, the Psilotales. The order includes a single family, the Psilotaccae. The family comprises of two genera-Psilotum and Tmesipteris. Genus Psilotum has described here in detail.
Common Name: The common name of Psilotum nudum is Whisk Fern
The systematic position of Psilotum is mentioned below:
- Division – Psilopyotz
- Class – Psilotopsida
- Order – Psilotales
- Family – Psilotaceae
- Genus – Psilotum.
The plant is slender and shrubby, rarely as much as a meter in height. It possesses green, ridged and dichotomously branched stems. The stems are perennial and somewhat xerophytic in structure.
The stems merge below the surface into slender dichotomous rhizomes which branch freely and may be one meter or more long. There are no roots. The rhizome generally contains a mycorrhizal fungus. Any dichotomy of rhizome may develop into a green aerial shoot. The rhizomes remain covered with hair-like absorbing structures or rhizoids.
The green aerial shoots of epiphytic plants are commonly pendant and those of terrestrial plants are usually erect. The aerial shoot regularly has several dichotomous branching’s. The basal portion of the shoot is cylindrical. The aerial green portion of Psilotum nudum is radially symmetrical and longitudinally ribbed. In P. flaccidum the aerial shoot is markedly flattened.
The aerial bear small, scale-like leaves which remain more or less irregular in distribution. These leaves are without veins. These minute leaves are not found on the rhizomes and the extreme base of the aerial shoot.
Anatomy of Psilotum nudum
Anatomy of Aerial Stem
In section the aerial stem structure shows the well-marked epidermis, one cell in thickness. This layer consists of elongated cells possessing greatly thickened external walls. The stomata are found in the grooves of the surface.
The cortex consists of three zones. The outermost zone consists of cells which are thin-vertically elongated and having well-defined intercellular spaces among them. This zone is an assimilatory tissue and contains abundant chloroplasts. This zone is composed of two to five cells in breadth.
Inside the photosynthetic zone is a broad consisting of four to five layers of sclerenchymatous cells. This band of sclerotic cells represents the main mechanical tissue of the plant. Inside the sclerenchymatous zone there lies a zone of thin walled parenchymatous cells without intercellular spaces.
The well-marked endodermis surrounds the stele. There lies a core (pith) of sclerenchymatous cells in the center of the stele. This central sclerenchymatous core (pith) remains surrounded by a narrow band of xylem elements. The xylem cylinder that surrounds the sclerotic pith is stellate or star-shaped when seen in transverse section. The xylem is exarch and the protoxylem lies at the-tip of each ray. The metaxylem tracheids are composed of scalariform xylary elements while the narrow protoxylem elements show spiral thickenings. In between the endodermis and the xylem there lies a mass of thin-walled cells that represent the phloem. The phloem is of very indefinite character. The cells of the phloem are tubular but they do not possess the structure of sieve tubes. The pericycle is ill-defined. This is a siphonostelic type of structure, though the pith is sclerenchymatous and not parenchymatous as found in ferns.
In the upper part of the stem the central group of sclerenchyma is not formed and there are not xylem rays, so that the structure here is typically protostelic.
Anatomy of Leaf
The leaves receive no vascular tissue. They possess a very simple structure. It remains surrounded by a typical cuticularized epidermis. The stomata are not present. Inside the epidermis there lies the parenchymatous tissue which possesses well developed (e.g., in P. nudum) or little developed (e.g., in P. flaccidum) intercellular spaces.
Anatomy of Rhizome
The epidermis is inconspicuous. All the cells of the outermost cortical layer extend into two celled absorptive rhizoids.
The major portion of the cortex is thin-walled and mycorrhizal. However, outside the endodermis there are three or four layers of thick, brown-walled cells.
The stele is of protostelic type. It has no pith and the xylem mass is usually circular in outline and do not bear any protoxylem rays. There is very little phloem and the endodermis is conspicuous.
Apical Growth: It takes place by the activity of an apical cell both in the case of rhizome and aerial shoot.
Vegetative Reproduction In Psilotum nudum
The vegetative reproduction takes place by means of gemmae develop both on rhizomes (sporophyte) and prothalli (gametophyte).
(a) Gemmae on sporophyte
The gemmae develop freely on the rhizomes. Each such gemma on being detached develops into a new rhizome. In starved conditions the upper cell of many of the rhizoids developed on the rhizome divides and produces a small nodular gemma. These gemmae are small oval bodies’ one cell in thickness. The cells remain filled with starch.
(b) Gemmae on gametophyte
The gemmae also develop on the surface of the prothallus (Holloway, 1939, Bierhorst, 1953). In structure of these gemmae resemble those which have been developed on the rhizome. These gemmae on being detached develop into prothalli.
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Spore Producing Organs
The sporangia are borne in triads on minute appendages subtended by a bract. The sporangia remain fused with one another, and therefore, the group is called a synangium. The so-called synangium is generally interpreted (Bower, 1935, Eames, 1936) as a sporangiophore fused with a subtending bract. The sporangia seem to be borne on the adaxial side of the appendage at the point of dichotomy and are slightly raised on broad short stalks. The slender branches of the fertile appendage are more or less erect and embrace the synangium. The fertile appendages are mostly grouped on the upper part of the stem, but both these regions are not strictly limited and sterile appendages may occur here and there among the fertile ones.
Structure of Mature Synangium
The synangium is three lobed when examined from the outside internally it remains divided into three chambers. The wall is composed of several layers of cells. The sporangial wall cells have thickened considerably, except along one vertical line running from the distal end to the base of each sporangium. These are the lines of dehiscence, along which the synangium opens into three segments releasing spores. There is no true tapetum. The spores within the sporangium are bean-shaped with finely reticulated walls.
Development of Synangium
The development is apparently of the eusporangiate type. The earliest stages of development have shown, that each loculus arises separately from a single epidermal cell of the sporangiophore. It shows, that this structure is a union of three sporangia, the synangium. The primary initial divides periclinally separating a jacket initial and an archesporial initial.
The jacket initial gives rise to a wall three to five cells in thickness, while the archesporial initial
produces a central mass of sporogenous cells. Unlike most other pteridophytes, neither the outermost sporogenous cells nor the innermost jacket cells develop into a tapetum. The three sporangia are grouped around a central sterile tissue, the sporangiophore axis.
Division and redivision of the archesporial cell produce a large number of sporogenous cells. As the sporogenous tissue matures, some of the sporocytes are disorganized forming a fluid that nourishes the surviving cells and spores develop. The spores are bean shaped with finely reticulated walls. Simultaneously the cells thickened, except along one vertical line from the apex to the base of the sporangium; these act as the lines of dehiscence.
The Spore and its Germination
The spores are of equal size. Usually the spores are bean-shaped but some of them are found to be of the tetrahedral type. Each spore possesses cell a narrow ridge that joins the two ends of the curve. There is a median slit extending for about three fourth of its length. The surface spore is delicate and finely reticulate.
The spores germinate after about four months by rupture of its wall along the median slit and the spore contents-project as a small protuberance covered with a thin membrane, the intine. Later on the extruded portion is cut off by a transverse wall so that the basal portion remains with in the spore wall as a large spherical cell. The apical cell divides by oblique walls and a club-shaped filament is resulted. An apical meristem is established which apparently continues growth indefinitely resulting in a cellular body. This elongates into a cylindrical, slightly branched prothallus, covered with brown rhizoids. The prothallus grows by means of apical meristem. At this stage it closely resembles a portion of the rhizome. The prothallial tissue is colorless, saprophytic and mycorrhizal as in the sporophyte but it lacks vascular tissue, though cases are on record (Holloway, 1939) in which tracheids were found in large prothalli.
The gametophyte is monoecious that it bears both antheridia and archegonia. The organs are irregularly distributed on the prothallus. The number of archegonia is lesser that of antheridia.
Development of Antheridia
The antheridia appear first. They are prominently projecting, spherical bodies, with a wall of single layer of cells.
The antheridium arises from a superficial cell of the gametophyte the antheridium arises. This cell acts as the antheridial initial. Periclinally the antheridial initial divides into an outer cell, the jacket initial, and an inner cell, the primary androgonial. The primary androgonial cell divides and redivides forming a considerable number of androgonial cells, the last generation of which are androcytes. The androcytes metamorphose into spirally coiled multiflagellate antherozoids. In the meantime jacket initial also divides anticlinally forming a jacket layer one cell in thickness which lies spores ridge outside the outer face of androgonial tissue. Their lies a triangular opercular cell in the centre of the antheridium which by its disintegration provides an opening for the escape of antherozoids.
The Archegonium and its Development
The archegonium develops from a superficial cell of the gametophyte. This cell is known as archegonial initial. First of all this initial divides periclinally forming a primary cover cell
and a central cell. Thereafter the primary cover divides anticlinatly twice giving rise to four quadrately arranged neck initials. Now these neck initial divide periclinally forming an archegonial neck four to five cells in height and composed of four vertical rows of cells.
In the meantime, the central cell also divides periclinally forming a primary canal cell and primary venter cell. As the neck develops the primary canal cell elongates vertically. According to Holloway (1939) and (1917) the division of neck canal cell is uncertain it divides and gives rise to two neck canal nuclei which disintegrate later on. The behavior of the primary cell is also doubtful. It is reported that it functions directly as the egg and does not undergo the usual division forming venter canal, cell and egg.
As the archegonia approach maturity, the cell walls of the lowermost tier of neck cells become cutinized and thickened. With the result the sloughing off of the upper portion of the neck takes place. Simultaneously. the neck canal cell or cells also disintegrate bearing a passage way for the entrance of antherozoids in the venter of the archegonium. One of the antherozoids penetrates the egg and the fusion of male and female nuclei takes place forming the oospore.
Development of Embryo
The development of embryo is very simple. The fertilized egg enlarges downward. It divides periclinally and the upper cell develops into the axis, the lower cell into a foot, which sends out finger-like projections into the prothallial tissues. No root or cotyledon is formed. There is a marked line of division between axis and foot. When the developing axis breaks through the surface of the prothallus the foot and axis separate, the foot remains attached to the prothallus.
The first division in development of the epibasal cell into a foot is vertical. Thereafter the divisions take place in an irregular sequence in the two halves of the foot. As development of the foot continues it becomes a cylindrical structure of equal length and breadth. Superficial cells of the foot elongate into projections that enter the prothallus like haustoria.
The development of shoot takes place as follows. The epibasal daughter cell of the zygote divides vertically. The two daughter cells thus formed divide transversely. Subsequent divisions take place in irregular way. The short portion of a young embryo is hemispherical. It is differentiated vertically into two halves, derived, respectively from the first two cells of the shoot. The axis elongates vertically before it becomes free and develops an apical cell in one or in both halves of the young shoot. The apical cell always lie in the middle between base and apex of the half in which it is formed. Further development of the shoot takes place because of the activity of the apical cell or cells. When there are two apical cells the axis is dichotomous from the beginning, but in any case the first dichotomy soon takes place.
Early Stages Of The Development Of Embryo
In the early stages of the development of embryo the prothallus tissue also grows up like a calyptra round the young axis, but the growth activity of the embryo soon breaks through it. The protruding portion of the sporophyte soon develops rhizoids on its surface and becomes infected with a mycorrhizal fungus. Now this becomes a new independent plant. The rhizome continues to branch either dichotomously or by the formation of adventitious side branches. The original branch or some of the adventitious branches later on grow above the soil and develop into aerial shoots.
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