Psilotum is a genus in the order Psilotales. This fern-like vascular plant is mainly found in tropical and subtropical regions.
Psilotum is commonly known as whisk fern due to its whisk-like upward branches and lack of fern.
Distribution of Psilotum
Psilotum is a genus of two species. These two species are well-defined but polymorphic (having two or more different morphs or forms arising from the same genotype).
P. nudum (Syn. P. teiquertum) is very common and widely distributed in tropical and sub-tropical regions of both the eastern and western hemispheres. But P. flaccidum (Syn. P. complanatum) is a rare species found only in tropical islands such as Mexico, Java, Jamaica, Bermuda, etc.
Psilotum is a xerophytic plant, although it grows in various habitats.
The species of Psilotum usually occur on tree ferns and palms as epiphytes. They may also grow terrestrially in soil or in humus-rich crevices of rocks.
P. nudum is a predominantly terrestrial species, while P. flaccidum mainly grows as an epiphyte.
Sporophyte of Psilotum
The plant body of Psilotum is sporophytic.
External Morphology of Psilotum
The sporophytic plant body is differentiated into two parts: a subterranean branched rhizome and an erect aerial stem.
Rhizome
The rhizome is the horizontally growing underground portion of the sporophyte. It is dichotomously branched and brown in color. The branching may be lateral or irregular.
The rhizome contains an endophytic fungus and is covered with many long, multicellar (i.e., 2-3 celled), hair-like rhizoids instead of roots.
Rhizoids anchor the plant body and also help in absorbing water and minerals from the soil.
Aerial Stem
The aerial stem is slender and may be erect (P. nudum) or pendulous (P. flaccidum). It arises from the rhizome. The tip of any branch of the rhizome turns upward and undergoes several dichotomies (apical growth) to form a green aerial shoot.
The aerial shoots are perennial, regularly dichotomously branched, and green in colour. They function as photosynthetic organs.
The basal part of the axis is cylindrical, while the upper green portion may be longitudinally ribbed (e.g., P. nudum) or flattened (e.g., P. flaccidum).
The aerial branches bear numerous small, scale-like appendages, which are often called leaves. The leaves are scattered over these branches.
A few leaves are restricted to the upper portions and bear a group of sporangia (spore-bearing structures) in their axil, which is called the synangium.
Internal Morphology of Psilotum
Internal morphology, or the internal structure of the plant body, is the anatomy of Psilotum.
Anatomy of Aerial Stem
A transverse section of the aerial stem is circular in outline from the base, pentagonal near the first dichotomy, and triangular between successive dichotomies.
Internally, the stem of Psilotum is differentiated into the outermost epidermis, followed by the extensive cortex and the central stelar region.
Epidermis
The epidermis is the outermost layer of the aerial stem. It is single-layered and consists of closely packed cells. The outer tangential cell walls are heavily cutinized and covered by a layer of cuticle, which is composed of cutin and waxes.
The epidermis is interrupted regularly by stomata. The stomata are sunken in nature and are restricted to the grooves between the longitudinal ridges. They have no subsidiary cells.
Cortex
The extensive cortex is differentiated into three zones: the outer, middle, and inner cortex.
The outer cortex is chlorenchymatous and situates directly beneath the epidermis. It is 2–5 layers thick and consists of loosely arranged, slightly lobed, thin-walled, elongated chlorophyllous cells with intercellular space. The cells of the outer cortex contain starch grains and chloroplasts.
The middle cortex lies internally next to the outer cortex. It is composed of 4-5 layers of vertically elongated, thick-walled sclerenchymatous cells with little or no intracellular spaces. The walls of these cells apparently become lignified in the basal portion of the aerial stem. The cells of the middle cortex provide mechanical support to the stem.
Further inward, there is the inner cortex, which forms the major portion of the stem. It is made up of a few layers of parenchymatous cells without intracellular spaces. These cells show a gradual reduction in the thickness of cell walls towards the centre (i.e., the stelar region). The cells of the inner cortex contain more starch grains.
The cortex is internally delimited by a single-layered endodermis, which consists of vertically elongated cells. These cells have conspicuous casparian strips on the radial and end walls.
Stelar Region
The centre of the aerial stem is occupied by a lobed or flattened cylinder of primary xylem with protoxylem elements at the tip of each lobe. The xylem cylinder may have as many as ten lobes near the transition region from rhizome to aerial stem. The number of lobes is reduced to 5 or 6 in the more distal parts of the stem.
A single-layered pericycle is present just below the endodermis, which consists of thin-walled parenchymatous cells.
The nature of the stele varies in the different portions of the aerial stem. At the extreme base of the stem, the stele is the actinostelic protostele. In the middle part, the stele is siphonostelic, as the centre of the xylem is occupied by a sclerotic pith (a patch of elongated sclerenchymatous cells).
The xylem is exarch (in which protoxylem is directed towards the periphery and metaxylem towards the centre) and composed of only tracheids. The tracheids are scalariform or pitted in the metatoxylem, while the protoxylem has spiral tracheids.
The phloem is internal to the pericycle, lies between the lobes of the xylem, and forms irregular patches. It is composed of sieve tubes and phloem parenchyma.
Anatomy of Rhizome
A transverse section of the rhizome also shows an outermost epidermis, a cortex, and a stelar region.
The epidermis is inconspicuous and consists of thin-walled cells. Rhizoids appear as prolongations of the epidermal cells. Stomata are absent.
The cortex is extensive and divisible into three zones. The outer cortex is parenchymatous and characterised by the presence of intracellular endophytic mycorrhizal fungus.
The middle cortex also consists of parenchymatous cells. But the cells are large and rich in starch grains.
The parenchymatous cells of the inner cortex are often dark brown in colour due to the presence of phlobaphene (an oxidation product of tannins).
The stele is protostelic (haplostelic or actinostelic type) and is surrounded by a distinct endodermis, which is followed by a single-layered pericycle.
The xylem is exarch and completely enclosed by phloem.
Internal Structure of Leaf or Appendage
Structurally, the leaf or appendage of Psilotum is composed of parenchymatous chlorophyllous cells (mesophyll) surrounded by a single layered cutinized epidermis. Stomata are absent in the epidermal layer.
There is no vascular trace in the leaf of P. nudum. But in P. flaccidum, a minute leaf trace appears from the stem, which terminates at the leaf base.
Reproduction in Psilotum
Psilotum shows the alternation of generations of the sporophytic phase and the gametophytic phase. Both sporophytic and gametophytic generations are independent.
The sporophyte of Psilotum reproduces vegetatively as well as asexually. Sexual reproduction occurs in the gametophyte.
Vegetative Reproduction
Vegetative reproduction is usually seen in the sporophyte of Psilotum. Gametophyte also reproduces vegetatively.
In P. nudum, vegetative reproduction occurs through the formation of gemmae or brood bodies (Holloway, 1939; Bierhorst, 1954).
The gemmae are small, multicellular, and oval structures. They are produced in large numbers on the surface of the rhizome in the sporophyte or on the prothallus in the gametophyte. The cells of each gemma are thin-walled and rich in starch grains.
The gemmae of the sporophyte get detached from the rhizome and germinate into new sporophytes. Although the gemmae of gametophyte, after detachment, give rise to new prothallus.
Asexual Reproduction
Asexual reproduction in Psilotum takes place in the sporophytic plant body. It occurs through the formation of spores (i.e., asexual reproductive units).
The spores are produced in complex trilobed sporangia called synangium.
Synangium
Synangia are the asexual reproductive organs in Psilotum.
They are borne in the axils of minute bifid leaves on the upper portions of the aerial branches.
Development of Synangium
In P. nudum, the development of each sporangium of the synangium is of the eusporangiate type (arising from several epidermal cells, not from a single cell).
The sporangium develops from a group of superficial initial cells (Bower, 1935). Each initial cell divides periclinally to form an outer primary jacket initial and an inner primary sporogenous cell.
The primary jacket initial undergoes a number of periclinal and anticlinal divisions to form a four to five layered thick wall of the synangium.
Meanwhile, the sporogenous cell divides in various planes and forms a mass of sporogenous cells. Some sporogenous cells later develop into spore mother cells. These cells contain dense, granular cytoplasm. The remaining sporogenous cells gradually degenerate.
Each spore mother cell is diploid and undergoes reduction division (i.e., meiosis) to produce four haploid spores, arranged tetrahedrally (spore tetrad).
Structure of Synangium
The mature synangium is a short-stalked, trilobed structure. Each lobe of the synangium corresponds to a single sporangium.
A distinct bilobed, curved foliar appendage is present at the base of the synangium, which surrounds the synangium stalk. The stalk is about 1–2 mm in diameter.
The wall of the synangium is composed of 4–5 layers of cells. The outer wall is thick and consists of relatively long cells. The inner wall separates the three locules, or chambers. Each of the three locules is filled with numerous spores.
Dehiscence of Synangium
At maturity, the synangium dehisces along three vertical lines and liberates the spores.
Spores
Psilotum is homosporous (i.e., it produces spores of one kind) in nature. The spores are of equal size and shape.
Each spore is a bilaterally symmetrical, colourless, kidney-shaped structure. It remains surrounded by two wall layers: the outer exine and the inner intine. The exine is thin and reticulate.