Psilotum – Structure, Morphology, Anatomy, Reproduction, Life Cycle

Distribution of psilotum

The distribution of Psilotum, commonly referred to as whisk fern, primarily encompasses two species: P. nudum and P. flaccidum. Understanding their geographic spread and ecological niches offers valuable insights into their adaptability and survival strategies.

• Psilotum nudum is the more widespread of the two species.
  • It is distributed throughout tropical and subtropical regions, with notable occurrences in various locations including Florida, Hawaii, New Zealand, and parts of India, such as Assam, Bengal, and Kulu.
  • This species is characterized by its erect and slender form, typically reaching heights of up to 25 cm.
  • P. nudum commonly inhabits crevices in rocks, which provide a stable microenvironment conducive to its growth.
  • Additionally, it can occasionally be found as an epiphyte, thriving on other plants without deriving nutrients from them.
  • Due to its unique characteristics, P. nudum is often cultivated in greenhouses and botanical gardens, showcasing its appeal in horticultural settings.
• In contrast, Psilotum flaccidum is a rarer species with a more limited distribution.
  • This species can be found in Mexico, Jamaica, and several Pacific islands, where it inhabits similar ecological niches as P. nudum but is less commonly encountered.
  • P. flaccidum is notable for its pendulous growth form, which can extend up to 90 cm in length, allowing it to adapt to various vertical spaces within its environment.
  • It differs from P. nudum primarily through its flattened stem structure, which presents distinct morphological characteristics that are key to its identification.
• Both species typically exhibit epiphytic growth patterns, predominantly found on trees.
  • They often colonize the trunks of coconut trees or grow near the bases of larger trees, where they benefit from the moisture and nutrients available in the microhabitat provided by their hosts.

The occurrence of Psilotum

The occurrence of Psilotum, commonly known as whisk fern, is significant due to its unique adaptations and ecological roles. This plant is characterized by its leafless stem, which performs all the necessary functions for survival and reproduction. The following points elucidate the primary aspects of Psilotum‘s occurrence.

• Psilotum is recognized for its distinctive morphology, often described as a fern without leaves.
  • The absence of leaves allows the stem to engage in photosynthesis, a critical function that supports its growth and energy needs.
  • This adaptation highlights the plant’s evolutionary strategies, as it thrives in environments where traditional foliage might not be as advantageous.
• This genus is primarily found in humus-rich soils.
  • These soils, abundant in organic matter, provide essential nutrients that facilitate the growth and development of Psilotum.
  • The presence of humus contributes to moisture retention, which is vital for the plant’s survival, especially in the often warm and humid tropical and subtropical climates where it is commonly located.
• The occurrence of Psilotum is closely linked to specific geographic regions.
  • It predominantly inhabits tropical and subtropical areas, where environmental conditions align with its growth requirements.
  • These regions offer the warm temperatures and high humidity levels that support the health and reproduction of whisk ferns.
• Some species within the Psilotum genus exhibit epiphytic growth.
  • As epiphytes, they can be found growing on the trunks of trees, utilizing the vertical space offered by their hosts.
  • This growth strategy allows them to access sunlight and moisture that may be less available at ground level, showcasing their adaptability to various ecological niches.

External Morphology of Psilotum

The external morphology of Psilotum, commonly known as whisk fern, showcases its unique adaptations and structural characteristics that enable its survival and reproduction. The plant body is predominantly sporophytic and consists of two main components: the subterranean rhizome and the erect aerial shoot.

• Rhizome
  • The rhizome of Psilotum is cylindrical in shape and exhibits an irregular, extensively branched, and dichotomous structure.
  • It typically grows in a prostrate manner, allowing it to occupy space within the soil.
  • Characteristically, the rhizome is brown and lacks true roots; instead, it features rhizoids.
  • Rhizoids serve essential functions, including absorption of nutrients and anchorage in the substrate.
  • Additionally, the rhizome is often associated with an intracellular mycorrhizal fungus. The hyphae of this fungus penetrate through the rhizoids to reach the cortex, facilitating nutrient exchange and enhancing the plant’s ability to thrive in nutrient-poor soils.
  • Immature rhizoid tips can occasionally develop into gemmae, which play a crucial role in vegetative propagation, allowing the plant to reproduce asexually.
  • As the rhizome branches approach the soil surface and are exposed to light, they ultimately transform into aerial shoots.
• Aerial Shoot
  • The aerial shoots are slender, green, and upright, featuring a ridged surface and a dichotomous branching pattern.
  • In terrestrial forms of Psilotum, these shoots grow upright; however, in epiphytic forms, they hang down from their host plants, adapting to their environments.
  • The basal portion of the aerial shoot is generally smooth and cylindrical, while the distal part exhibits distinctive features: P. nudum has longitudinally ribbed surfaces, whereas P. flaccidum displays a flattened morphology.
  • Aerial shoots of P. nudum are relatively short, ranging from 15 to 20 cm in height, while those of P. flaccidum can reach lengths of up to 90 cm.
  • In the absence of true leaves, these green aerial shoots perform the vital function of photosynthesis, compensating for the lack of foliage by utilizing their extensive surface area for light absorption.
• Appendiages
  • The aerial shoots are devoid of true leaves but bear small, scale-like appendages. These appendages do not contain vascular traces or stomata, reflecting their limited physiological role compared to true leaves.
  • In P. nudum, these appendages are arranged spirally or may be distributed irregularly along the stem, contributing to the plant’s overall appearance.
  • Conversely, P. flaccidum exhibits appendages arranged in subopposite pairs, a distinguishing feature of this species.
  • It is noteworthy that appendages are absent on the rhizome and at the extreme basal parts of the aerial shoots.
  • In the terminal region of the shoots, fertile appendages can be observed, which contain trilobate sporangia known as synangia in their axils. These structures are essential for the reproductive cycle of the plant, as they house spores critical for propagation.

Anatomy of Psilotum

The anatomy of Psilotum, or whisk fern, is characterized by a well-defined structure that supports its physiological functions and adaptations to its environment. This plant comprises three primary components: the aerial shoot, the rhizome, and the appendages. Each of these components exhibits unique anatomical features that contribute to the overall functionality of Psilotum.

• Aerial Shoot
  • The transverse section of the aerial shoot presents an irregular outline, primarily due to the presence of ridges and grooves. It consists of three main parts: the epidermis, cortex, and stele.
  • Epidermis
    • The epidermis is a well-developed, single-layered structure.
    • Cells within the epidermis are elongated and protected by a thick cuticle, which aids in water retention.
    • Sunken stomata, indicative of xerophytic characteristics, are present.
    • These stomata lack subsidiary cells, a condition comparable to those observed in gymnosperms.
  • Cortex
    • The cortex is well-developed and divided into three distinct zones:
      • Outer Cortex: Comprising 2 to 5 layers of thin-walled, elongated, chlorophyll-containing cells, this zone features intercellular spaces and serves a photosynthetic function.
      • Middle Cortex: This layer consists of 4 to 5 layers of closely packed, thick-walled sclerenchymatous cells, providing mechanical support to the aerial shoot.
      • Inner Cortex: The broadest zone consists of thin-walled parenchymatous cells that lack intercellular spaces. These cells are rich in starch reserves, contributing to the plant’s energy storage.
    • The endodermis is well-developed and contains Casparian strips, which regulate the movement of water and nutrients into the vascular system.
  • Stele
    • In the rhizome, the stele is a protostele featuring a central core of xylem completely surrounded by phloem. This xylem core is neither star-shaped nor lobed, and there is no pith present.
    • In the transition zone between the rhizome and the aerial shoot, the xylem becomes lobed, with as many as ten lobes observable. Here, the protoxylem occupies the tips of the lobes, while the phloem occurs in irregular patches between them. At this stage, the stele is referred to as actinostelic protostele, still without pith.
    • In the middle section of the aerial shoot, the number of xylem lobes is reduced to 5 or 7, and pith appears at the center, marking the change from protostele to siphonostele.
    • Throughout these variations, the xylem is exarch and consists solely of tracheids. The protoxylem contains spiral tracheids, while the metaxylem features scalariform or pitted tracheids.
    • The phloem is composed of sieve tubes and phloem parenchyma, with vertically elongated sieve tubes that exhibit slight lignification and possess sieve plates.
• Rhizome
  • The anatomy of the rhizome is somewhat similar to that of the aerial shoot, but very small rhizomes (less than 1 mm in diameter) lack vascular tissues and are uniformly parenchymatous. As the diameter increases, differentiation of the stele occurs, particularly in rhizomes over 2 mm in diameter.
  • The structures observable in larger rhizomes include:
    • Epidermis: A single-layered and uninterrupted structure made of thin-walled cells.
    • Cortex: This region is well-developed and differentiated into three layers:
      • Outer Cortex: Contains numerous hyphae from endophytic mycorrhizal fungi, which assist in nutrient absorption.
      • Middle Cortex: Comprises parenchymatous cells that store starch.
      • Inner Cortex: Composed of two to four layers of cells that are brown due to the deposition of phlobaphene, a compound formed from the oxidation and condensation of tannins.
    • Endodermis: This layer is distinct and contains Casparian strips, enhancing the regulation of water and nutrient uptake.
    • Pericycle: A single-layered structure composed of parenchymatous cells.
    • Stele: It is a protostele where the xylem is surrounded by phloem, but there is no differentiation between metaxylem and protoxylem, and pith is absent.
• Appendages
  • The appendages of Psilotum are anatomically simple, consisting of an epidermis and mesophyll.
    • Epidermis: Characterized by a single-layered, cutinized structure with absent stomata, which reflects its adaptation to its environment.
    • Mesophyll: Composed of chlorophyll-containing parenchymatous cells that lack differentiation into palisade and spongy tissues.
    • There are more intercellular spaces in Psilotum nudum compared to Psilotum flaccidum, indicating a difference in cellular arrangement.
    • Given the absence of stomata and vascular traces, the appendages do not function in photosynthesis.

Reproduction of Psilotum

The reproduction of Psilotum, commonly known as whisk fern, occurs through both vegetative means and by the production of spores. Understanding these processes provides insight into the life cycle and ecological strategies of this unique plant.

• Vegetative Reproduction
  • Vegetative reproduction in Psilotum is achieved through the formation of ovoid, minute, one-cell-thick, multicellular outgrowths known as gemmae. These structures develop on the rhizome and contain starch, serving as an energy reserve.
  • Gemmae can germinate while still attached to the parent plant or after detaching and falling onto a suitable substrate. This flexibility enhances the plant’s capacity to colonize new areas.
  • Similar gemmae-like structures can also form on the gametophyte. These structures are functionally analogous to those on the sporophyte and germinate to produce new prothalli, thus perpetuating the reproductive cycle.
  • Furthermore, vegetative reproduction occurs through the natural death of older parts of the rhizome. As the older portions die, younger parts separate and grow into independent plants, ensuring the continuity of the species.
• Reproduction by Spores
  • As Psilotum matures, it engages in spore production, a process that is vital for its reproductive strategy. Spores are generated within trilobate sporangia, referred to as synangia, which are formed by the fusion of two or more sporangia.
  • Each synangium develops on a small fertile appendage, which is supported by a forked bract. Collectively, these structures constitute a sporangium complex.
  • Structure of Synangium
    • The synangium is a trilocular (three-chambered) spore-bearing structure. It represents the fusion product of three sporangia and is located in the axils of fertile appendages on the aerial shoot.
    • The wall of the synangium consists of 3 to 4 layers. The thick outer wall forms the epidermis, while the inner wall separates the three chambers. Each locule is filled with a large number of homosporous, bean-shaped spores.
    • Spores are released when the synangium splits along three longitudinal lines of dehiscence, facilitating dispersal.
  • Development of Sporangium
    • The development of the sporangium in Psilotum is classified as eusporangiate. In the early stages, each of the three sporangia develops from a single epidermal cell.
    • This initial epidermal cell divides periclinally to yield an outer primary jacket cell and an inner primary archesporial cell. The jacket cell then undergoes multiple divisions, resulting in a 4 to 5 cells thick jacket layer, while the archesporial cell divides to form sporogenous cells, which differentiate into sporogenous tissue.
    • Notably, Psilotum differs from many other pteridophytes in that neither the outermost sporogenous cells nor the innermost jacket cells develop into a tapetum.
    • Some cells within the sporogenous tissue continue to divide and differentiate into spore mother cells (SMCs), while others contribute to a plasmodial mass of spores that nourishes the sporocytes. Meiosis occurs within the SMCs, resulting in the formation of spore tetrads, which are homosporous, colorless, and kidney-shaped.
    • The spores are subsequently dispersed along the dehiscence line of the sporangium.
• Spores in Psilotum
  • The spores produced by Psilotum are homosporous, with mature spores exhibiting a distinctive kidney shape, averaging 0.065 to 0.032 mm in size.
  • Each spore has two curved ends connected by a narrow slit, and thick, smooth borders, referred to as lips, are present on either side of the slit. The outer coat is known as the exine, while the inner coat is termed the intine.
• Development and Structure of Gametophyte
  • The germination of the spores is a gradual process, taking approximately 3 to 4 months. Upon absorbing water, the germinating spore swells, increasing internal turgor pressure and causing the exine to rupture along the median slit.
  • As a result, the inner contents, encased by the thin intine membrane, project outward as a conical mound. Following this, a transverse wall forms, separating a basal, large spherical cell from the upper extruded cell.
  • The upper cell subsequently divides through two intersecting oblique divisions to produce an apical cell. This apical cell gives rise to the prothallus, the gametophyte stage of Psilotum.
  • The prothallus is a simple, irregularly dichotomizing, cylindrical structure that measures 0.5 to 2 mm in diameter and can reach lengths of up to 18 mm. It ranges in color from pale yellow to dark brown and is covered with hair-like rhizoids.
  • Scattered over the surface of the prothallus are the male and female sex organs, antheridia and archegonia, which develop without a specific arrangement. In early development stages, the prothallus is penetrated by an endophytic fungus that aids nutrient absorption, rendering the prothallus saprophytic and lacking vascular tissues.
• Sex Organs
  • The gametophyte is monoecious, meaning it possesses both male and female sex organs.
  • Antheridium: This male organ is emergent, exhibiting a semi-spherical shape. It is surrounded by a well-defined, single-layered jacket, enclosing a mass of spirally coiled, multiflagellate antherozoids, which are unicellular and uninucleate. The jacket contains only one opercular cell, which, upon disintegration, creates a passage for the liberation of the antherozoids.
  • Archegonium: This female organ is generally embedded within the prothallus and features a projecting neck. The neck typically consists of six tiers of four cells each, containing two neck canal nuclei and a venter region, which houses a venter canal cell and an egg cell. Two cover cells are positioned at the neck’s tip.
• Fertilization
  • Fertilization in Psilotum requires water, as antherozoids are attracted to the archegonium through chemotactic responses. A chemical substance released from the open archegonium serves as a sperm attractor, containing significant amounts of organic and inorganic compounds, particularly malic and fumaric acids.
  • The neck canal nuclei and venter canal nuclei disintegrate, creating a free passage for the entry of antherozoids. During fertilization, antherozoids swim down this canal to fuse with the egg, resulting in the formation of a diploid zygote.
• Development of Sporophyte
  • The zygote marks the initiation of the sporophytic generation. As it enlarges, it fills the venter cavity and divides into an outer epibasal cell and an inner hypobasal cell.
  • The epibasal cell undergoes repeated divisions, forming the shoot, while the hypobasal cell develops into the foot. This embryogenic process, where the shoot-forming cell is directed towards the neck of the archegonium, is known as exoscopic embryogeny.
  • After the epibasal and hypobasal cells are established, both undergo longitudinal divisions to create four cells, with further irregular divisions following (Holloway, 1939). Some cells within the foot undergo transverse divisions, forming finger-like projections that penetrate the gametophytic tissue.
  • Regular divisions of the epibasal cell lead to the formation of a three-sided apical cell, which subsequently branches dichotomously, giving rise to aerial shoots. This process culminates in the development of a new sporophytic plant, completing the life cycle of Psilotum.

Life cycle of Psilotum

The life cycle of Psilotum, commonly known as the whisk fern, illustrates a fascinating alternation between two distinct generations: the diploid sporophyte and the haploid gametophyte. This cycle is characterized by specific structures and processes that ensure the continuation of the species.

• The plant body of Psilotum is a diploid sporophyte, which represents the dominant phase of its life cycle.
• Within the sporophyte, the synangium serves as the trilobate sporangium that bears the spores necessary for reproduction.
• Inside the synangium, diploid spore mother cells (SMCs) undergo meiosis, a critical process that reduces the chromosome number by half, resulting in the formation of haploid spores.
• These haploid spores subsequently germinate, giving rise to the haploid gametophyte known as the prothallus. This prothallus is monoecious, meaning it possesses both male and female reproductive organs.
• The male reproductive organ, called the antheridium, produces antherozoids (sperm), while the female reproductive organ, the archegonium, produces eggs.
• Fertilization occurs when antherozoids swim toward the archegonium, facilitated by water, to fuse with the egg, leading to the formation of a diploid zygote.
• The zygote develops into an embryo, which then matures into the diploid sporophyte, completing the life cycle of Psilotum and allowing the cycle to repeat.

This cyclical process emphasizes the significance of both generations in the life cycle of Psilotum, highlighting the transitions between the haploid and diploid stages and their roles in the plant’s reproduction and survival.