Exploring the Morphology, Life Cycle, and Evolutionary Significance of Psilopsida (Psilotum)

Class Psilopsida, represented by genera such as Psilotum and Tmesipteris, exhibits several general characters, classification features, and evolutionary trends. Let's break down the provided information.

 

General Characters:

1. Resemblances to Extinct Early Vascular Plants:

   • Members of Psilopsida share fundamental characteristics with extinct early vascular plants like Rhynia, Cooksonia, Horeophyton, and Psilophyton.

2. Rootless and Leafless Sporophyte:

   • The plant body is a sporophyte without roots and leaves.
   • Differentiation into an underground rhizome and an aerial erect shoot is observed.

3. Dichotomous Branching:

   • Both underground rhizomes and aerial erect shoots exhibit dichotomous branching.

4. Rhizoids and Mycorrhizal Association:

   • Rhizome attaches to the substratum through rhizoids, which absorb water and nutrients.
   • Rhizome forms a mycorrhizal association with fungal hyphae.

5. Aerial Appendages:

   • On the aerial shoot, there are scale-like (e.g., Psilotum) or leaf-like (e.g., Tmesipteris) appendages.
   • Appendages are arranged spirally.

6. Stele and Pith

   • Stele is protostelic or siphonostelic.
   • Sclerenchymatous pith is present.

7. Secondary Growth Absent:

   • No secondary growth occurs in the plant.

8. Homosporous Reproduction:

   • Spores are of equal size and shape, indicating homospory.
   • Sporangia are borne in groups, forming trilocular synangia.

9. Multiflagellated Antherozoids:

   • Antherozoids (sperm cells) are multiflagellated.

A Comparative Account of Evolutionary Trends:

• Resemblances between Psilotum and Tmesipteris:
  1. Growth Habit:
     • Both genera can grow in soil or as epiphytes.
  2. Plant Body Differentiation:
     • Both have a lower subterranean rhizome, giving rise to an aerial axis.
  3. Aerial Branching:
     • Aerial branches grow from rhizome apices.
  4. Rhizome Features:
     • Rhizome bears numerous multicellular rhizoids.
  5. Foliar Appendages:
     • Foliar appendages in both genera are arranged in spirals or pseudospirals, sometimes opposite.
  6. Stem Anatomy:
     • Similarities exist in the anatomy of stems in both genera.

6. Sporangium Development and Structure:

   • The development and structure of the sporangium are similar in both genera, although some differences exist.

7. Prothallus Characteristics:

   • The prothallus in both genera is subterranean and saprophytic.
   • It is covered with numerous rhizoids.

8. Sex Organ Development and Structure:

   • The structure and development of sex organs in both genera are similar.

9. Embryo Development:

   • The development of the embryo in both genera is similar to a greater extent.

10. Basic Chromosome Number:

    • The basic chromosome number in both genera is believed to be X=13.

11. Prevalence of Polyploidy:

    • Polyploidy is prevalent معمولی in both genera.

This comparative account highlights the shared features and evolutionary trends within the Psilopsida class, particularly between the genera Psilotum and Tmesipteris.

Differences between Tmesipteris and Psilotum:

1. Aerial Branching:

   • Tmesipteris: Aerial branches are sparingly branched and show only dichotomies.
   • Psilotum: Aerial shoots are profusely branched and exhibit repeated dichotomies.

2. Foliar Appendages:

   • Tmesipteris: Foliar appendages are larger in size, green, and possess a distinct single vein in the center.
   • Psilotum: Foliar appendages are minute and avascular.

3. Stomata and Xylem Development:

   • Tmesipteris: Stomata are present in the leaves, and xylem development is called Mesarch. (arrangement of xylem elements in a plant stem where the first-formed xylem is located in the center, surrounded by later-formed xylem layers.)
   • Psilotum: Stomata are absent, and xylem development is not specified.

4. Xylem Structure:

   • Tmesipteris: The xylem is not star-shaped, consisting of varying numbers of xylem groups near the apex. It is a siphonostele (a type of plant vascular tissue arrangement characterized by a central cylinder of conducting tissues (xylem and phloem) surrounded by a pith) in the middle, and the xylem is U-shaped or a protostele (a simple type of plant vascular tissue arrangement characterized by a solid core or cylinder of conducting tissues (xylem and phloem) without a pith. at the base).
   • Psilotum: The specific details about the xylem structure are not provided.

5. Sporangium and Tapetal Layer:

   • Tmesipteris: The sporangium is bilobed, and the tapetal layer is absent.
   • Psilotum: The sporangium is trilobed, and the tapetum is not specified.

6. Antheridia and Archegonia:

   • Tmesipteris: Antheridia are much more numerous than archegonia.
   • Psilotum: Details about the relative numbers of antheridia and archegonia are not provided.

7. Archegonia Arrangement:

   • Tmesipteris: Archegonia occur in densely populated groups.
   • Psilotum: The arrangement of archegonia is not specified.

8. Size of Antheridia and Archegonia:

   • Tmesipteris: Antheridia and archegonia are larger in size.
   • Psilotum: Details about the size of antheridia and archegonia are not provided.

9. Zygote Development:

   • Tmesipteris: The zygote enlarges sufficiently before dividing. The embryonic shoot possesses two apical cells.
   • Psilotum: Details about zygote development and embryonic shoot structure are not provided.

10. Aerial Shoot Termination:

    • Tmesipteris: The aerial shoot is terminated by a leaf.
    • Psilotum: Details about the termination of the aerial shoot are not provided.

The affinities and phylogeny (history of evolution) of psilophytes have been a subject of debate among botanists, leading to several theories explaining their position within the plant kingdom. Here are the four theories proposed:

1. Psilophytes-Psilotales Theory:

   • Proponents: Kidson and Lang (1917–1921)
   • Key Points:
     • Psilotales and psilophytes are closely related.
     • Similarities include rootless rhizomes, protostelic stems, small leaves, and dichotomously branched shoots.
     • Differences are primarily in the position and number of sporangia.
     • Fossil Psilophytales, like Rhyniaceae, exhibit similarities in vegetative traits, and fructification refers to the process of fruit formation in plants.

2. Algae-Psilotales Theory:

   • Proposed by: Arber
   • Key Points:
     • Psilotales have a late origin along independent lines from algae.
     • Considered an entirely distinct group from Lycopods or Sphenophyllales.

3. Psilotales-Sphenophyllales Theory:

   • Hypothesized by: A.P.W. Thomas, formerly suggested by Bower and Scott
   • Key Points:
     • Suggested an affinity with the Sphenophyllales.
     • Ruled out due to significant differences between Psilotales and Sphenophyllales.

4. Psilotales-Lycopods Theory:

   • Proposed by: Lawson (1917)
   • Key Points:
     • Psilotales are essentially lycopods.
     • Differences include bulged-out antheridia in Psilotales compared to embedded antheridia in Lycopods.

5. Psilotales-Ferns Theory:

   • Suggested by: Bierhost (1968, 1969-1970)
   • Key Points:
     • Psilotaceae (Psilotum and Tmesipteris) are included under the order Filicales.
     • Placed next to Stromatopteridaceae (a family of extinct fern-like plants known from the Late Devonian and Early Carboniferous periods, characterized by their distinctive winged structures.)
     • Similarities include underground axial gametophytes and indistinguishable underground saprophytic stems in both families.

     • Gametophyte Characteristics:

       • Similarity: The gametophytes bear septate rhizoids in both families.

     • Growth Periods:

       • Similarity: Alternate periods of growth in both the sporophyte and the gametophyte produce short and long cells.

     • Archegonia Dehiscence:

       • Similarity: The archegonia in both families was dehisced by the decapitation of the neck.

     • Embryo and Spore Characteristics:

       • Similarity: The embryos are rootless, and the spores are monolete. (refers to a type of spore that possesses a single, continuous groove or line, known as a monolete suture, which aids in the release of the spore from the sporangium.)

     • Sporangial Development:

       • Similarity: There are similarities in the development and wall structure of the sporangia.

     • Leaf and Pinna Development:

       • Similarity: The terminal leaf of Tmesipteris and the terminal pinna of Stromatapteris develop from the apical meristem and consume it.

     • Antheridia Characteristics:

       • Similarity: large superficial antheridia with lateral opercular cells, which break down at maturity.

     • Vascular Tissue in Smaller Axes:

       • Similarity: discontinuous vascular tissue in the smaller axes of the sporophyte. Synchronous xylem maturation occurs in smaller subterranean stems.

     • Xylem Characteristics:

       • Similarity: Mesarch xylem without protoxylem elements in the large subterranean stems.

     • Stem Dichotomies:

       • Similarity: Successive dichotomies with determinate apices in the vertical subterranean stem system.

     • Chromosome Behavior:

       • Similarity: A laxity لچک of chromosomes coiling at meiosis so that chromosomes in anaphase resemble early prophase ones, and nucleoli at mitosis persist up to the anaphase stage.

Phylogeny of Psilophytes:

The affinities of two classes of Psilophyta have been discussed in detail. The division Psilophyta is of great evolutionary significance as it includes two closely related classes of extinct and living plants (Psilophytopsida and Psilotopsida). These are the oldest known simple vascular plants that possess no roots and are either leafless or possess rudimentary leaves. They have been regarded as connecting links between the aquatic algae and the terrestrial and more complex vascular plants like the lyopods, horsetails, and ferns.

This division of plants offers a number of pieces of evidence that have great evolutionary importance. These can be listed as:

1. They throw light on the origin of the root, as suggested by Prof. Lignier, from that portion of the axis that became negatively geotropic and entered the soil, where it became adapted to perform the functions of anchorage and absorption. The rootless and subterranean rhizomes of Psilophytales and Psilotales offer great support for this because they grow beneath the surface of the soil and serve to fix the plant and absorb nutrition.
2. They also suggest the origin of the microphyllous and megaphyllous leaves of lycopods and ferns. The leaf-like emergence of Asteroxylon and Psilotum might have developed into Protopteridium; the leaves are formed by flattening of the branch system and are supposed to have led to the development of large and megaphyllous leaves of ferns.
3. The anatomy of the shoot in the Psilophytales also suggests that protostele is the primitive type of vasculature and that siphonostele has originated from it by the appearance of pith.
4. A comparative study of the Psilophytales and the higher vascular plants also suggests the establishment of four distinct subdivisions, namely, Psilopsida, Lycopsida, Sphenopsida, and Pteropsida. The Psilophytales clearly indicate the three lines of evolution that led to the development of Lycopsida, Sphenopsida, and Pteropsida. This line of evidence suggests that plants with microphyllous and megaphyllous leaves have separate lines of evolution. The seed plants are included in the Pteropsida, along with ferns, because they have megaphyllous leaves. The ferns are closer to the seed plants (spermatophyta) than to the Lycopsida and the Sphenopsida. The evidence leads us to conclude that the psilophytes have originated from the green algae and have in turn given rise to the three subdivisions of the vascular plants, namely, the Lycopsida, Sphenopsida, and Pteropsida. It is also believed that the bryophyta also originated from green algae. It is also believed by some botanists that bryophytes may have originated as a result of retrogressive evolution accompanied by reduction from psilophytes. The Psilotales are regarded as the last living descendants of the Psilophytes. They have also been aptly regarded as living fossils.

PSILOTUM:

Psilotum is the most widely distributed genus of the Psilopsida class. The plant body is perennial and a slender shrubby. It commonly grows in humus-rich soil in tropical and subtropical regions. Two species of this genus have been discovered so far. These are Psilotum nudum and P. flaccidum. However, the most common and widely distributed species is P. nudum. It is commonly known as whisk fern. Some species grow as epiphytes. P. nudum may be terrestrial in habitat and grow in exposed rock crevices. The terrestrial forms of P. nudum grow erect and are dwarf, whereas epiphytic forms grow pendent, having 75–100 cm of height. P. flaccidum possesses a flattened stem and, in this way, can be differentiated from P. nudum.

The plant body is a rootless and leafless sporophyte and consists of a dichotomously branched underground rhizome and an upright aerial axis. Roots are absent, and instead of these, rhizoids are present, which are covered with cuticle and perform dual functions such as anchoring and absorption. The epidermal cells produce a lateral protuberance (outgrowth-like) that grows to form 1-3-celled rhizoids. The rhizome harbors an endophytic fungus. The endophytic fungus penetrates hyphae within cortical cells that come out throughout rhizoids and probably aid in absorption. The aerial branches are green, upright, slender, and dichotomously branched in three dimensions. However, in some cases, trichotomous branches were also reported. These are the principal photosynthetic organs because leaves are mere emergences and are without vascular supply. The aerial branches are developed from the rhizome. The branch tip of the rhizome turns upward and grows to form aerial branches through apical growth. Light is considered a major causative agent in inducing the formation of aerial branches from the rhizome. However, sometimes deeply seated tips also grow from the soil to form an aerial branch. The aerial axis may be cylindrical at the base, furrowed in the upper parts, but somewhat flattened with three longitudinal ridges at the top. The basal part is smooth, but the distal part bears small, scaly appendages and synangia. The appearance of the aerial axis is xerophytic; however, plants grow in moist conditions.

There are two unique features exhibited by the gametophyte of Psilotum:

1. The mature gametophyte shows a striking similarity to a piece of sporophytic rhizome. In the gametophyte of tetraploid P. nudum, the center is occupied by xylem with annular, scalariform, and reticulate tracheids, surrounded by phloem and an endodermis. Thus, Psilotum is the only plant in the plant kingdom where the vascular tissues develop in the gametophytic generation.

2. The external resemblance of the sporophytic rhizome and gametophyte, coupled with the presence of vascular tissue in the gametophyte, supports the homologous theory on the origin of the alternation of generations.

Here is the provided information, with key terms bolded:

### Life Cycle of Psilotum:

**Psilotum** life cycle shows regular heteromorphic alternation of generations. Both the generations (sporophyte and gametophyte) are morphologically dissimilar, and both can reproduce by vegetative means by producing **gammae**. Gammae are small, multicellular, and ovoid structures developing on the surface of the rhizome (in sporophyte) or prothallus (in gametophyte).

After detachment from the parent body, gammae of sporophyte may germinate to form a subterranean shoot, while the gammae of prothallus, on germination, form a new prothallus. The gametophytes of Psilotum are monoecious (i.e., homothallic). Sex organs, i.e., **antheridia** and **archegonia**, are superficial and scattered over the surface of the gametophyte. Generally, antheridia are more numerous than archegonia.

The antheridium contains **androcytes** surrounded by a jacket of sterile cells protective in nature. Each androcyte eventually becomes a spirally-coiled, multiflagellate **antherozoid** and escapes from the antheridium for fertilization. The archegonium is structurally similar to that of other land plants and produces a large egg in the center. At maturity, the cell wall of the lower tier of neck cells becomes thick-walled and cutinized. The apical tier, however, breaks away in the presence of water, and the mucilaginous contents of the neck cells are released. Thus, a free passage is formed for the entry of the antherozoids.

Fertilization is accomplished by the union of a multiflagellate sperm and egg, resulting in the formation of a diploid zygote, which is the mother cell of the sporophytic generation.

 

https://www.plantscience4u.com/2014/04/lifecycle-of-psilotum.html

An Explanation of Important Terms

1. Stele:

   • The term "stele" refers to the central cylinder or core of vascular tissue in the stems and roots of higher plants. It is a structural component that includes both the xylem and phloem tissues and is responsible for the transport of water, nutrients, and sugars throughout the plant. The arrangement and organization of vascular bundles within the stele can vary among different plant species.

2. Pith:

   • The "pith" is a soft, spongy, and often cylindrical or centrally located tissue found in the center of certain plant stems. It is composed of parenchyma cells and is surrounded by vascular tissues, such as xylem and phloem. The pith functions for storage and support. In some plants, especially in herbaceous stems, the pith may be prominent, while in woody stems, it can become less conspicuous over time as secondary growth occurs.
3. A trilocular synangium is a three-chambered structure found in certain fern allies like Psilotum, housing sporangia that produce spores for reproduction. The term "trilocular" signifies the presence of three compartments within the synangium.

 

• Unlocking Nature's Treasure: A Comprehensive Guide to the Economic Significance of Algae - Solved MCQs with In-Depth Explanations

• Unraveling Algae Kingdom: A Comprehensive Exploration through Fritsch, Smith, and Lee's Classifications

•