Systematics, Reproduction and Phylogeny of Psilotum
Psilotum:-
1. Systematics:-
Division:- Psilophyta
Class:- Psilotopsida
Order:- Psilotales
Family:- Psilotaceae
Genus:- Psilotum
2. Habit and Habitat:-
> It is commonly called as whisk fern (because it is without fern and stem perform all function).
> Found in humus rich soil , in tropical and sub tropical regions.
> Some species grows as epiphytes ( tree trunk).
3. Study of sporophyte:- It is sporophyte and contains following parts:
i. Rhizome:-
> The horizontal portion is rhizome.
> Buried in soil or humus.
> Dichotomously branched.
> 2 celled rhizoids are present near the apices of the younger branches.
> These rhizoids absorb water and nutrients from soil for aerial branches.
ii. Aerial branch:-
> Rhizome bears aerial branches
> The branches are green, cylindrical and dichotomously branched.
> The leaves are small, scale like and are scattered over these branches.
iii. Sporangia:-
> The sporangia are borne in triads.
> They have very short stalks.
> They are borne in the axils of small bifid leaves on the aerial branches.
> This triad of sporangia is called a synangium.
> The two lobes of the leaf are closely united with the synangium.
> Stem Anatomy:- It is circular in outline from base, pentagonal near the first dichotomy and triangular between successive dichotomies . It has following parts:
a. Epidermis:-
- There is a single layer of epidermis present outside
- It is heavily cutinized
- Stomata are also present on epidermis, situated at the grooves
b. Cortex:- It is divided into following parts:
i. Chlorenchymatous cortex:- It is the outermost part of cortex and has 2 to 5 layers of cells. The cells are thin walled and are parenchymatous. They are photosynthetic as they contained chloroplast.
ii. Seclerenchymatous cortex:- Below the parenchymatous cells there are 2- 4 layers of sclerenchymatous cells. The cells are thick walled and provide support.
iii. Parenchymatous cortex:- They form the major portion of the stem. The cells are thin walled and no inter cellular spaces in them.
c. Endodermis:- There is well developed endodermis between the stele and the cortex. These cells has casparian bands on their radial walls.
d. Steler system:- Pith is absent. The stele of Psilotum consists of following parts:
i. Xylem:-
- The xylem is actinostelic and radial in outside in 6 rays, the protoxylem is located at the tip of the rays.
- In the center the metaxylem is present.
- The cells of xylem are thick walled and their main function is transport of nutrients.
ii. Phloem:-
- Between the endodermis and the xylem there is phloem.
- It is of thin walled cells.
- It consist of sieve cells and sieve areas in their oblique end walls. Nuclei disintegrate at maturity.
.png)
> Anatomy of rhizome:-
- In rhizome the epidermis is inconspicuous and all the cells of outermost layer of cortex extend into rhizoids.
- The cortex is thin walled and cells contain fungus.
- The endodermis is conspicuous.
- The stele in rhizome is protostele (xylem is surrounded by phloem)
- The pith is absent And xylem occupies center of the axis and surrounded by the phloem.
4. Reproduction:-
> It is characterized by alternation of generation.
> Both spore producing and gamete producing regeneration are independent.
> Sporophytes reproduces by formation of haploid reproductive units, called as meiospores, produced in complex trilobed structure synangium.
> Each synangium is trilobed, stalked structure borne at the apex of short lateral branch . A bilobed appendage is present at the base of each synangium that curve and surround the stalk of synangium.
> Nature of synangium:- There are different views:
i. The trilobed synangium is formed by fusion of two or more sporangia.
ii. One sporangium with 3 chambers (trilocular sporangium).
iii. Synanium is cauline (developed at the apex of stem) in nature and it is actually modified trilocular sporangium present on lateral branches.
iv. This concept was suggested by Bierhorst (1956) according to him each unit in synangium represents a condensed fertile axis. The synangium in Psilotum can be considered as homologus to fertile portion of ( Rhynia) where one arm is fertile and other is sterile . The condensation of fertile arm is modified into synangium .the bract modified to surround the synangium.
> Structure of synangium:-
- It coinsist of three chambers or locules.
- Wall of synangium is 3 – 4 layers
- Thick outer wall forms the epidermis
- Inner wall separates the three locules
- Each locule is filled up with large number of spore and these are homosporous in nature.
- Synangium splits up from 3 lines along the epidermis and dehiscence occurs.
.png)
.png)
.png)
.png)
> The gametophyte lives underground as a saprophyte, sometimes in a mycorrhizal association.
> When the gametophyte is mature, it produces both egg and sperm cells.
> The gametophyte of Psilotum is unusual in that it branches dichotomously, lives underground and possesses vascular tissue.
> The gametophyte of Psilotum is called as Prothallus. It contains parenchyma cells and there is strand of tracheids extending back from the apex.
.png)
.bmp)
.png)
.png)
.bmp)
.png)
.png)
5. Phylogeny:-
> Phylogeny:- The history of the evolution of a species or group, especially in reference to lines of descent and relationships among broad groups of organisms.
> Psilotum superficially resembles certain extinct early vascular plants, such as the rhyniophytes and the trimerophyte genus Psilophyton.
> The unusual features of Psilotum that suggest an affinity with early vascular plants include dichotomously branching sporophytes, aerial stems arising from horizontal rhizomes, a simple vascular cylinder, homosporous and terminal eusporangia and a lack of roots.
> Unfortunately, no fossils of psilophytes are known to exist.
> A careful study of the morphology and anatomy suggests that whisk ferns are not closely related to rhyniophytes, and that the ancestral features present in living psilophytes represent a reduction from a more typical modern fern plant.
> Significant differences between Psilotum and the rhyniophytes and trimerophytes are that the development of its vascular strand is exarch, while it is centrarch in rhyniophytes and trimerophytes.
> The sporangia of Psilotum are trilocular synangia resulting from the fusion of three adjacent sporangia and these are borne laterally on the axes.
> In the rhyniophytes and trimerophytes the sporangia were single and in a terminal position on branches.
> Molecular evidence strongly confirms that Psilotum is a fern (in the broad sense that includes horsetails) and that psilophytes are sister to ophioglossoid ferns.