Doc: Pteridium - Class 11 PDF Download

Pteridium 

Systematic Position  

Common name – " Sun Fern"   or  "Bracken Fern"

Habit And Habitat

1. Ferns commonly grow in moist and shady places. This is present throughout the world except for South America and Antarctica.                                                                                                                                               
2. They show 'gregarious' habit. This habit is due to vegetative reproduction. 

  It can be grown in the presence of less moisture and high light, so it is called "Sun fern". It is a xerophytic plant. It is a pioneer plant after forest fire.      

External Morphology

The plant body of Pteridium is sporophytic and it is differentiated in root, stem and leaves. Root and stem are underground and leaves are aerial. Plants are perennial.

I.    ROOT :             

• The primary root is short lived. It is soon replaced by adventitious roots which develop from the rhizome in Acropetal manner.
• The adventitious roots are thin, small, soft and less branched.
• They are endogenous in origin

II.  STEM (Rhizome)

• The stem of fern is underground rhizome.
• Rhizome grows horizontally in the soil. The main stem is dichotomously branched.
• The young rhizome particularly covered by multicellular brown hairs called as ramenta.

III. LEAVES :

• Pteridium is a megaphyllous plant. Its large leaf is known as 'frond'. leaves are borne only on small branches.
• The leaves may be 2-12 feet long (height) and in each leaf, the petiole and lamina are almost equal in length. They are developed on rhizome in alternate manner.                              
• Leaf lamina is divided into a number of leaflets. The leaves are compound, bipinnate and imparipinnate.      
• The young leaf shows typically open furcate venation. (The bifurcation of veinlets without attachment, at the anterior end is called open furcate venation).
• In these leaves, the young leaves of Pteridium are curled. Circinate ptyxis" is found. Each young leaf opens from the base to apex. Ramenta are also present on young leaves.

Internal Structure Of Root

In the centre, a stele is present which is surrounded by single layered endodermis. Below the endodermis pericycle is present which is one or two layered and made up of thin walled parenchymatous cells.

1. The stele is diarch with two xylem bundles alternating with two pholem bundles.
2. The xylem is exarch in position.

Internal Structure Of Rhizome :

Inside the epidermis, three-four layers of sclerenchymatous hypodermis is present. which is broken at two ends (Points). At these points, the parenchymatous cells of ground tissues reaches directly up to the epidermis.

1. Hypodermis is followed by parenchymatous ground tissues in which two rings of meristeles are present. The stele is polycylic dictyostele or polystele or Dictyostele type.
2. Both the rings of meristele are separated by two plates of sclerenchyma in dorsiventral position.
3. The meristeles of outer ring are smaller in size and more in number, while there are two or three large meristeles are present in inner ring .

Meristele :

1. Each meristele is surrounded by a single-layered endodermis.
2. Below the endodermis one or two layered parenchymatous pericycle is present.
3. Xylem forms the central core of the meristele, which is Mesarch in position i.e. protoxylem is surrounded by metaxylem.
4. Vessels in xylem and companion cells in phloem are absent. 
5. In meristeles vascular bundles are concentric and amphicribral or hadrocentric type.
6. Pseudo vessels are formed due to disintegration of septum from meta xylem tracheids.
7. Meristele is a complete stele because it has its own pericycle, endodermis, Xylem and phloem.
8. Secondary growth is absent in rhizome due to absence of vascular cambium.

Internal Structure Of Leaf :

It is differentiated into rachis and leaflet (pinnules).

(1) Rachis : In a transverse section the young rachis is hemispherical in shape.

• Many meristele are arranged in the mature rachis of Pteridium in horse-shoe shaped.
• In each meristeles vascular bundles are concentric, amphicribral and mesarch.

(2)  Leaflet pinnules :

• Mesophyll is present between two epidermal layers and mesophyll differentiated into palisade and spongy tissue.
• The stomata are present on lower epidermis.
• The epidermal cells of leaflet also bears chloroplast. Concentric vascular bundles are present in Pteridium leaflets.

Reproduction

1. Three different type of reproduction – Vegetative, by spore formation and sexual, are found in Pteridium.
2. The main plant body represents sporophytic phase and reproduces by vegetative and by spore formation.
3. Sexual reproduction only takes place in gametophytic phase i.e. in the prothallus.

Vegetative Reproduction :

(1)  By death and decay method 

(2)  By adventitious buds : Sometimes a bud is formed in the axis of persistent leaf base. These buds separate and grow into new plants

By Spore Formation : 

1. The main plant body of Pteridium is sporophytic and reproduces by the formation of spores.
2. The spores of pteridium are of one type and this condition is termed as homospory.
3. Leaflet/fronds at maturity bear small, rounded, reniform, brown or black coloured spots called sori. Sori bearing fronds are called as sporophyll. The sporophylls of Pteridium are photosynthetic in nature.
4. Each Sorus is protected by a covering sheath called indusium (bilipped) which may be true or false on the basis of origin.
5. A true indusium (Inner and small) arise from the placenta whereas false indusium (outer and large) does not arise from the placenta but it is formed by curved margin of the sporophyll.
6. The main function of indusium = Provide protection to sori (Singular – sorus).
7. Each sorus bears large number of sporangia and each sporangium produces many spores.

Position and Form of Sori :

1. Each sporangium develop on the abaxial (Lower) surface of the leaves
2. These sporangia arise in the groups i.e. in the form of sori.
3. These sorus  develop continuous, linearly arranged along under the margin of leaves is termed as continous linear sorus or coenosorus
4. The sorus that lies on the margin of sporophylls is termed as "marginal sorus"
5. Inner indusium is small and true and it arises from the placenta.
6. A sporangium has a long narrow, slender, multicellular stalk usually composed of three vertical rows of cells and a sporangial body or capsule.
7. The capsule is oval or biconvex (generally).
8. The radial and inner walls of the cells of specialized row of jacket becomes heavily thickened while their outer tangential wall remains thin.   
9. These cells constitute the "annulus". It starts from the base of the capsule on one side, run upwards curve on the other side and ceases almost near the middle.
10. Annulus covers 3/4 part of the sporangium. Below this, the cells of marginal row are thin walled and parenchymatous, consititute the stomium.
11. The annulus and stomium together help in dispersal of spores and dehiscence of sporangium. Sporangium break at the place of stomium during dehiscence of sporangium.
12. The cavity of capsule contain 8 or 16 spore mother cells or sporocyte which is diploid.
13. Spore mother cells divide meiotically to produce 32 or 64 haploid spores.

Dehiscence of sporangium :

Dehiscence of sporangium takes place in dry season. When the sporangium mature, the indusium dries and shrivels exposing the sporangia to dry air.

In dry weather, the annulus Cells of the sporangial wall loose moisture, dries up and shrink. The radial and tangential walls of annulus are thick so they do not shrink or bend easily. But outer wall of annulus is mainly thin which curve inwards and becomes concave. This inwards curving of the thin walls exerts pulling force on annulus cells.

Due to this pulling force (tension) thin cell of stomium tear up. So that a fissure once started from region of stomium and ultimately reaches up to the base of annulus on the opposite side of stomium.  

Due to the pulling force the entire annulus turns into the back side. Along the annulus most of the spores are attached. The space which is created by loss of water is filled by air and saturate. Because of this pulling force suddenly released from the annulus and annulus immediately comes in original position with a great force. The spores are attached on annulus, during this time, are thrown away. Dispersal of spores in Pteridium by "Catapult mechanism" (By annulus).  

The gametophytic phage begins with the formation of spores which are tetrahedral.

Prothallus :

Under the favourable conditions of moisture and temperature the spores germinates.

A gametophyte plant prothallus is produced by germination of spore. Prothallus is a heart shaped structure

It is independent and autotrophic. It is free from the sporophytic generation. It is unbranched. Diameter of adult prothallus is 3-8 mm.

It grows prostrate on the soil. The middle region of the prothallus is thick and, thin towards the marginal side. An apical notch is present at the anterior end.

A apical cell is present in the apical notch, by which growth takes place in the prothallus. It is nonvascular. Unicellular and unbranched rhizoids are located on the ventral surface of the prothallus.

They fix the prothallus with the substratum (soil). Sex-organs are developed on the ventral surface of the prothallus. Prothallus is monoecious which bears both sex organs –Antheridia and Archegonia

Archegonia borne on prothallus near the apical notch, while the antheridia are developed in the posterior region in between the rhizoids.

Main point

      (i)   Antheridia matures before the archegonia ie, prothallus is protandrous.

      (ii)  Always, cross fertilization takes place in Pteridium (Heterothallism)

Antheridium :

• Antheridium of Pteridium is simpler than Riccia. The presence of antheridium is the primitive character.
• Antheridium of Pteridium is a small sessile spherical structure.
• The jacket of the antheridium is composed of three cells. The two cells of the jacket lying one above and the other forming the base.
• Inside the antheridium there are 20-50 androcytes or antherozoid mother cells are present which are responsible to form spirally coiled multiflagellate antherozoids.
• Dehiscence of antheridium takes place in the presence of water.
• When the antheridium fully mature and water is available, its mucilage which is present in the cavity of antheridium, absorb moisture and swells up, resulting, the opening of the lid like cell at the top.
• The antherozoids liberated and after coming in contact with water, they swim freely.

Archegonium :

1. Archegonium is a flask-shaped sessile structure. Its swollen venter embedded in the tissue of prothallus and short curved neck projecting above the prothallus. Jacket is only found in neck region. Jacket of neck region is made up of four vertical rows of neck cells.
2. The venter has a large ovum or egg and a small venter canal cell.
3. In the cavity of the neck there is a single long neck canal cell which is usually binucleated. Cover cells are absent at the apex of neck.

Fertilization takes place in the presence of water.

Just before the fertilization the venter canal cell and neck canal cell of the archegonium disintegrate forming a mucilagenous substance which absorbs moisture, resulting the opening of mouth of the neck.   

Some of the mucilage oozes out at the tip of the archegonium and attracts the antherozoids on account of certain organic substance like malic acid present in it.

As a result of chemotactic response and lashing movement of cilia, some of the antherozoids enter the archegonium and ultimately one of them fuse with the egg. This fertilized egg is termed oospore or zygote.

Embryo And Its Development :

Zygote (oospore) is the first cell of the sporophytic generation.

1. The oospore first divides by a vertical division nearly parallel to the long axis of the archegonium, which divide the oospore into two half an Anterior or epibasal cell (lying the notch of prothallus) and posterior cell or hypobasal cell (lying towards the posterior end). The second division is also vertical but right angle to first one so that four-celled embryo is formed.
2. There are two epibasal or anterior cells and two hypobasal or posterior cells. This stage known as quadrate stage.
3. Third division is transverse and therefore eight cells are formed. This is known as octant stage. Therefore, two inferior hypobasal cells, two inferior epibasal cells, two superior epibasal cells and two superior hypobasal cells are formed.
4. Foot is embedded in the prothallus and help in providing nourishment to the embryo. The young sporophyte depends upon gametophyte.
5. Both the inferior hypobasal cells gives rise to radicle. The radicle grows first and pierce into the soil and form the primary root. Firstly, the primary root comes out from the prothallus but it is short lived. It soon dies and is replaced by adventitious root developed from the stem.
6. The cotyledons and apical bud of stem comes out in air from the lower surface of the gametophyte. Plumule grows immediately to form an underground rhizome. Foot and prothallus dies later on.

Note :

• Lateral polarity is exhibited by the embryo of Pteridium.
• The venter of the archegonium forms a protective covering, called calyptra, around the developing embryo. Its is haploid and part of gametophyte.

Primary leaves are simple and lives for one year and later it dies off. Later on special leaves arises from the rhizome. New leaves are developed each year, new leaves are compound and pinnate.

Independent alternation of generation is found in Pteridium.

In its life cycle two independent phases are present. Out of them one is main plant which is sporophyte. This reproduces through spore formation.

A haploid gametophyte plant body prothallus develops from the germination of spore.

Gametophytic generation begins from the spores till the formation of the gametes.

1. Sporophytic generation begins with zygote or oospore formation. Spore mother cells are last cell of sporophytic generation. Spore is the first cell of gametophytic generation.
2. The life cycle of Pteridium is known as Diplo-haplontic type.
3. Sympodial branching found in Pteridium rhizome
4. Branches are three types in pteridium.
5. Long shoots, Short shoots, Intermediate shoot. Short shoots bear fronds.

Main feature of ferns :

1. Presentce of ramenta on young leaf and rhizome

2. Circinate ptyxis of young leaf.

Pteridium, Adiantum, Nephrolepis (Sword fern) & Dryopteris etc are leptosporangiate fern.

Megaphyllous pteridophytes – With large pinnatified leaves having a complex series of vein. They form prominent leaf gaps in the stem stele.

eg. Pteris, Pteridium and Dryopteris.

Tapetum cells are present in sporangium of pteridium which provide nutrition to sporogenous mother cell. It is Diploid.