Doc: Spirogyra | Plant Kingdom | NEET

Biology Class 11

Created by: Sushil Kumar

NEET : Doc: Spirogyra

The document Doc: Spirogyra is a part of the NEET Course Biology Class 11.

Spirogyra

"Pond Scum", "Pond Silk" or "Water Silk" or "Mermaid's tresses".

Systematic Position 

Kingdom

Plantae

Sub kingdom

Thallophyta

Division

Chlorophyta

Class

Chlorophyceae

Order

Conjugales/Zygnemales

Family

Zygnemaceae/Zygnemataceae

Genus

Spirogyra

 

Habit And Habitat 

It occurs in stagnant fresh water as free floating. It occurs in water of changing temperature.

  • Spirogyra adnate Occurs in stream and river.

Structure

Doc: Spirogyra

Spirogyra is a multicellular, unbranched and filamentous algae. It is olive-green in colour.

All the cells are similar in shape, size and structure. These are arranged in a linear row.

Hold-fast is normally absent but S.fluviatalis contains hold-fast.

Cell-Wall is double-layered, outer layer is made up of pectose (which dissolves in water to form a mucilagenous sheath) and inner layer is made up of cellulose.

Note: Filament is silky smooth due to presence of pectin in the outer layer. Due to this reason epiphytes are not attached with filament.

  • Each cell contains a large central vacuole.
  • Cytoplasm occurs in the form of a thin layer around the vacuole. Thin layer of cytoplasm is called "primordial utricle".
  • Cytoplasm contains a large nucleus and it is held by cytoplasmic-fibres.
  • Each cell contains spirally coiled or ribbon-shaped chloroplasts, with a left-handed twisting (sinistral).
  • Number of chloroplasts varies from one to sixteen in each cell (according to species). (Single chloroplast was recorded in S.Venkataramanii). and 16-chloroplasts in S-rectispora)
  • Chloroplast has many Pyrenoids.
  • Spirogyra grows in length by intercalary growth.

Note: Some species contain rhizoid like outgrowths.

e.g. S.affinis, S.dubia ,S.rhizobrachialis 


These are of following types:

Vegetative Reproduction

Fragmentation: Fragmentation takes place by -

(i)   Accidental breakage 

(ii)  Dissolution of middle-lamella 

(iii) By the formation of H-shaped segments - 

E.g. Spirogyra colligata 

  • In some species cell-wall between two cells form H-shaped thickening, called replicate wall/replicate septum.
  • Middle lamella of these cells dissolves and both cells remain connected by replicate wall.
  • Replicate wall facilitates fragmentation without dissolution of cells. 
  • Normally asexual reproduction is absent in Spirogyra. It takes place only in some species by the following methods-
  • are totally absent in Spirogyra. Its reproductive structures are non-flagellated.

    Doc: Spirogyra 


(A) By aplanospore - e.g. S.aplanospora

(B) By akinete  

  • Under unfavourable conditions, the cells of a filament develop into thick walled structures. Such thick walled structures are called as akinetes.
  • Thick cell-wall of akinete is made up of cellulose and pectose. In akinetes, food has been accumulated.
  • When favourable conditions return, akinete directly germinates to form a new thallus.

            e.g., S. farlowii.

     Doc: Spirogyra

(C) By Parthenospores or Azygospores - 

  • When gametes fail to fuse they form parthenospore. Sudden variation in environment leads to the formation of parthenospores.
  • artificially induced the formation of parthenospores in S.varians by placing it in sugar solution.
  • In S.daedalea both the gametes form parthenospores in situ (at their original place)

Note: In S.rhizoides parthenospores are developed from rhizoids.

Sexual Reproduction

  • It takes place towards the end of growing season-usually in the late spring.
  • It takes place by conjugation. Gametes are non-ciliated or non-flagellated and oval shape.
  • Eye-spot is absent in gametes.
  • Gametes perform amoeboid movement during conjugation.
  • Gametes of Spirogyra are called aplanogametes (due to the absence of cilia or flagella).
  • anisogamous type of sexual reproduction is found in Spirogyra.
  • Following conditions which favour the process are:

(i)   Nitrogen deficiency
(ii) The change in pH of the aquatic medium to alkaline side.

  • Types of Conjugation 

(i)   Scalariform or Ladder like conjugation
(ii) Lateral conjugation

Scalariform Conjugation 

  • It is most common type of conjugation and occurs in "heterothallic" species of Spirogyra.
  • It mainly takes place during night.
  • Two filaments arranged parallel to each other.
  • After some time cells of two filaments opposite to each other form cylindrical out growths which fuse to form a conjugation tube.
  • Cells united by conjugation tube are called aplanogametangia.
  • Protoplast of gametangium constricts to form gamete.
  • In the cells of one filament, male gametes are formed and in the cells of another filament female gametes are formed.
  • Male gamete perform amoeboid movement and migrate to female gametangium through conjugation tube, it fuses with female gamete to form a diploid zygote.
  • After this a three-layered wall is formed around the zygote, now it is called zygospore.
  • In scalar form conjugation cells of one filament become empty and the other filament contains zygospore in its cells.
  • Zygote is initially green but soon after chloroplast of male gamete degenerate followed by the degeneration of chloroplast of female gamete. Due to this reason, the filaments of Spirogyra do not remain green after fusion.

Doc: Spirogyra

Lateral Conjugation 

  • It takes place between adjacent cells of the same filament. 
  • Lateral conjugation occurs in homothallic species of Spirogyra. Lateral conjugation is of two types-
  • Indirect lateral conjugation
  • Direct lateral conjugation

(a)  Indirect lateral conjugation / chain conjugation-

  • Protoplast of cells constricts to form gametes.
  • Male and female gametes are formed in adjacent cells of a filament.
  • Each cell form tubular/papilla-like outgrowth in their lateral wall.
  • Outgrowth of two cells fuse to form conjugation tube.
  • Male gamete migrates through conjugation tube and fuses with female gamete to form zygospore.
  • After the completion of conjugation the filament contains empty and zygospore containing cells in alternate manner. 

Ex.: S. affinis.

Doc: Spirogyra

(b)  Direct lateral conjugation-

  • This type of conjugation was studied by M.O.P. lyengar (Father of Indian phycology/Algology) is Spirogyra jogensis.
  • Male gamete migrates to adjacent cell through a conjugation pore in transverse wall between two cells.
  • Male gamete fuses with female gamete to form a diploid zygote.

Doc: Spirogyra

Structure Of Zygospore 

Wall of zygospore is three layered-

  • Exosporium – Thin layer of cellulose (Mainly) or pectose.
  • Mesosporium – Brown, thick ornamented and chitinised.
  • Endosporium – Thin & made up of cellulose.

Germination Of Zygospore 

  1. Germination takes place in favourable conditions.
  2. Four haploid nuclei are formed inside the zygospore as a result of meiosis.
  3. Three nuclei are degenerate and only nucleus remains functional.
  4. Now functional nucleus of zygospore undergoes repeated mitotic division to form a haploid filament of Spirogyra.
  5. During germination of zygospore in spirogyra exosporium and mesosporium reptures and endosporium emerges in form of cylindrical tube.
  6. Life cycle of Spirogyra is "haplontic type".

Doc: Spirogyra

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