NEET BIOLOGY NOTES IN PDF-SPIROGYRA-NTA NEET 2020
“Pond Scum”, “Pond Silk” or “Water Silk” or “Mermaid’s tresses”.
HABIT AND HABITAT
It occurs in stagnant fresh water as free floating. It occurs in water of changing temperature. Spirogyra adnata = Occurs in stream and river.
NEET BIOLOGY NOTES IN PDF
- 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 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)Spirogyra grows in length by intercalary growth.
- Note: Some species contain rhizoid like out growths.
- e.g. S.affinis S.dubia S.rhizobrachialis
These are of following type –
By 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
Zoospores are totally absent in Spirogyra. It’s reproductive structures are non flagellated.
(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.
(C) By Parthenospores or Azygospores –
- When gametes fail to fuse they form parthenospore. Sudden variation in environment leads to the formation of parthenospores.
- “Kleb” 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.
- 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).
- Physiological 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.