Ulothrix - Class 11 PDF Download

Systematic Position 

Habit And Habitat

Ulothrix mainly found in fresh water.

The majority of species of Ulothrix occurs in cold slow moving streams.

Marine species, eg., Ulothrix flacca. (intertidal zone of sea and saline marshes)

Lithophyte species. – Ulothrix implexa

Freshwater species. – Ulothrix zonata.

Ulothrix zonata is the main species of Ulothrix which is found in India

(A) Ulothrix Filament

 (b) T.S. of a cell

(C) Structure of chloroplast

Thallus  Structure

The thallus of Ulothrix is multicellular, filamentous, unbranched and bright green in colour.

All the cells of the filament are similar but basal cell and apical cell are different. All cells present between basal cell and apical cell are called intercalary cells.

Shape of the cells are cylindrical but they looks rectangular in external appearance.

The upper most or apical cell is somewhat dome-shaped and basal cell is elongated and narrow at the base.

• The filament attached to the substratum with its specialized cell which is called Holdfast or Rhizoidal cell or Hapteron.
• At the base of holdfast develops fine processes or fingers like structure are called Haptera.

The holdfast cell of the thallus is living and non-green. Chloroplast absent in hold fast.

• The cell wall of the filament is double layered.
• The outer layer is Protopectin. (Precursor of pectin and insoluble in water). The inner layer is cellulose.

A large vacuole is present in the centre of the cell. Surrounding the vacuole, the cytoplasm present as a thin layer and to form a primordial utricle.

• Chloroplast contains two or three pyrenoids. (Protein particles which are surrounded by starch.)
• These pyrenoids are concerned with storage of starch.

REPRODUCTION

Ulothrix reproduces by three methods :

      (1) Vegetative reproduction.

      (2) Asexual reproduction.

      (3) Sexual reproduction.

(1) Vegetative Reproduction

• It takes place generally in favourable conditions.
• Fragmentation :The filaments break up into two or more pieces. They are called fragments.
• Each fragment divides to form a new Ulothrix filament.
• Fragmentation occurs due to-
• Incidental reasons.
• Death of intervening cells or middle lamella.        
• Strong waves of water.
• Change of pH or temperature

(2) Asexual Reproduction 

•  It takes place in favourable and unfavourable conditions.
  In favourable conditions : By Zoospores. 
• Zoospores formation occurs generally near the apex and progresses towards the base of filament. (Basipetal order)
• Zoospores are of two types –

1. Micro zoospores.
2. Macro zoospores.

(1)  Micro zoospores : They are oval shaped. They may be biflagellate or quadriflagellate. 

• Eyespot lies at the anterior lateral position.
• Asexual Repoduction by macrozoospores formation
• Their swimming period is 2-6 days. Their number in mother cell varies from 8 to 32.
• Their germination begins from the anterior end.

(2)  Macro zoospores : They are spindle or pear shaped with a pointed posterior end.

• They are always quadriflagellate.
• Eye spot lies at the anterior in middle region. Their swimming period is about 24 hours (1 day).
• The number of macro zoospores in sporangium from 4-8.
• Their germination begins from the posterior end.

Structure Of Zoospores 

• Zoospores are naked. It means cell wall is absent in them. Cup-shaped chloroplast is present in which one pyrenoid is present.
• A contractile vacuole is present which is concerned with osmoregulation.
• A red colour eyespot is found in zoospores for reception of light, which is made up of carotenoids. (Hematochrome)

Note : Contractile vacuole is absent in zoospore of Ulothrix flacca.

Germination Of Zoospores

• Zoospores, after the liberation from zoosporangium swim in water for sometime.
• When they tired then settle down on any solid object.
• Micro zoospore attached with their anterior end and macro zoospore attached with their posterior end.
• The zoospore withdraws its flagella and secretes a wall around it. During this period it elongated and divides.
• First of all transverse division takes place, it is asymmetrical after that two cells are formed.
• The lower long cell transform into hold fast and upper cell divides continuous transversely to form Ulothrix Filament.

(ii)  Asexual Reproduction in Unfavourable condition : 

(A) By Aplanospore :

• When the unfavourable condition returns slowly, then all the cells of thallus except holdfast form aplanospore.
• Aplanospores are thin walled and Non-flagellated structure.
• On the approach of favourable condition the aplanospore may germinates by two different ways:

(i)   Direct germination :

• Aplanospores germinates directly, when the availability of water is less.
• Aplanospores absorb water and swell up and their covering breaks and aplanospores become free then transverse division takes place.

(ii)  Indirect germination :

• It takes place by formation of zoospores. When water is available in more amount
• Each alpanospore act as a zoosporangium and each zoospore produce new Ulothrix filament.
• Alpanospore shows "in situ" germination also

(B) By Hypnospores :

• When highly adverse conditions are coming fast then the protoplast of the Ulothrix cell shrinks and collect in the centre.
• Shrinked protoplast secretes a thick wall around itself. These thick walled non-motile structures are called Hypnospores.
• They are more resistant as compared to aplanospores. These are perennating bodies.
• On the approach of favourable conditions hypnospores also germinates directly or indirectly.

Example – U. fimbriata.

(C) Akinetes :

• When adverse conditions come suddenly then vegetative cell transform into thick walled structure, in which food has been accumulated, called akinete.
• They shows perennation. When favourable conditions come they germinate either directly or indirectly.
  Example : U. idiospora, U. zonata, U. subtilissima.   

(D) Palmella stage :

• This type of reproduction take place in those Ulothrix species, which lies at the banks of reservoirs.
• The water start drying in the unfavourable conditions, Ulothrix faces deficiency of water step by step.
• Certain cells of the thallus divides in uncontrolled manner during such type of changes and to form group of a large number of small daughter cells.
• The cell wall of cells dissolves and to form a mucilaginous envelope. Later it dries and form a protective covering.
• On the approach of favourable conditions, mucilage layer dissolves and cells give rise to new thallus through direct or indirect germination.

Note : Above stage is identical to an another algae-Palmella, therefore it is known as Palmella stage.
 Note : In Ulothrix, colony formation occurs during palmella stage.

• One zygospore   4-16 Ulothrix filament.
• Germination of zygospore is direct or indirect in favourable conditions and meiotic division takes place during germination.

Fusion of motile gametes is a thought of luck.. When these gametes fail to fuse. They settle down, withdraw their flagella, round off and secretes a thick wall around itself. These are called as azygospores or parthenospores.

• Their germination is through the mitotic cell division. Homogenetic thallus is formed through the germination of azygospore.

Special point :  

Ulothrix is heterothallic. It means fusing isogametes have different strains genetically.

Sexual reproduction of Ulothrix begins after the growth periodof Vegetative season.

Sporophytic generation is represented only by the zygote. In the life cycle of Ulothrix, Zygote is very short in duration.

Gametes are formed in semi drying condition and released in the morning.

In Ulothrix primary division of labour is found. The work of the holdfast is to make the filament stable on the base. For food it depends on other cells, which does the work of food formation and reproduction.

• Ulothrix zonata  shows the origin of Sexual reproduction.