Thallus Organization in Algae (6 Types of Thallus)

In this article we will discuss about range of thallus organization in algae:- 1. Unicellular forms of thallus organization in algae, 2. Colonial forms of thallus organization in algae, 3. Filamentous forms of thallus organization in algae, 4. Heterotrichous forms, 5. Siphonous forms of thallus organization in algae and 6. Parenchymatous forms of thallus organization in algae

• The plant body in algae is not differentiated in root, stem and leaves.
• The undifferentiated plant body is called a thallus
• The thallus of algae ranges from simple unicellular to highly organized differentiated multicellular forms
• The various forms of thallus organization in algae are : 1. Unicellular forms, 2. Colonial forms, 3. Filamentous forms, 4. Heterotrichous forms, 5. Siphonous forms and 6. Parenchymatous forms

6 Types of Thallus Organization in Algae

1. Unicellular forms

• They exist as single celled organisms.
• Unicellular thalli are found in all classes, except of the class Phaeophyceae.
• They can be either motile or non-motile.
• Motile unicellular algae have the ability to move from one place to another.
• They can be further classified into two categories based on the presence or absence of flagella.

A. Motile Form of Unicellular Algae:

• This category includes algae that have flagella and use them for movement.
• Examples of motile unicellular algae
  1. Chrysamoeba (Chrysophyceae) lack flagella, but are able to perform amoeboid movement by means of cytoplasmic growth.
  2. Rhizochloris (Xanthophyceae) lack flagella, move through water using their pseudopodia.
  3. Dinophyceae have two flagella of unequal length. Transverse flagellum and Longitudinal flagellum help for movement.

B. Non-Motile Form of Unicellular Algae

• These algae do not have flagella and cannot move.
• However, they may become motile during reproductive stages.
• Examples of Non-Motile Form of Unicellular Algae
  1. Coccoid algae, which have a rigid cell wall and belong predominantly to the Xanthophyceae class (e.g., Prochloron, Aphanocapsa, and Synechococcus)
  2. Spiral algae, which have a spiral filament (e.g., Spirulina).

2. Colonial forms

• Group of individual cells forms colony and each cell of colony is capable of doing life processes.
• They are aggregate by common mucilaginous matrix
• Types of Colonial Forms: Coenobial, Palmelloid, Dendroid nad Rhizopodial

A. Coenobial Forms

• This type of colony consists of a fixed number of cells with a specific shape and arrangement.
• Coenobium colonies can be further divided into motile and non-motile types.
• Examples of Motile Coenobia:
  1. Volvox: Spherical, hollow colony with numerous flagellated cells
  2. Gonium: Plate-like colony with a few to many cells
  3. Pandorina: Spherical colony with 16-32 cells
• Examples of Non-Motile Coenobia:
  1. Hydrodictyon: Net-like colony
  2. Pediastrum: Disc-shaped colony
  3. Scenedesmus: Plate-like colony

B. Palmelloid Forms

• The algal cells are aggregated together in a gelatinous matrix of indefinite shape
• Palmelloid can be further divided into two types.
  1. Permanent Palmelloid Forms: maintain the palmelloid state throughout their life cycle. E.g. Tetraspora, Chlorosaccus and Phaeocystis
  2. Temporary Palmelloid Stage: adopt this state as a survival strategy in response to adverse conditions. E.g. Chlamydomonas

C. Dendroid Forms

• This is also a non motile colony of unfixed number of cells
• Cells joined end-to-end, resembling a branched tree
• E.g., Prasinocladus, Mischococcus etc.

D. Rhizopodial Forms

• The individual cells are linked together by rhizopodia is called rhizopodial forms
• E.g. Chrysidiastrum, Chlorachnion

3. Filamentous forms

• The cells are attached end to end in a linear fashion and form a filament.
• The filamentous form may be branched, unbranched or falsely branched.
• There are two main types of filamentous forms:
  1. Uniseriate: Single row of cells. E.g. Oscillatoria (Simple, unbranched filament), Nostoc (Heterocystous, branched filament), Ulothrix (Unbranched filament with basal holdfast) and Oedogonium (Branched filament with specialized reproductive cells)
  2. Multiseriate: Multiple rows of cells. E.g. Ulva (Flat, sheet-like thallus with multiple layers of cells)

4. Heterotrichous forms

• The filaments which contains both prostrate system and erect system is called Heterotrichous form.
• The thallus was differentiated into creeping system and projecting system.
• Creeping system consists of branched filaments attached to the substratum.
• Projecting system arises from the creeping system and grows erect.
• It is characteristic feature of Chaetophorales order of class Chlorophyceae, in many Phaeophyceae, Rhodophyceae, in some Chrysophyceae and Dinophyceae.
• E.g. Fritschiella, Ectocarpus, Dinoclonium, Stigeoclonium, Trentepholia, Coleochaete, Drapanaldiopsis etc.

5. Siphonous forms

• The thallus is coenocytic non-septate, multinucleate siphon like structure.
• It may be Branched or unbranched morphology
• Eg. Vaucheria and Protosiphon

6. Parenchymatous forms

• This is a highly advanced type of thallus
• The plant body of algae made up of parenchyma cells.
• During the cell division septation leads to the formation of a parenchymatous body
• This thallus may appear flat, leaf like or cylindrical or well branched.
• Examples
  1. Ulva, Punctaria and Porphyra – flat and leaf like parenchymatous thalli
  2. Enteromorpha – Tubular parenchymatous thallus
  3. Sargassum – Thallus Differentiated into Root-like holdfast, Stem-like stipe and Leaf-like blades