THALLUS ORGANIZATION IN ALGAE

( Habit and structure)

Dr.B.Adinarayana,M.Sc;Ph.D.

Asst.Professor of Botany

Govt. Degree College

Rajampeta

Kadapa Dist.

• In Chlorophytes there is no differentiation of root, stem and leaves; hence the plant body is thalloid in nature.

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• The thallus may be single-celled or many-celled and shows evolutionary progression from simple to more complex types of construction.

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• Chlorophytes are a heterogenous group exhibiting a wide range in their thallus structure and morphology beginning from simple microscopic motile unicellular forms through multicellular flagellated or non flagellated colonies, palmeloid forms, dendroid forms, filamentous forms, heterotrichous forms, siphonous forms to well developed parenchymatous thalli. 

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I. Unicellular thallus :

• It is the simplest type of construction within Chlorophyta. Unicellular thallus is of two types:
• (i) Unicellular motile thallus: Vegetative cells have two or four flagella and are motile (e.g., Chlamydomonas, Tetraselmis, Sphaerella)
• (ii) Unicellular non-motile thallus: These cells do not possess flagella, eyespot etc., meant for locomotion. (e.g., Chlorella, Chlorococcus).

Figure:  Chamydomonas cell- unicellular motile thallus

II. Colonial thallus

The colonial habit is achieved by aggregation of the products of cell division within a mucilaginous mass, by aggregation of motile cells or juxtaposition of cells subsequent to cell divisions.

(i) Coenobium: A colony with a definite shape, size and arrangement of cells. The number of cells in a coenobium is determined at the juvenile stage and subsequently there is only increase in size. Coenobia may be motile with flagellated cells (e.g., Eudorina, Pandorina, Volvox) or non-motile having cells without flagella (Pediastrum, Hydrodictyon).

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Figure: Coenobia of Eudorina, Pandorina and Volvox

Figure : Hydrodictyon

(ii) Palmelloid: In contrast to coenobial forms, in a palmelloid colony the number of cells, their shape and size is not definite. The cells remain irregularly aggregated within a common mucilaginous matrix, but they are independent and function as individuals. In some palmelloid forms it is a temporary phase (e.g. Chlamydomonas), whereas in others it is a permanent feature (e.g., Tetraspora)

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(iii) Dendroid: The colony appears like a microscopic tree. The number, shape and size of cells is indefinite and a mucilaginous thread is present at the base of each cell. Threads of different cells are united to form a branched structure (e.g., Ecballocystis).

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III. Filamentous forms :

(i) Filamentous unbranched: Simple unbranched filaments may be free floating (e.g., Spirogyra) or attached to some substartum (e.g., Ulothrix, Oedogonium)

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(ii) Filamentous branched: In Cladophora simple branched filaments remain attached to the substratum by a basal cell. In such filamemnts branches arise just below the septa between any two adjacent cells except the basal cell.

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(iii) Heterotrichous: The thallus is very much evolved and differentiated into prostrate and erect systems (e.g., Fritschiella, Coleochaete, Stigeoclonium, Draparnaldiopsis). Both the prostrate and the erect systems may be well developed or there is progressive elimination of prostrate or erect systems.

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(iv) Siphonaceous: The thallus is made up of branched, aseptate, coenocytic, tubular filaments as the nuclear divisions are not accompanied by wall formation (e.g. Protosiphon, Codium)

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(v) Parenchymatous: Thallus is formed by the divisions of cells in two or more planes. The daughter cells do not separate from the parent and give rise to parenchymatous thalli of various shapes (e.g., Ulva, Enteromorpha). Leaf like or foliaceous thallus as seen in Ulva has evolved due to transverse as well as longitudinal septation in the filament.         

Figure:  Range of thallus structure in Chlorophyta.

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A: Pterosperma  B: Nephroselmis  C: Palmophyllum D: Tetraselmis  E:Chlorella  F: Oocystis  G:Haematococcus  H:Pediastrum  I: Bulbochaete J: Chaetophora  K: Ulothrix L: Ulva  M:Cladophora ´ N: Boergesenia O: Acetabularia P: Caulerpa Q: Klebsormidium R: Spirotaenia T: Micrasterias   U: Coleochaete

Table: Thallus organization in different genera

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