The algae exhibit a great diversity in the organization of the plant body. The plant body shows no differentiation into true root, stem and leaves and is called a thallus. This term is used event if the plant is a unicell.
Various forms of thalli can be distinguished into following types:
I. UnicelluarThallus:
The simplest form of the thallus is a unicell. These unicellular forms may be motile or non-motile.
i) Motile Unicellular Thallus:
These unicellular structures possess flagella for locomotion. They may be spherical, oval or pear shaped. It is considered to be most primitive type of thallus. Chlorophyceae, Euglenophyceae, Chrysophyceae, Xanthophyceae, Cryptophyceae are classes where this thallus is common. Ex. Clamydomonas.
These unicellular structures lack any kind of motility structures. Ex. Chlorella, Chlorococcales, Desmids etc.
iii) Rhizodal Unicellular Thallus:
In this types no locomotive structures are present but the cells move with the help of cytoplasmic outgrowth or rhizopodia. Xanthophyceae, Chrysophyceae, Dinophyceae are the classes in which this thallus is present. Ex. Chrysamoeba, Rhizolekane.
II. MulticelluarThallus:
In this type more than one cell is present. It is again divided into non-filamentous and filamentous
I) Non-filamentous Thallus:
In this following types are present-
A) Colonial Thallus:
These type of thallus consists of loose assemblage of indefinite cells mechanically held together in a common gelatinous envelope. It is known as colony. The cells in the colony have little or non dependence upon one another.
Motile colonial thallus:
it consists of motile, unicellular Chlamydomonas like cells held together in a common mucilaginous sheath. The cell act together and bring about movement of the entire colony. Ex. Volvox.
Non-Motile Colonial Thallus:
In this type of thallus, indefinite number of small, non-motile cells are held together to form non-motile colonies. They are free floating colonies Ex.-Pediastrum, Hydrodictyon.
B) Coenobial Colonies:
These types of thallus consists of definite number of cells enveloped in a mucilaginous sheath and arranged in a particular manner. The cells of a coenobium are independent of each other. It may be motile or non-motile.
Motile CoenobialThallus:
The definite number of motile cells are embedded in a mucilaginous. Ex,- Gonium ( 4 to 32 cells), Eudorina ( 4 to 12 cells).
Non-Motile CoenobialThallus:
Palmelloid colonies:
In this type of thallus, the vegetative cells are non-motile and are embedded in a mucilaginous matrix of irregular shape and size. The cells becomes motile during reproduction. The matrix is formed from the walls of individual cells. Ex. Tetraspora, Palmella, Stichogloea.
Dendroid colonies:
In this type of thallus, the cells are united in a branching manner by localized production of mucilage at the base of each cell. The whole colony looks like a tree in habit. Ex.-Ecballocytes, Chrysodendron, Dindrobryon.
Rhizoidal Colonies:
The cells are united through rhizopodia Ex.-Chrysidiastrum, hrysorachnion.
III) Multicellular FilamenotusThallus:
In this type of thallus, the cells are arranged in linear rows called the threads or filaments. The filament is the result of repeated cell division of a non-motile cell in a single plane. The daughter cells in the chain remain united and further keep on dividing in the same plane forming a filament.
i) Unbranched Simple Filaments:
In this type, the cells are arranged from end to end in a single file and held together firmly. It is long, thread like, unbranchedtpe of multicellular thallus. The cells of the filament are alike, self-sufficient and independent of one another.
Ex.- Spirogyra, Ulothrix, Oedogonium.
ii) Branched Filaments:
This type of thallus is formed when an occasional cell in the filament divide in second plane. The branches thus arise as lateral outgrowths from the main filament. Ex.-Cladophora, Bulbochaete.
The filamentous forms evolved in two directions resulting in Pseudoparenchymatous and Parenchymatous forms. The Pseudo thallis may be fromed by juxtaposition of the branch system of a single axial thread or many axial filaments. If branches from a single filament are evolved as in Batrachospermum, the thallus is called uniaxial. If branches of many axial filament aggregate , the thallus is mlti axial, Eg. Nemalion.
The foliaceous parenchymantous thalli (Eg.Ulva) are of a very large size in brown sea weeds (Pheaophyceae).
The thallus is differentiated into central medulla, middle cortex and outer meristoderm. Ex.-Laminaria, Microcystis.
Heterotrichous Thallus:
It is the most advanced and evolved type of thallus. It consists of two parts – the basal portion of the thallus that creeps along the substratum and is called prostrate system and upright filaments constituting the erect system.
In Stigeoclonium, both the erect and prostrate systems are equally developed.
The heterotrichousthallus has undergone modification by reduction or elimination of one or other of the systems. The disappearance of erect system has resulted in the discoid form of thallis of Coleocheate.
Coleocheate
Drapanaldia and Draparnaldiopsis are examples in which there is complete disappearance of the prostrate system and a corresponding elaborate development of the erect system.
Siphanceous or Coenocytic Thallus:
Acetabularia
Characium is tubular and Acetabularia is umbrella-shaped. In Caulerpa the thallus is more elaborate and mimics the creeping shoots of aerial plants resembling a large moss or a fern. It is differentiated into a creeping structure resembling a rhizome. The rhizome gives rise to holdfast from the under face and erect leaf shoots from its upper face.
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