Range of thallus organization

in Algae

Prof. Gauri Rane

P. G. Research Centre Dept. of

Botany

M. J. College, Jalgaon

Range of thallus organization in Algae

1)Unicellular Motile (Flagelloid) Forms:

The unicellular motile forms are the simplest

type of thallus in algae. The flagellated

unicellular forms are seen in various classes

of algae. The flagellated unicelled structures

are distinctive of certain classes e.g.,

Euglenineae, Cryptophyceae, Chrysophyceae

and Dinophyceae. Flagellated vegetative cells

are absent in Cyanophyceae, Phaeophyceae,

Rhodophyceae, Bacillariophyceae. The

unicellular plant body may be spherical,

oblong or pear-shaped and sometimes

elongated and approximately circular in cross

section.Eg. Chlamydomonas, Chlorogonium,

Ochromonas, Chromulina

2) Unicellular Non-motile (Protococcoidal)

Forms:

These cells do not possess flagella, eyespot

etc., meant for locomotion. (e.g., Chlorella,

Chlorococcus).

Unicellular non-flagellated cells show many

morphological variations e.g.,

Bacillariophyceae (Diatoms), in many

Chlorophyceae (Chlorellti, Cosmarium)

Cyanophyceae (Synechococcus), and in some

forms of Xanthophyceae, Dinophyceae and

Rhodophyceae (Porpliyridium).

They are simple spherical or elongated cells

e.g., Microcystis, Cylindrocystis, Pinnularia

(Bacillariophyceae); triangular as in

Tetragonidium (Cryptophyceae) and

Triceratium (Bacillariophyceae). The epiphytic

or attached forms have a basal disc

3) Colonial Forms:

A further evolution of the unicellular types from

occasional and indefinite type of colony like structures—

with independent individual cells inside it to a well-

defined colony prasinocladus with interlinks among the

cells results in a true colonial habit.

Here varying numbers of unicells aggregate together in

different ways, often within a mucous envelope. Colonial

forms are seen among Chlorophyceae. Chrysophyceae,

Bacillariophyceae, Dinophyceae, Xanthophyceae etc. The

colony may be (a) motile or (b) non-motile.

(a) Motile Colonial Forms:

Motile flagellated cells aggregate

together to form motile colonies.

Colonies vary in shape and size and

in the number of cells.

The movement of the colony is

effected by the conjoint and uniform

flagellar action by all the cells. In

Chlorophyceae, the colony is made

up of Chlamydomonas like cells and

the cells arc arranged just below the

mucilaginous surface. The colonies

are either “plate-like” (e.g.,

Gonium) or spherical (e.g., Volvox).

The cells may be connected by

cytoplasmic strands, (e.g..

Volvox). Though in the majority

of cases all the individual cells

are alike, a few forms have some

larger cells for reproductive

functions; the rest of the cells

being purely vegetative (e.g.,

Volvox) Mostly they are coenobia

(sing, coenobium) i.e., colonics

composed of definite number of

cells arranged in a defined

manner.

(b) Non-Motile Colony:

Aggregations of non-motile

cells in the form of a colony

(non-motile) are common only

in Chlorophyceae. Here the

cells are, more or less, fused

together (e.g., Hydrodictyon). It

may be plate like e.g..

Scenedesmus or net-like as in

Hydrodictyon.

Non Motile- 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)

Non Motile- 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).

4. Filamentous Forms:

A further development would involve a more closely

knit structure, i.e., the division of the single cell into

many daughter cells with septa between the divided

cells and common lateral walls derived from the mother

cell.

If the plane of cell division is transverse to the long axis

of the thallus i.e., elongation followed by division, a

filamentous type of construction would be formed. This

type of multicellular thallus organization is seen in the

filamentous types, common to most of the algae. Under

the filamentous habit several types are possible.

Filaments may be branched or un-branched.

(i) Un-branched Filaments:

Simple un-branched filaments are found in

many forms. They are either free-living e.g.,

Spirogyra or attached, at least initially e.g.,

Oedogonium or aggregated in colonies e.g.,

Nostoc

In many Cyanophyceae it consists merely of a

row of cells connected closely (e.g.,

Oscillatoria). In the simpler forms e.g.,

Ulothrix, Spirogyra, there is no division of

labour. The cells are all alike, structurally and

functionally, may take part in growth and cell

division and in reproduction. The cells of

filaments may be uninucleate (e.g., Spirogyra)

or multinucleate (e.g., Cladophora).

(ii) Branched Filaments:

Branched filamentous structures may be put into three

categori es:

(i) Simple,

(ii) Heterotrichous

(iii) Pseudoparenchymatous.

They are put according to the shape and nature of the

thalli, a result of different types of cell behaviour

concerning growth and division.

(i) Branched Simple:

A simple branched filament with single

row of cells and a basal attaching ceil,

holdfast or hapteron is common with

many types e.g., Ulothrix, Oedogonium

In many, the branches arise immediately

below the cross walls, and the growth and

divisions are restricted to the end-cells of

the branches e.g., Cladophora. Simple

branched filaments are also seen in

Xanthophyceae, Chrysophyceae. A

peculiar form of branching, known as

„false‟-branching is observed in

Cyanophyceae e.g., Scytonema.

(ii) Heterotrichous:

This most highly evolved type of plant-body,

showing a good amount of division of labour, is

characteristic of the Chaetophorales among

Chlorophyceae, in many Phaeophyceae,

Rhodophyceae, in some Chrysophyceae and

Dinophyceae (e.g., Dinoclonium).

The plant-body consists of two distinct parts:

(1) A basal or prostrate creeping system, and

(2) An erect or upright system.

The prostrate system is attached to some

substratum, grows apically and gives rise to

numerous photosynthetic and rhizoidal

filaments. Rhizoidal filaments sometimes

penetrate the substratum (e.g., Fritschiella). The

erect system develops from the prostrate system

and is composed of one or more and usually

branched photosynthetic filaments.

(iii) Pseudoparenchymatous forms:

As indicated by the term „pseudo‟ = false, the plant

body gives the appearance of parenchymatous

construction. Parenchyma is a tissue composed of thin

walled closely associated cells which has arisen by the

division of a common parent cell. Whereas the

pseudoparenchymatous structure is a secondary

development, close association of cells is a result of

interweaving of filaments.

Through the establishment of secondary intercellular

connections the cells of pseudoparenchymatous algae

may be densely packed and firmly coherent (e.g.,

Dumontia, Rhodophyceae) or, the association may be

loose and the component filaments can easily be

separated by pressure (e.g., Castanea, Phaeophyceae).

Pseudoparenchymatous thallus

Two types are recognised in the

construction of the

pseudoparenchymatous thallus.

The body may have (1) a single

colourless central axial filament

(uniaxial construction) or (2)

many filaments (multi-axial)

around which photosynthetic

filaments are supported

The uniaxial construction in

simple form showing clearly the

filamentous nature as seen in

Batrachospermum

Multi-axial construction is seen in

Polysiphonia.It is interesting to note

that in many forms, such types of

constructions can be traced to a

primary heterotrichous condition in the

ontogeny of the thallus, one or many

threads uni- or multi- of the erect

system taking part in the production of

the mature thallus.

Secondary filamentous structures also

develop in many genera either

externally or internally.

Such internal filaments by close

association give a solid core like

structure in many forms. Secondary

external filaments (cortication) in

many cases increase the thickness of

the primary thallus (e.g.,

Desmarestia). Besides giving

rigidity to the body they play a

considerable role in the formation of

attaching discs (Fucales) and

branched haptera (Laminariales) in

many parenchymatous forms.

5. Siphonaceous Forms:

In a number of algae, belonging to

Siphonales e.g., in Vaucheria, Botrydium,

the growth of the plant body takes place

without the usual cross-wall formation

except during formation of reproductive

organs. Thus a „tube‟-like multinucleate

structure, or a coenocyte, is produced.

This structure is interpreted as a

multinucleate or coenocyte cell by some

and as acellular by others. The simplest

organization is in the form of a small un-

branched vesicle. It contains a central

vacuole with chloroplasts and nuclei in the

peripheral cytoplasm.

It is anchored by branching rhizoids (e.g., Botrydium). An

irregular branching system with rhizoids or haptera and

occasional septa formation in cutting off old siphons

andcytopia reproductive organs is found in Vaucheria

6. Parenchymatous Forms:

Parenchymatous thallus organization also

is a modification of the filamentous habit,

with cell division in more than one plane.

Depending upon the nature of cell division,

the parenchymatous thalli may be „leaf-

like‟ or foliose, tubular or highly

developed structure.

Flat, foliose or tubular thalli are formed by

the division of the cells two or three planes.

Common examples of flat and foliose

structures in Viva (Chlorophyceae),

Punctaria (Phaeophyceae) and Porphyra

(Rhodophyceae).

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