Algae are a group of autotrophic, non-vascular thalloid plants with chlorophyll and without any formation of embryo. They are mostly photosynthetic in nature. The plant body may be unicellular, colonial or multicellular.
Algae possess a simple plant body which is called thallus. The thallus is not differentiated into true root, stem and leaves. They also lack vascular tissue system, so xylem and phloem are absent in algae.
The size of algae is highly variable. Some members are microscopic single celled forms, while some members are large multicellular sea weeds. The giant sea weeds may grow upto about 60 meter in length.
Sex organs in algae are simple in structure. They are generally unicellular and when multicellular, they are not surrounded by any sterile jacket layer. After gametic fusion embryo is not produced, and this is a major difference from higher plants.
Algae are not a single natural taxonomic group. They are polyphyletic in origin, because different algal groups have developed from different evolutionary line. Most algae are eukaryotic, but cyanobacteria or blue-green algae are prokaryotic and are traditionally studied with algae.
Algae are mainly aquatic plants. They are found in freshwater and marine water. Some members also occur in moist soil, on rocks, tree bark, hot springs and snow fields.
Some common examples of algae are Chlamydomonas, Volvox, Spirogyra, Ulothrix, Ulva, Sargassum and Laminaria.
What is the thallus organization of algae?
The plant body of algae is known as thallus. It is not differentiated into true root, stem and leaves. The vascular tissue system is also absent, so xylem and phloem are not present in algal thallus.
The thallus organization in algae is highly variable. It ranges from simple microscopic unicellular body to large multicellular sea weeds. On the basis of structure and arrangement of cells, algal thallus is divided into different forms.
Unicellular thallus is the simplest type of thallus organization. In this type, the plant body is made up of single cell and it performs all the life activities independently. It may be motile by flagella or amoeboid movement, or it may be non-motile.
In some algae, many cells remain together and form colonial thallus. The colony may be fixed in number and arrangement of cells, which is called coenobium. Sometimes the colony is irregular and the cells are held together by common mucilaginous matrix.
Filamentous thallus is formed when cells divide in one plane and remain attached end to end. The filament may be unbranched or branched. In some advanced forms, the filament becomes heterotrichous, where there is one prostrate system for attachment and one erect system for growth.
Siphonaceous thallus is also called coenocytic thallus. In this type, the thallus is tubular and without internal cross walls. The cytoplasm is continuous and contains many nuclei, so the whole thallus behaves like a multinucleate structure.
Parenchymatous thallus is formed by cell division in two or more planes. It gives rise to flat leaf like body, tubular forms or large complex body as found in brown algae. In giant kelps, the thallus becomes much differentiated but true root, stem and leaves are still absent.
Pseudoparenchymatous thallus looks like parenchymatous tissue. But it is not formed by division in many planes. It is formed by compact arrangement and fusion of filaments and their branches.
Types of thallus organization in Algae
- Unicellular forms – It is the simplest structural level of algal thallus. The thallus consists of single independent cell. These forms may be motile by flagella or amoeboid rhizopodia, or they may be non-motile like simple spherical or spiral forms.
- Multicellular Forms
- Colonial forms – In this type the cells remain grouped together within a common mucilaginous matrix. It includes coenobial colonies with definite shape, size and cell number. It also includes palmelloid, dendroid and rhizopodial colonies.
- Filamentous forms – In this type the thallus is thread like chain. It is formed when cells divide and remain attached end to end. It may be unbranched or branched, and advanced forms include heterotrichous filaments and pseudoparenchymatous forms.
- Siphonaceous forms – It is also called coenocytic form. These are large, hollow and tubular structures without internal cross walls or septa. So it forms a single continuous cellular compartment with many nuclei.
- Parenchymatous forms – It is complex, solid and three dimensional thallus. It is formed when cells divide in many planes. This type produces flat leaf like sheets, tubular structures or highly differentiated complex body as seen in giant kelps.
The algal thallus organization can be classified in these following groups such as;
A. Unicellular Forms
- Unicellular algae are acellular form of algae where the whole plant body is made up of only one cell. This single cell performs all metabolic, sensory and reproductive functions. The cell may be spherical, pear shaped or elongated in form.
- Unicellular forms are found in almost all algal groups. But they are absent in Phaeophyceae and Charophyceae. They represent the simplest type of thallus organization in algae.
- In rhizopodial or amoeboid unicells, the cell wall is absent and the protoplast remains naked. The cell is soft and it can change its shape during movement. Flagella are absent in this type.
- In this form, movement and food capture takes place by cytoplasmic extensions called pseudopodia or rhizopodia. Common example is Chrysamoeba.
- Flagellated unicells are motile unicellular forms. They have one or more flagella for movement. The number and type of flagella may vary in different algae.
- Some flagellated unicells are enclosed by rigid cell wall, as found in Chlamydomonas. Some are periplastic forms and cell wall is absent, as found in Euglena.
- Non-motile coccoid unicells are small, simple, spherical or ovoid unicellular forms. They have a rigid cell wall. They do not have locomotory organs like flagella or pseudopodia, so active movement is absent during vegetative phase.
- Chlorella is a well-known coccoid unicellular algae. It remains non-motile in vegetative condition.
- Non-motile spiral unicells are special type of non-motile unicellular form. The body is elongated, helical or spirally coiled. It looks like multicellular filament in low magnification, but actually it is one continuous and undivided cell.
- Spirulina is the example of non-motile spiral unicellular form. Its body is spirally coiled but not divided into many cells.
1. Motile form of Unicellular Algae
Motile unicellular algae are single celled independent forms which can move from one place to another. The whole thallus is represented by one cell. They are mainly of two types-
a. Rhizopodial or amoeboid unicells
In this type rigid cell wall is absent. The protoplast remains naked and it is surrounded by a soft periplasmic envelope. Due to absence of rigid wall, the cell can change its shape and size.
Flagella are absent in them. They move by cytoplasmic projections called pseudopodia or rhizopodia. This type of movement is called amoeboid movement.
Examples are Chrysamoeba, Rhizochloris and Rhizochrysis.
b. Flagellated unicells
In this type the cells are spherical, pear shaped, elongated or ovoid. The cell may be covered by rigid cell wall as in Chlamydomonas. In some forms cell wall is absent and they are periplastic as in Euglena.
These forms resemble motile gametes and zoospores. They move actively with the help of flagella. The flagella may be one to many in number. They may be equal or unequal in length and may be tinsel or whiplash type.
They are present in almost all major algal groups except Cyanophyceae, Phaeophyceae, Rhodophyceae and Bacillariophyceae.
Examples are Chlamydomonas, Euglena, Phacotus, Chlorogonium and Cryptomonas.
2. Non Motile form of Unicellular Algae
Non-motile unicellular algae are single celled independent forms which cannot move actively from one place to another. They lack locomotory structures like flagella, pseudopodia and eyespot. The whole thallus is represented by one cell only.
They are mainly of two types-
- Coccoid forms
In this type the cells are usually small and simple. They may be spherical, ovoid or slightly elongated in shape. The cell wall is rigid and flagella are absent during the vegetative phase. These forms remain as solitary independent cells or sometimes they may remain together inside a gelatinous matrix. Some coccoid forms may become motile only during reproductive stage. Common examples are Chlorella, Chlorococcum, Prochloron, Synechococcus, Aphanocapsa and Gloeocapsa. - Spiral forms
In this type the body is elongated, helical or spirally coiled. It is a specialized type of non-motile unicellular form. Due to spiral body, it may look like multicellular filament under low magnification. But actually it is made up of single continuous and undivided cell. The common example is Spirulina.
B. Multicellular Forms
Multicellular forms of algae are those forms in which the thallus is made up of many cells. The cells remain together and form different types of plant body. The main multicellular forms are colonial forms, filamentous forms, siphonaceous forms and parenchymatous forms.
1. Colonial Forms
Colonial forms are formed when many unicellular algal cells remain together in a common mucilaginous matrix. The cells may show definite or indefinite arrangement. The main types of colonial forms are as follows-
- Coenobial colony
In this type the colony has definite shape, size and arrangement of cells. The number of cells is also fixed. It may be motile as in Volvox or non-motile as in Hydrodictyon. - Palmelloid colony
In this type the cells are independent and non-motile. They are irregularly embedded in a common gelatinous matrix. The colony has no definite shape and size. Examples are Tetraspora and Palmella. - Dendroid colony
In this type the cells are united by mucilage at their base in a branching manner. Due to this arrangement, the colony appears like a small tree. Examples are Dinobryon and Chrysodendron. - Rhizopodial colony
In this type the cells of the colony are connected with each other by fine cytoplasmic threads or rhizopodia. Example is Chrysidiastrum.
2. Filamentous Forms
Filamentous forms are thread like thallus formed by repeated division of cells in one plane. The cells remain attached end to end and form a row. The main types of filamentous forms are as follows-
- Unbranched filament
In this type the cells form a simple chain without any lateral branches. It may be free floating as in Spirogyra or attached to substratum by basal holdfast cell as in Ulothrix. - Branched filament
In this type the filament produces lateral outgrowths. The branching may be true as in Cladophora or false branching as in Scytonema. False branching is formed when broken trichome comes out from the sheath. - Heterotrichous form
It is a highly developed branched filamentous form. The plant body is differentiated into prostrate creeping system and erect vertical system. Examples are Ectocarpus and Fritschiella. - Pseudoparenchymatous form
In this type the central filaments and their lateral branches are densely interwoven and fused together. Due to compact arrangement, it gives false appearance of solid tissue. Example is Polysiphonia.
3. Siphonaceous Forms
Siphonaceous forms are also known as coenocytic forms. The thallus is large, hollow and tubular. Internal cross walls or septa are absent.
The cytoplasm remains continuous and contains a central vacuole with many free nuclei. Examples are Vaucheria and Botrydium.
4. Parenchymatous Forms
Parenchymatous forms are solid and three dimensional thallus. It is formed when cells divide in two or more planes. This type of thallus forms flat leaf like sheets, tubular structures or highly differentiated bodies.
Examples are Ulva and giant Laminaria kelps.
1. Colonial forms of algae
Colonial forms of algae are formed when many individual vegetative cells remain together in a common mucilaginous matrix. The cells are grouped together, but they may remain physiologically independent. This type of thallus organization is more advanced than simple unicellular form.
The following are the main types of colonial forms in algae-
a. Coenobial Colonies
Coenobial colony is also called coenobium. It is a highly organized type of colony with definite shape, size and fixed number of cells. The number of cells is fixed in the juvenile stage and after that the colony increases mainly by enlargement of cells, not by adding new cells.
Coenobial colonies are of two types-
Motile coenobia
In this type, the cells have flagella. The whole colony can move in a coordinated manner. Examples are Volvox, Pandorina and Eudorina.
Non-motile coenobia
In this type, the cells do not have flagella. The cells may form definite geometric arrangement like net like structure or flat star like plates. Examples are Hydrodictyon, Pediastrum and Scenedesmus.
b. Palmelloid Colonies
Palmelloid colony is formed by non-motile cells which are irregularly embedded in a loose gelatinous matrix. The colony has no definite shape, size and fixed number of cells. Each cell remains independent in its function.
This condition may be permanent as in Tetraspora, Aphanothece and Palmella. It may also be temporary survival stage during unfavourable condition as in Chlamydomonas.
c. Dendroid Colonies
Dendroid colonies show branched and microscopic tree like appearance. In this type, mucilage is secreted mainly at the base of each cell. Due to this basal mucilage, the cells remain attached with each other in branching stalk like manner.
Examples are Dinobryon, Chrysodendron and Prasinocladus.
d. Rhizopodial Colonies
Rhizopodial colony is a type of colony where individual cells are connected with each other by fine cytoplasmic threads. These cytoplasmic projections are called rhizopodia. They help the cells to remain united and communicate with each other.
Common example is Chrysidiastrum.
2. Filamentous Forms
Filamentous forms of algae are formed when the cells divide repeatedly in single plane and remain attached end to end. Due to this arrangement, a linear chain or thread like body is formed. This thread like structure is called trichome. The adjacent cells are usually separated by transverse cross walls or septa.
a. Unbranched Filaments
Unbranched filaments consist of a single straight row of cells. Lateral branches are absent in this type. The cells remain arranged one after another and form a simple filament.
In free-floating type, the filament has no special attachment structure. It floats freely in water and all the cells are almost similar in appearance. Example is Spirogyra.
In attached type, the filament is fixed to the substratum. The attachment is done by a modified basal cell called holdfast. This basal cell is non-photosynthetic. Examples are Ulothrix and Oedogonium.
In colonial type, simple unbranched filaments remain together inside a common mucilaginous envelope. Thus they form a colony like mass. Example is Nostoc.
b. Branched Filaments
Branched filaments are formed when some cells divide in second plane and produce lateral outgrowths. Due to this, branches are formed from the main filament. Branching may be false branching or true branching.
In false branching, the trichome breaks internally. This may occur due to death of an intercalary cell or at the place of heterocyst. The broken end comes out through the mucilaginous sheath and gives appearance of a branch. Example is Scytonema.
In true branching, the branches arise directly as lateral outgrowth from the cells of main filament. In simple true branched form, the whole thallus remains attached by basal holdfast. Lateral branches may arise from any cell except the basal attachment cell. Example is Cladophora.
c. Heterotrichous Forms
Heterotrichous form is a highly developed type of true branched filament. In this type, the thallus shows division of labour. The plant body is differentiated into two systems, one is prostrate system and another is erect system.
Prostrate system is horizontal and creeping in nature. It helps in anchoring the plant body to the substratum. Erect system grows vertically upward into water or air and helps mainly in photosynthesis.
In some algae both the systems are well developed, as in Ectocarpus and Fritschiella. In some forms, one system may be gradually reduced or eliminated, as seen in Coleochaete.
d. Pseudoparenchymatous Forms
Pseudoparenchymatous form is a complex filamentous organization. In this type, one or more central axial filaments and their lateral branches become densely interwoven and fused together. Due to this close arrangement, the thallus looks like solid three dimensional parenchymatous tissue.
In uniaxial type, the thallus is formed by branching and compaction of single central filament. Example is Batrachospermum.
In multiaxial type, the thallus is formed by many independent central filaments growing closely together. Example is Polysiphonia.
3. Siphonaceous forms
Siphonaceous forms are also called coenocytic forms. In this type of thallus organization, the algal body becomes large without formation of normal cross walls. It is a special type of organization in algae.
The plant body is made up of large, hollow and tube like structure. This structure is called coenocyte. The thallus is aseptate, because internal cross walls or septa are absent during normal vegetative growth.
In this type, the nucleus divides repeatedly but the cytoplasm does not divide. This is called free nuclear division or karyokinesis without cytokinesis. Due to this, many nuclei remain freely present in the same continuous cytoplasm.
The cytoplasm forms a peripheral layer. It contains many free nuclei and chloroplasts. This continuous cytoplasm surrounds a large central vacuole, which is single and uninterrupted.
Septa are generally not present in the vegetative thallus. They may be formed only during formation of reproductive organs or when any damaged part is to be separated from the remaining plant body.
The siphonaceous thallus may show different forms. It may be simple sac like or saccate, unbranched tube or branched tube. In some forms, the thallus becomes differentiated into subterranean and aerial portions.
In Caulerpa, the body becomes very much complex and macroscopic. It shows parts which look like root, stem and leaves. But they are not true root, stem and leaves.
This type of organization is mainly found in Chlorophyceae, Xanthophyceae and Ulvophyceae. Common examples are Vaucheria, Botrydium, Caulerpa, Bryopsis and Codium.
4. Parenchymatous Forms
Parenchymatous forms are highly advanced type of thallus organization in algae. In this type, the plant body becomes solid and three dimensional. It is not a simple filamentous body.
This type of thallus is formed when the cells of primary filament divide in two or more planes. The daughter cells do not separate from each other. Due to this multidirectional division, a true tissue like body is formed.
The parenchymatous cells remain tightly attached with each other. They are held by common extracellular matrix (ECM). The cells also remain connected by intercellular connections like plasmodesmata or sieve pores.
The growth of parenchymatous thallus may be of different types. In diffused growth, all cells of the thallus can divide. In intercalary growth, the dividing region remains in the middle position, as found in Laminaria.
In trichothallic growth, the growth occurs by a special meristem present at the base of terminal hair. Example is Porphyra. In apical growth, growth takes place by one or more apical cells present at the tip, as found in Fucus.
Parenchymatous thallus shows different structural forms. It may be flat and leaf like foliose sheet. Such thallus is usually made up of one or two cell layers, as found in Ulva or sea lettuce and Porphyra.
Some parenchymatous algae form hollow tubular structures. The tube has solid parenchymatous wall. Examples are Enteromorpha and Scytosiphon.
In some brown algae, the thallus becomes very large and highly differentiated. It shows cormus like body and looks like root, stem and leaves of higher plants. This type is found in Sargassum, Laminaria, Macrocystis and Fucus.
Cheatsheet on Thallus Organisation in Algae
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