In this article we will discuss about the thallus organisation found in algae.
The plant body in algae is always a thallus. It is not differentiated in root, stem and leaves. Algae range in size from minute unicellular plants (less than 1 µ in diameter in some planktons) to very large highly differentiated multicellular forms e.g., some sea-weeds.
Their forms may be colonial (loose or integrated by inter-connections of protoplasmic strands), filamentous (branched or un-branched), septate (branched or un-branched), non-septate or branched, multinucleate siphonaceous tube where the nuclear divisions occur without usual septa formation.
Structural and cellular organizations are important characters in the classification of algae and in establishing the inter-relationship among them. Similarities of some morphological structures are seen among various classes of algae. The unicellular types which are seen in all groups of algae except the brown algae are considered to be the basic type from which, through evolution, other types of thalli developed.
The range of thallus organization in algae may be classified as follows:
1. Unicellular
Motile and non-motile
2. Aggregates
Palmelloid and Dendroid
3. Colonial
(a) Colony motile
(b) Colony non-motile
4. Filamentous
(a) Un-branched
(b) Branched
(i) Simple
(ii) Heterotrichous
(iii) Pseudoparenchymatous.
5. Siphonaceous.
6. Parenchymatous.
1. Unicellular Type:
The unicellular types are seen in all groups of algae with the exception of the class Phaeophyceae. The unicellular types may be amoeboid motile or non-motile, the motility being due to the movement of the flagella attached at the anterior end.
The number of flagella may be one or more, usually two or in multiples of two. The cells may have a rigid cell-wall or the outer layer of the protoplast forms do the ‘periplast‘ giving a definite shape to the cell or may allow changes in shape (rhizopodial or amoeboid type).
(a) Unicellular Amoeboid Forms or Rhizopodial Forms:
These algae lack flagella, the organs of motion, but are able to perform amoeboid movement by means of cytoplasmic growth e.g., Chrysamoeba (Fig. 1).
(b) 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.
Nature of flagellation, type and number of flagella and the attachment of flagella, is an important character in classification. In most of the Chlorophyceae members, the flagella are usually two or in multiples of two in number, equal in size, of whiplash type and anteriorly attached (Fig. 2).
The unicellular plant body may be spherical, oblong or pear-shaped and sometimes elongated (Fig. 2); and approximately circular in cross section. In some cases, the plain body proper and peripheral layer is separated by a space. This envelope is spherical, rigid, variously shaped and is provided with apertures for the protrusion of the flagella.
Such types are termed ‘en-capsuled’ e.g., Haematococcus etc. The common examples of unicellular flagellated forms are Chlamydomonas, Chlorogonium, (Chlorophyceae), Ochromonas, Chromulina (Chrysophyceae) etc.
The unicellular flagellated forms can be ‘naked’ and ‘en-capsuled’ e.g., Crypto-monas. Euglena (naked) and Chrysococcus (encapsuled). A feature common with the motile of flagellated forms is the presence of eye-spot (stigma).
(c) Unicellular Non-motile (Protococcoidal) Forms:
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 (Fig. 3).
Cell-wall ornamentations of various types may occur e.g., in the Desmids (Conjugates), they are in the form of ribs, spines or warts and more complex among the Diatoms (Bacillariophyceae) due to the poroid nature of their siliceous wall.
Though non-flagellated, many diatoms may be motile by the extrusion of mucilage through the pores, some Desmids and Cyanophycean members also show slight motility. In a very large number of cases the motile flagellated types, at times, lose their flagella and become non-motile.
This is a period of rest in the changeover from vegetative phase to reproductive phase. This feature gets further elaborated in many forms where the vegetative phase is completely non-motile. They are motile in the re-productive phase only. Such developments ultimately lead to palmelloid, dendroid and coccoid habits.
2. Aggregates:
Aggregates are formed by the collection of single cells to make thallus. Unlike the coenobium the aggregation of cells does not have fixed number of cells shape or size.
The cells are aggregated into more or less irregular colony like mass. When the cells divide, the daughter cells remain in same gelatinous mass. Thus there is increase in the number of cells after division. The aggregates can be palmelloid, dendroid and rhizopodial in form.
(a) Palmelloid Habit:
In a large number of genera this habit is a permanent one e.g., Tetraspora (Chlorophyceae), Phaeocystis (Chrysophyceae) or is a temporary phase, in the life-cycle e.g., Chlamydomonas (Chlorophyceae) Chromulina, (Chrysophyceae). The habit is named after the genus Palmella (Palmeflaceae, Chlorophyceae) (Fig. 4).
Here the individual lion-flagellated cells have mucilaginous sheaths around them and are enveloped in a common gelatinous matrix of indefinite shape which may be microscopic or macroscopic in size.
Such ‘colony’ like or ‘filamentous’ structures (e.g., Hydrorus) in which the individual cells are loosely aggregated in a common gelatinous matrix and behave independently are termed Palmelloid. (‘Palmelloid-stage’ means Temporary phase and ‘palmelloid-habit’ or ‘palmelloid-form’ denotes permanent habit).
(b) Dendroid Forms:
A variation of the palmelloid condition is seen in dendroid colonies. Here the mucilage is produced locally, generally at the base of the cell. This polarity differentiates the two habits. Dendroid forms are seen e.g. Prasinocladus (Chlorophyceae), Mischococcus (Xanthophyccae) and in Chrysophyceae and Euglenineac (Fig. 5 A, B).
(c) Rhizopodial Forms:
Variable number of amoeboid cells formed together by cytoplasmic projections give the shape of roots or rhizoids e.g., in Chrysidiostum (Chrysophyceae).
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) (Fig. 6). Mostly they are coenobia (sing, coenobium) i.e., colonics composed of definite number of cells arranged in a defined manner.
Thus, a clear advancement in the organization of the thallus is seen in the sequence gradually from unicellular motile types to aggregates in a colony with a regular pattern, where the cells are inter-connected and behave as a single unit (multicellular organism).
(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) or connected by mucilaginous threads (e.g., Dictyosphaerium) and the colony may be of various shapes (Fig. 7). It may be plate like e.g.. Scenedesmus or net-like as in Hydrodictyon.
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 (Fig. 9), or aggregated in colonies e.g., Nostoc (Fig. 8 B).
The most simple type of filament construction is seen among Ulotrichales. The filament is the most elementary type of thallus as seen in genera Ulothrix, Spirogyra (Chlorophyceae) (Fig. 9), Tribonema (Xanthophyceae), Nematochrysis (Chrysophyceae).
In many Cyanophyceae it consists merely of a row of cells connected closely (e.g., Oscillatoria) (Fig. 8 A). 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 categories:
(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 (Fig. 9).
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 (Fig. 10). 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 (Fig. 10).
(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.
However, there are several types of heterotrichous habit on account of the elaboration and reduction of one system over the other. In Stigeoclonium and Trentepohlia (Fig. 11) these two systems are equally developed and easily distinguished.
Whereas in Coleochaete (Chlorophyceae) (Fig. 11) and Ascocyclus (Phaeophyceae) the prostrate system is highly elaborated and the erect system is reduced.
This gives the body a discoid type of appearance. On the other land, in Draparnaldiopsis (Chlorophyceae), in many species of Ectocarpus (Fig. 11), the prostrate system is reduced, and the erect system is well developed. This gives the body a crust or cushion type of appearance.
In many cases the reduction of one system has gone to the limit of practical elimination. Thus in an endophytic species, Endoderia and in Chaetopeltis (Chlorophyceae) only the prostrate system is developed. Complete absence of prostrate system is noticed in Microthamnion, and a comparable condition in Drapanaldiopsis (Chlorophyceae), Sphacelaria (Phaeophyceae) and Batrachosperumum (rhodophyceae).
The production of thread-like structures from the cells of the multicellular thallus (cortication) is a method of increasing rigidity among many Phaeophyceae and Rhodophyceae and in some members of Chlorophyceae (e.g., Draparnaldiopsis).
(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).
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 (Fig. 12, 13).
The uniaxial construction in simple form showing clearly the filamentous nature as seen in Batrachospermum (Fig. 12). Uniaxial pseudoparenchymatous types are also seen in Dasycladaceae (Chlorophyceae) and in Phaeophyceae (e.g., Spermatochnus) and in Polysiphonia (Rhodophyceae, Fig. 13).
Multi-axial construction is seen in Codiaceae (Chlorophyceae) and in many Ectocarpales (Phaeophyceae). 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 (Fig. 13).
5. Siphonaceous Forms:
In a number of algae, belonging to Siphonales e.g., in Vaucheria, Botrydium, the growth of the plant body lakes 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 (Fig. 14). In other Siphonales, two basic patterns are seen.
In one the siphon has become increasingly large and branched (e.g., Caulerpa) and the other is an intricate system of interwoven vesicles and tubes (e.g., Codium). Most of the tropical Siphonales are calcified which, in a way, provide the mechanical support to the thallus (e.g., Halimeda).
6. Parenchymatous Forms:
Parenchymatous thallus organization also is a modification of the filamentous habit, with cell division in more than one plane (Fig. 15 A, B). Depending upon the nature of cell division, the parenchymatous thalli may be ‘leaf-like’ or foliose (Fig. 15 A), tubular or highly developed structure (Fig. 15 B).
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).
The example of tubular structure is Enteromorpha (Chlorophyceae). In some phaeophyceae e.g., in lictocarpales and Sphacelariales, the parenchymatous form develops by abundant separation of primary filament. In Phaeophyceae (e.g., Sargassum) cells of the thallus are differentiated into central medulla, middle cortex and outer merislodenn (Fig. 15 B).