Range of thallus structure of algae:
the vegetative structure of algae shows a wide variety and it ranges form from unicellular to complex multicellular thalli. Their size ranges from one micron to several meters. on the basis of thallus organisation, algae are divided into the following five groups:
I) Unicellular forms:
Simple unicellular forms are found in all groups algae except charophyceae and pheophyceae. These forms are some times referred to as acellular since they function as complete living unit without any cellular differentiation. Unicellular forms can be classified into the following sub-groups.
a) Rhizopodial unicells. These forms lack a rigid cell wall. They possess cytoplasmic projections which help them in amoeboid movement. In the absence of rigid cell wall, these forms are periplastic; e.g., Rhizochrysis, Rhizochloris, Chrysamoeba.
b) Flagellated unicells. Flagellated vegetative cells are found in all groups of algae except cyanophyceae, pheophyceae and rhodophyceae. They vary in the number and type of flagella. For example, in chlorophyceae, vegetative cells, zoospores and gamets have two equal flagella. Their number may sometimes vary. In dinophyceae and xanthophyceae there are two unequal flagella; in the former they emerge in different planes and in the latter in the same plane. In euglenophyceae there is only one flagellum, inserted at the anteior end.
Flagellated unicells may be periplastic without cell wall or with a distinct cell wall. In phacotus, there is a thick calcareous covering around the cell wall.
c) Spiral filamentous unicells. Some unicellular algae form spiral or coiled structures, e.g., spirulina.
d) Non-motile unicells. These are non-motile coccoidal algae which do not possess flagella, eye spot, etc. meant for locomotion. The simplest non-motile forms are found in cyanophyceae where they do not possess well organised nucleus and plastids, eg., chroococcus. the non-motile unicells of bacillariophyceae, such as diatoms, are made up of two halves or theca, joined by a girdle band.
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| unicellular algae |
II) Colonial forms:
The colonial habit is achieved by aggregation of the products of cell divisions within a mucilagenous mass, by aggregation of motile cells or juxtaposition of cells subsequent to cell divisions. Therefore, all members of a colony have similar structure. These association are usually loose, and as such a colony may break into smaller pieces. In some colonial forms, all members of a colony are connected with each other by cytoplasmic connections, hence they cannot break into segments. On the basis of morphology, colonial organisation may be divided into the following four types:
a) Coenobial. A colony with a definite shape, size and arrangement of cells is known as coenobium. 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 or non motile; in the former cells are flagellated and in the latter without flagella.
b) Palmelloid. Unlike 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 mucilagenous matrix, but they are independent and function as individuals. In some palmelloid forms it is a temporary phase, where as in others it is a permanent feature as in tetraspora, aphanotheca.
c) Dendroid. Here the colony looks like a microscopic tree. As in palmelloid colony, the number, shape and size of the cells is also indefinite in dendroid colonies. A mucilagenous thread is present at the base of each cell, and the threads of different cells are united to form a branched structure. It gives a tree like appearance to the whole colony.
d) Rhizopodial. In a rhizopodial colony the cells are united through rhizopodia.
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| colonial algae |
III) Filamentous forms:
Filamentous forms are developed by repeated transverse divisions of cells. The daughter cells do not separate and they remain attached one upon the other in a definite sequences to form a filament. The filaments may be branched or un branched, and the cells in a filament may be arranged in a single row or more than one rows.
a) Unbranched filaments. Simple unbranched filaments are found only in a few groups of algae. such filaments may be free floating, attached to some substratum or form colony.
b) Branched filaments. Branched filaments are formed by repeated transverse divisions of lateral outgrowths of cells. The branching of filaments may be false or true. A false branch does not arise as lateral outgrowth, but the trichome may break either due to death or decay of intercalary cells or at the point of heterocyst. The broken end emerges out of the mucilagenous sheath in the form of a branch, e.g., scytonema. False branches in scytonema arise almost always in pairs at some distance from the heterocyst.
True branches, which arise as lateral outgrowths, may result in the following three types of filaments:
i) Simple filaments. Simple branched filaments remain attached to the substratum by a basal cell. In such filaments branches may arise from any cell except the basal cell. In cladophora branches arise just below the septa between two adjacent cells.
ii) Heterotrichous. In this type the thallus is very much evolved and differentiated into prostrate and erect systems. Both the prostrate and the erect system may be well developed or there is a progressive elimination of prostrate or erect system.
iii) Pseudoparenchymatous. In many filamentous forms one or more central or axial filaments, together with their branches, form a parenchymatous structure. If a pseudoparenchymatous thallus is formed by the branches, formed by the branches of only one filament, it is called uniaxial, and if branches of more than one filaments are involved, it is said to be multiaxial.
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| filamentous algae |
IV) Siphonaceous forms. Here the thallus is made up of branched, aseptate, ceonocytic, tubular filaments as the nuclear divisions are not accompanied by wall formation, e.g., vaucheria
V) Parenchymatous forms:
In this type the flat foliose or tubular thalli are 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, like flat, tubular or complex . The growth of such thalli is apical, intercalary or trichothallic.
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| siphonaceous and parenchymatous algae |
