Morphology of algae, Biology tutorial


Algae are positioned in Kingdom Protista all along with protozoa. Earlier they were categorized with plants as they are photosynthetic autotrophs possess chloroplasts and chlorophyll and superficially appear similar to plants. As their gametes don't encompass protective cells around them they are no longer categorized with plants. 

The morphology of algae is simple in structure, deficient in differentiation, algae represent great diversity in appearance and size. Their size ranges from simple microscope to giant thallus extending some meters in length as in kelps. Algal morphology differs from simple unicellular form to complex thallus as found out in the seaweed. 

Algae are broadly distributed in nature if there is plenty of water and sunshine. They as well take place abundantly on wet rocks, wet ground and a pool of water. They as well inhabit the harsh habitats.

Algal Morphology:

The science or study of algae is termed as 'Phycology'. One who specializes in the study of algae is termed as 'Algologist' or 'Phycologist'.

The body of an alga is termed as thallus. In unicellular algae it is simple comprising of a single cell. All multicellular organisms begin their life as single cells. Whenever a cell splits and the daughter cells form a packet surrounded in a mucilaginous mass, a colony is made. Whereas the division of a cell constantly in the similar plane having daughter cells sticking altogether, outcomes in a row of cells make a filament. A few cells of a filament divide or split only once by a vertical plane followed through transverse divisions repeatedly and therefore produce filamentous branched thallus. Moreover, if all the cells of a filament experience divisions in cross and vertical planes it outcomes in a sheet of one or more cells in thickness. Such multicellular thallus might represent complicated differentiation as in the seaweed. All multicellular algae represent the above phases all through their development.

Morphologically algae can be differentiated as unicellular, colonial, filamentous, heterotrichous, thalloid and polysiphonoid forms. Each of such kind is explained below.

1) Unicellular Forms:


Single cells, cylindrical, short or long; at times very long snake forms. Cells divide or split by constriction, the two daughter cells get separated, and they rarely remain altogether to form a 2-celled filament.

Individual single cells might encompass their own mucilaginous cover around them. Some of such cells might be enclosed in common colorless mucilage providing the impression of a colony. 


The single celled alga includes a nucleus, a cup-shaped chloroplast in which 1 pyrenoid is generally present. The chloroplast on the anterior side exhibits 2 to 3 rows of fatty red colored granules. This is termed as stigma or eyespot which is useful for the alga to respond to light. The cell-wall is firm and dissimilar. A small contractile vacuole is found at the base of every flagellum.

Chlamydomonas cells beneath partially dry conditions splitted and the daughter cells devoid of flagella remain enclosed through a common mass of mucilage. Such a colony is termed as palmella phase of Chlamydomonas. This is merely a temporary phase and on flooding with water individual cells build up flagella and escape swimming away from the colony. Therefore the starting of the colony construction found in the Volvox can be observed in Chlamydomonas.

2) Colonial Algae:

Whenever a cell splits or divides and the daughter cells formed remain altogether within a common mucilage mass, it is termed as a colony. A colony might include large number of cells. At times it might be so big that one can view it with unaided eyes.


This is a colonial alga, most general in polluted lakes and ponds. At times the colonies are very big and can be viewed by unaided eyes. They gather on the surface of water forming fairly a thick layer in some seasons (that is, water blooms).

Single cells are spherical and colony is build up because of loose aggregates of several thousand cells held by means of mucilage. The colonies float on the surface of water since of the presence of lengthens cylindrical gas vesicles within the individual cells.


The colonies of Volvox are spherical, ball-like and big adequate to be observed with unaided eye. Each colony includes l000 to 5000 cells arranged on the exterior of a mucilaginous ball termed as coenobium. Coenobium is a colony in which the number of cells is fixed at the time of development. No further addition of cells takes place. Usually the cells are as well in a special arrangement.

In Volvox the entire cells of a colony are derived from the single parental cell. They are organized on the surface of mucilaginous ball, joined with other cells using cytoplasmic connections. A few cells behave as sex cells meant for reproduction while others remain vegetative and finally grow old and die. This differentiation into vegetative and reproductive cells is an extremely significant feature in the growth of multicellular organisms.

3) Filamentous Forms:

Whenever a cell divides or splits cross-wise and the daughter cells don't separate from one other, it outcomes in a linear row of cells as in Nostoc, Ulothrix and Oedogonium. Though, the three algae exhibit dissimilar levels of differentiation.


This is a simple, single row of cells, uniseriate and of filamentous form. Some of the filaments of Nostoc are usually enclosed in common mucilage envelop to prepare a colony. A few cells in between the vegetative cells are transformed into heterocysts. Heterocyst is a highly differentiated cell in some of the filamentous blue green algae which is a site of nitrogen fixation. All the vegetative cells are able of building into spores termed as akinetes. Akinete is a thick1walled, non-motile reproductive cell found in the algae.

4) Heterotrichous Forms:

Whenever a few cells of a filament divide or split vertically it outcomes in a branch. Most of the filamentous forms exhibit extensive branching of the main filament giving it a bushy look.

In several algae the branches at the base remain horizontal, joined to the substratum termed as prostrate system from which erect system of vertical branched filaments occur. This kind of body is termed as heterotrichous habit. Heterotrichous habit is the most highly build up filamentous construction in the algae.


It is a kind of heterotrichous alga that exhibits greater differentiation in plant body. The prostate system is very much diminishing. The major axis includes long intermodal cells alternating by short nodal cells. The short nodal cells bear a bunch of short branches. A few of the side branches might build up into long colorless setae or hairs. The major axis produces at the base long multicellular colorless rhizoids in large number to prepare a type of cortex.

Their major function is to join the alga to the substratum.


It is the other heterotrichous alga. The prostrate system that attaches the alga to the substratum is build up of branched filaments. The erect system is in the form of uniseriate (that is, single row of cells) branched filaments making loose tufts of 1mm to 10 mm or more. The branches occur just beneath the cross walls of the cells of the main filament. Most of such branches terminate in the elongated hairs.

5) Thalloid Forms:

Whenever the cells of a filament divide or spilt in more than one plane that is not just cross-wise however as well length wise it outcomes in a sheet of cells. The thallus might be one cell or many cells in thickness.


Ulva is a very general alga found on the rocky coasts of sea. The thallus is joined to the substrate like rocks through rhizoids at the base.


Fucus is brown algal seaweed very general on the rocky coasts of sea in temperate countries. The body of Fucus is big around half a meter or so in length. It consists of a basal discoid holdfast, a short stipe and long flat and dichotomously branched blades and fronds. Dichotomous branching pattern is one in which the two arms of the branch are more or less equivalent in length. At the tip of the blade are found air bladders that make up the plant float in water. 

6) Polysiphonoid Forms:

This kind of algae is more complicated than the earlier explained forms. It is found in the red alga Polysiphonia that is marine in habitat.


The algae exhibit in general heterotrichous habit. The prostrate system is in the form of a lengthened rhizoid which joins the algae to the substratum. The erect system is highly branched. The branches are of two types, some are long and some short and hair-like. The major filament grows by the division of a single apical cell. The mature plant body is made up of up central row of cells-central siphon, bounded by vertical rows of cells, 4 to 24 pericentral siphons.

All the pericentral cells are joined with the cells of central siphon and are as well joined with one other.

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