Algae are placed in Kingdom Protista along with protozoa. Earlier they were classified with plants as they are photosynthetic autotrophs-possess chlorophyll and chloroplasts and superficially appear like plants. Since their gametes do not have protective cells around them they are no longer classified with plants.

In this article on algae, you will study the morphology of algae. Although simple in structure, and lacking differentiation, algae exhibit great diversity in size and appearance.

Their size ranges from simple microscopy to giant thallus extending several meters in length as in kelps. Algal morphology varies from simple unicellular form to complex thallus as found in seaweed.

Algae are widely distributed in nature whenever there is plenty of water and sunshine. They also occur abundantly on wet rocks, wet ground, and a pool of water. They also inhabit harsh habitats.

Algal Morphology

The science or study of algae is called ‘Phycology’. One who specializes in the study of algae is called a ‘Phycologist’ or ‘Algologist’.

The body of an alga is called the thallus. In unicellular algae, it is simple and consists of a single cell. All multicellular organisms start their life as single cells. When a cell divides and the daughter cells form a packet enclosed in a mucilaginous mass, a colony is formed.

While the division of a cell continuously in the same plane, with the daughter cells sticking together, results in a row of cells forming a filament. Some of the cells of a filament divide only once by a vertical plane followed by transverse divisions repeatedly and thus produce filamentous branched thallus.

Further, when all the cells of a filament undergo divisions in the cross and vertical planes it results in a sheet of one or more cells in thickness. Such multicellular thallus may show complicated differentiation as in seaweed. All multicellular algae show the above stages during their development.

In the following account, you will study the specific examples of the above basic types of thallus in algae. It is to be noted that all the above forms may not be found in all algal divisions. Still, some are predominantly multicellular, some filamentous and some include only unicellular forms. A gradual complexity in form also indicates how the evolution of thallus has taken place, in algae.

Morphologically algae can be distinguished as unicellular, colonial, filamentous, heterotrichous, thalloid, and polysiphonoid forms. Each of these types is described below.

1. Unicellular Forms

i. Anacystis

Single cells, cylindrical, short or long; sometimes very long snake forms. Cells divide by constriction, the two daughter cell gets separated, but rarely do they remain together to form a 2-celled filament.

Individual single cells may have their own mucilaginous cover around them. Several such cells may be enclosed in common colorless mucilage giving the impression of a colony.

ii. Chlamydomonas

The Different Forms and Morphology of Algae

This single-celled alga contains a nucleus, a cup-shaped chloroplast in which one pyrenoid is commonly present.

The chloroplast on the anterior side shows 2 to 3 rows of fatty redcoloured granules. This is known as eyespot or stigma which is helpful for the alga to respond to light. The cell wall is firm and distinct. A small contractile vacuole is found at the base of each flagellum.

Chlamydomonas cells under partially dry conditions divide and the daughter cells without flagella remain enclosed by a common mass of mucilage. Such a colony is known as palmella stage of Chlamydomonas.

This is only a temporary stage and on flooding with water individual cells develop flagella and escape swimming away from the colony. Thus the beginning of the colony construction found in Volvox can be seen in Chlamydomonas as presented in the images below.

iii. Colonial Algae

When a cell divides and the daughter cells formed remain together within a common mucilage mass, it is known as a colony. A colony may contain a large number of cells. Sometimes it may be so big that one can see it with unaided eyes.

iv. Microcystis

This is a colonial alga, most common in polluted ponds and lakes. Sometimes the colonies are big and can be seen by unaided eyes. They accumulate on the surface of water forming quite a thick layer in some seasons (water blooms).

Single cells are spherical and colony is formed because of loose aggregates of several thousand cells held by mucilage. The colonies float on the surface of the water because of the presence of elongated cylindrical gas vesicles inside the individual cells.

v. Volvox

The colonies of Volvox are spherical, ball-like, and big enough to be seen with an unaided eye. Each colony contains l000-5000 cells arranged on the outside of a mucilaginous ball called coenobium.

Coenobium is a colony in which the number of cells is fixed at the time of formation. No further addition of cells occurs. Generally, the cells are also in a special arrangement. Two types of cells can be seen generally, vegetative or somatic and gonidia.

In younger colonies cytoplasmic connections – plasmodesmata between individual cells can be seen under the microscope.

Vegetative cells are more or less like ChIamydomonas with two flagella, cell walls, a single cup-shaped chloroplast, an eyespot, pyrenoid, contractile vacuole, and a nucleus. The cells on the posterior side of the colony may be larger than in the front.

In Volvox, all the cells of a colony are derived from a single parental cell. They are arranged on the surface of mucilaginous ball, connected with other cells by cytoplasmic connections.

Some cells behave as sex cells meant for reproduction whereas others remain vegetative and ultimately grow old and die. This differentiation into vegetative and reproductive cells is a very important feature in the development of multicellular organisms.

2. Filamentous Forms

When a cell divides always crosswise and the daughter cells do not separate from each other, it results in a linear row of cells as in Nostoc, Ulothrix, and Oedogonium. However, the three algae show different levels of differentiation.

i. Nostoc

This is a simple, filamentous form, a single row of cells, uniseriate (Fig.1.3a). Several filaments of Nostoc are generally enclosed within a common mucilage envelop to form a colony.

Some cells in between the vegetative cells are modified into heterocysts. Heterocyst are a highly differentiated cell in some filamentaous blue-green algae that is a site of nitrogen fixation. All the vegetative cells are capable of developing into spores called akinetes. Akinete are a thick-walled, nonmotile reproductive cell found in algae.

3. Heterotrichous Forms

When some cells of a filament divide vertically it results in a branch. Many filamentous forms show extensive branching of the main filament giving it a bushy appearance.

In some algae the branches at the base remain horizontal, attached to the substratum known as prostrate system from which an erect system of vertical branched filaments arises.

This type of body is known as heterotrichous habit. Heterotrichous habit is the most highly developed filamentous construction in algae.

i. Draparnaldiopsis

It is a heterotrichous alga which shows greater differentiation in plant body. The prostate system is very much reduced. The main axis contains long internodal cells alternating with short nodal cells.

The short nodal cells bear a bunch of short branches. Some of the side branches may develop into long colourless hairs or setae.

The main axis produces at the base long multicellular colourless rhizoids in large number to form a kind of cortex. Their main function is to attach the alga to the substratum.

ii. Ectocarpus

It is another heterotrichous alga. The prostrate system which attaches the alga to the substratum is made of branched filaments. The erect system is in the form of uniseriate (single row of cells) branched filaments forming loose tufts of 1mm to 10 mm or more.

The branches arise just below the cross walls of the cells of the main filament. Most of these branches terminate in elongated hairs.

4. Thalloid Forms

When the cells of a filament divide in more than one plane that is not only cross-wise but also lengthwise it results in a sheet of cells. The thallus may be one cell or many cells in thickness.

i. Ulva

Ulva is a very common alga found on rocky coasts of sea. The thallus is attached to the substrate such as rocks by rhizoids at the base.

Fig.: Ulva lactuca; a) habit of growth b) Fucus vesiculosus- morphology of the thallus.

ii. Fucus

Fucus is a brown algal seaweed common on the rocky coasts of sea in temperate countries. The body of Fucus is large about half a metre or so in length.It has a basal discoid holdfast, a short stipe and long flat and dichotomously branched fronds or blades.

Dichotomous branching pattern is one in which the two arms of the branch are more or less equal in length. At the tip of the blade are found air bladders which make the plant float in water.

5. Polysiphonoid Forms

This form of algae is more complex than the earlier described forms. It is found in the red alga Polysiphonia which is marine in habitat.

i. Polysiphonia

The algae shows in general heterotrichous habits. The prostrate system is in the form of an elongated rhizoid which attaches the algae to the substratum. The erect system is highly branched.

The branches are of two kinds, some are long and some short and hair-like. The main filament grows by the division of a single apical cell. The mature plant body is made up of a central row of cells – a central siphon, surrounded by vertical rows of cells, 4 to 24 – pericentral siphons.

All the pericentral cells are connected with the cells of the central siphon and are also connected with each other.

When the cytoplasm of one cell is connected to the cytoplasm of the neighboring cell through a pit in its wall, it is known as a pit connection. In Polysiphonia although all the cells are separate, their cytoplasm is connected by means of pit connections.

Fig: Polysiphonia; habit showing multicellular construction of several interconnected rows of siphons

New branches may develop from the cells of central siphon or from the pericentral cells. The trichoblasts which are simple or branched hair-like lateral branches arise from the pericentral cells.

In summary, Algae are diverse in their group and form. They can be distinguished as unicellular, colonial, filamentous, heterotrichous, thalloid, and pohysiphonoid forms. The unicellular algae are simplest in form while the thalloid are sheet-like.

Algae are diverse organisms ranging from microscopic unicellular to giant thalloid forms anchored to rocks in the sea. Morphologically they can be distinguished as unicellular, colonial, filamentous, hetertrichous, thalloid, and polysiphonoid forms.

The unicellular algae are simplest in morphology. Some advancement is observed in colonial forms. The cells of a colony may communicate through plasmodesmata. There is division of labour between cells, some remain vegetative while others take part in reproduction.

Some algae have a prostrate system attached to the substratum and an erect system of vertical branches. This is called heterotrichous habit.

Thalloid forms are sheet-like polysiphonoid forms that are more complex. They possess rhizoids and a branched erect system. The mature thallus consists of a central row of cell-central siphon surrounded by pericentral siphons.