There are several distinct divisions of the plant kingdom, which although they are not really closely related, are conveniently discussed together because they have certain features in common. 

Even though some of these basically simple plants appear to have bodies that have true roots, stems or leaves because they lack the vascular tissues and other characteristics of such structure in higher plants. 

The sex organs and sporangia are usually one celled, or if multicellular, the gametes and spores are not enclosed within a wall formed by a layer of sterile (non reproductive cells).

The members of these groups are generally called cryptogams.

Algae are Cryptogams. Cryptogams are flowerless, seedless plants. They form three main groups.

1. Thallophyta: algae, fungi and lichens.

2. Bryophyta: Liver worts e.g. marchantia and mosses e.g. funaria (polyrichum).

3. Pteridrophyta: ferns e.g. Dryopteris.

Various Kinds of Algae

There are various kinds of algae base on their morphology and t’ c pigment they contain.

On the basis of their pigments, the following groups have been identified

• Blue green algae e.g. Nostoc, Oscillatoria

• Green algae e.g. Spiroygra

• Brown algae e.g. Fucus, sargassum

• Red algae e.g. Bartrachospermum On the basis of morphology

• Euglenoid algae e.g. Euglena

• Bacillairophytes e.g. diatoms e.g. Pinnula a, Diatoma

By the end of this article, you should be able to explain what algae, List the characteristics of algae, describe the structure of algae, explain the reproduction of algae, State the importance of algae and List the differences between algae and fungi.

Now let us commence our study of algae by examining the general characteristics of this group

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Characteristics of Algae

1. They are simple plants without roots, stems and leaves.

2. All algae have chlorophyll. Some have blue, yellow, brown and red pigments which mask the chlorophyll.

3. Majority are non -cellular while few are multicellular. Some, such as sea weeds are large. 4. They are mainly aquatic, with a few on damp soils and shady places. Examples are Spirogyra, Anabaena and Sargassum.

5. The body of an alga in composed of a true parenchymatous tissue.

6. The cell wall of an alga is composed of true cellulose.

7. Reserve carbohydrate is usually starch and not glycogen as in fungi.

Classification of Algae

Class I – Mycophyceae or cynophyceae or blue green algae e.g. include Nostoc, Anabeana, Oscillatoria. 

Class 2 – Euglenophyceae e.g. Euglena

Class 3 – Chlorophyceae or green algae e.g. volvox, spirogyra, witella, Zygnema

Class 4 – Bacillariophyceae or diatoms.

Class 5 – Phaeophyceae or brown algae, e.g. Fucus

Class 6 – Rhodophyceae or red algae e.g. Batrachospermum

Spirogyra (Chlorophyceae)

Occurence: Spirogyra belongs to the family chlorophyceae and the order conjugates or zygnematales. It is a cosmopolitan plant which forms a tangled mass of filaments floating on stagnant fresh water, especially in ponds, ditches, springs and streams.

Some species grow in running water. Such species produce a short unicellular organ of attachment, called hapteron, for anchorage on sea weeds. Spirogyra is commonly found as bright green masses of thread-like or filamentous structures on the surfaces of waters, and is often referred to as a “pond scum”.

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Structure of a Spirogyra

Mature filament is unbranched and consists of single row of identical cylindrical cells joined end to end. Cell wall is made of cellulose and pectin.

External cell wall is covered by mucilage, making it slimy.

Cytoplasm is a thin layer with spiral bands of chloroplast.

Nucleus is suspended at the centre by strands of cytoplasm.

Chloroplast contains small nodular protoplasmic bodies called the pyrenoids.

Starch grains are deposited around each pyrenoid.

Gametes are without cilia and are called aplanogametes.

Gametes are morphologically isogamous (identical in structure) but physiologically anisogamous (ie. gametes are different in terms of appearance and behaviour).

Reproduction in Spirogyra

Reproduction in spirogyra occurs in two ways,

1. Asexual reproduction.

2. Sexual reproduction.

Asexual Reproduction:Asexual reproduction is in the form of vegetative fragmentation. The filament breaks into pieces by wave action against solid objects. Each piece grows into mature filament. Sexual Reproduction:

Sexual reproduction is by the process of conjugation or fission of two similar reproductive units or gametes i.e. isogametes. Conjugation usually takes place between the cells of two filaments or even three; this is called scalariform (or ladder-like) conjugation.

When conjugation takes place between the gametes of the same filament, this is called LATERAL CONJUGATION . Do you know how the process of conjugation occurs? During conjugation, two spirogyra filaments come to lie side by side. Conjugation tubes are formed between

The cells in one filament act as male gametes, while those in the other act as female gametes. The male gamete migrates into and fuses with the female gamete in the opposite cell to form thick-walled zygospores. Now what happens to the zygospores that are formed?

Germination of  Zygospores

The zygospore is provided with a thick cellulose wall, composed of three layers, of which the middle one contains chitin. With the rapid decay of the parent filament all the zygospores are set free and they sink to the bottom of the pool of water. They undergo a period of rest till the next favourable season and then they germinate.

The protoplast of each zygospore at first increase in size, then its outer layers burst and the inner one with the protoplast grows out in the form of a short tube which ultimately forms a new filament. The filament escapes and floats on the surface of water. The cell divides and the filament increase in length, soon the floating filament takes to conjugation again, and the life-cycle begins again.

What happens when conjugation does not take place?

The gametangia becomes converted into thick-walled bodies identical with zygospores, these bodies which are thus formed parthenogenetically are called a zygospores or parthenospores. They germinate like the zygospores.

Importance of Algae

The green algae probably evolved from motile unicells similar to Chlamydomonas.

1. It exhibits a variety of forms and it has different reproduction.

2. This group presents a clear example of radiating evolutionary development.

3. The evolution of sexuality has been emphasised because of the great importance of this type of reproduction in the production of variation which is significant in the survival of any group.

4. Green algae are also thought to have given rise to all of the higher green plants by evolution.

5. The chloroplast in both fresh and salt water comprise food supplies for animals.

6. Algae (e.g. ulva) are utilised by mains food.

The photosynthetic process of algae supplies the oxygen that is necessary for life in water.

7. Green algae along with other algae, occassionally cause problems when they become abundant in water.  

They impart obnoxious odours and flavours to the water, and make water treatment difficult.

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