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
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".
Read On: Mosses: Characteristics, Reproduction and Economic Importance
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.
Read On: Fungi: Classification, Characteristic, Structure, Reproduction and Facts
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