Plant Groups
Introduction
PLANT GROUPS
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Introduction to Algae
The term algae have been derived from Latin word algae (means
seaweeds).
The science study algae and subsequently is called Algology or
phycology consists of two Greek words: Phykos means seaweeds (algae)
and logos mean discourse (study).
General characteristic of the Algae
Algae are small autotrophic plants that fail to show any cellular
differentiation and their sex organs are unicellular and if multicellular all
cells are fertile.
Fritsch (1935) define algae all holophytic organisms that fail to reach
the level of differentiation characteristics of archegoniate plants. Smith
(1955) offered another definition of algae based on characters of sex organs.
He said in algae the sex organs are usually unicellular and when they are
multicellular all cells are fertile.
In higher plant and bryophytes the sex organ and sporangia which
have outer layer sterile and multicellular and this does not occur in algae
except in members of charales.
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Plant Groups
Introduction
However, the algae are primitive plant, chlorophyll bearing
organisms, make their food by photosynthesis (Autotrophic organisms),
weak differentiation have no vascular tissues, have no true roots, stems and
leaves, uptake food material from all body surface.
Reproduction
Reproduction in the algae occurs in three general ways, with
variations in certain species:
 Vegetative Reproduction: is common to all algae and includes
multiplication by cell division or by splitting of colony or by
fragmentation. In short that uses only vegetative cell and does not
employ any specialized structure.
 A sexual Reproduction: (under favorable condition) is accomplished
by using differentiated or specialized cells or organs capable of
directly multiplying the plant without uniting with any others cells.
Many different kinds of spores are involved in asexual reproduction
particularly the swimming zoospores are equipped with one, two,
four or many flagella and commonly contain a bit of chloroplast and
a pigment spot.
The zoospores are produced frequently and often in large number, and
their motility enables them to distributed the species, a zoospore maybe
produced in unspecialized cells or in differentiated is sporangia,
Zoospore have no wall (naked).
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Plant Groups
Introduction
Aplanospores many of which are produced within a cell is enclosed
by a wall and non-motile. When an Aplanospores form an especially
thick wall, suitable for dormancy it is called hypnospore.
Endospore a special kind of Aplanospores it is formed when the entire
content of a cell or only a portion of the protoplast is cut in to small
reproductive non motile element. Usually they are naked.
 Sexual Reproduction: comes at the end of a growing season or
may be induced by unfavorable condition or critical changes in
the environment. In sexual reproduction specialized elements are
produced either in unspecialized vegetative cells or in male sex
organ (Antheridium) or female sex organ (Oogonium). These sex
cells are incapable of producing a new plant by themselves but
must unite (Syngamy) with another cell to form a zygote. Such
cells are called gametes.
When the uniting gametes come from the same plant is called
Homothallic.
When they come from different plant the species are called
Heterothallic.
The sexual Reproduction can be categorized in to two types:
 Isogamous: common occurrence among unicellular algae.
When the two uniting gametes are similar in size, shape and
behavior and do not show any recognizable maleness or
femaleness but differ physiologically they are called
Isogametes. This type is primitive, both of them motile.
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Plant Groups
Introduction
 Heterogamous: can be divided in to two types
 Anisogamous
 Oogamous
Anisogamous both the fusing gametes are motile but male gametes are
smaller in size and more active while female gametes are slightly
sluggish in movement.
Oogamous: being an advanced type of sexual reproduction, it is quite
common among higher algae.
The male gametes are motile and formed in large number whereas
female gametes are non-motile and bigger in size and produced few in
number. Male gametes (Antherozoids) are produced in the
Antheridium (Male Reproductive Organ). Whereas female gametes an
egg or ovum is produced in Oogonium (Female Reproductive Organ).
Fertilization
In Isogamous and Anisogamous types of sexual reproduction
both gametes are first release from their respective gametangia and the
fertilization takes place outside gametangia. This process called
External fertilization.
In Oogamous the male gametes are liberated from male
gametangium which swims to Oogonium, fertilization takes inside the
Oogonium, the process called Internal Fertilization.
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Plant Groups
Introduction
Zygote: the two gametes after fusion form the zygote. Zygote being
diploid cell represents the Sporophyte of the life cycle.
Germination of Zygote
The germination of zygote may be direct or in direct type:
 Direct type: In such type of germination the zygote, develop
directly into a new plant without producing any intermediate
spore like Spirogyra.
 Indirect germination: it may be of two types
 Zygote divides meiotically and forms motile or nonmotile haploid spore of which each develops into a
new plant like Chlamydomonas
 Zygote divides mitotically and forms a diploid plant.
Later plant reproduced by meiotic division and
produces spore. Each spore develops into a new plant
like Ectocarpus.
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