Introduction to Algae

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.
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).
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.
? 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.

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 non-motile 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.