Biology Reference
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organelles, the blue-green algae were considered closer to bacteria (Cohn, 1872, 1875). Subsequent
to the publication of taxonomic keys on heterocystous forms (Bornet and Flahault, 1888) and the
Oscillatoriaceae (Gomont, 1892), a comprehensive account on the taxonomy of these organisms has
been presented by Geitler (1932) wherein 145 genera encompassing 1300 species have been classifi ed
into 20 Families under three Orders. The “Geitlerian” approach provided new impetus for a number
of taxonomists to classify them (Fritsch, 1945; Drouet and Daily, 1956; Golubic, 1967; Bourelly, 1970
and many others) from time to time.
Drouet and Daily (1956) revised Geitler's classifi cation and brought down the number of genera
to 24 with 62 species based on their observations on the blue-green algae collected from all over
the world. The taxonomic criteria developed for identifi cation have been based on morphological
features and they further attributed the diversity in morphology due to the existence of ecophenes
or ecotypes. According to them, the blue-green algal species is highly polymorphic and the
phenotypic changes lack a genetic basis. That is polymorphism is assumed to be purely phenotypic.
The ecophenes thus represent form-variations caused by environmental factors rather than due to
genetic differences. Thus the species concept envisaged by Drouet predicts that a particular species
should be morphologically identical when grown under the same environmental conditions and
shows a similar change in morphology with changing environmental conditions while at the same
time the genotype remains unchanged. On the basis of taxonomic keys available from time to time
(Geitler, 1932; Drouet and Daily, 1956; Desikachary, 1959; Drouet, 1981; Komárek and Anagnostidis,
1989), a number of workers enumerated the cyanobacterial fl ora from wide variety of locations and
habitats from all over the world.
The classifi cation proposed by Fritsch (1945) has been mainly based on morphological characters.
Of the fi ve orders recognized, three have been placed together on the basis of absence of hormogones
(Chroococcales, Chamaesiphonales and Pleurocapsales) and the other two Orders Nostocales and
Stigonematales that form the hormogones have been placed together. Chroococcales are characterized
by unicellular or colonial (commonly palmelloid) forms that multiply by cell division and by endospore
formation. The members included in the Order Chamaesiphonales are unicellular or colonial epiphytes
or lithophytes exhibiting marked polarity and reproduce by the formation of endospores or exospores.
The Pleurocapsales comprise of heterotrichous fi lamentous types without heterocysts and reproduce by
the formation of endospores. The members of Nostocales are either non-heterocystous or heterocystous
fi lamentous forms. Some of these show false-branching. Multiplication is by means of hormogones,
hormocysts and akinetes. Order Stigonematales comprises of heterotrichous fi lamentous types that
exhibit true branching and possess heterocysts. The presence of pit-connections between the cells is
another characteristic feature of these forms. The multiplication is by hormogonia, hormocysts and
more rarely by akinetes. A few representative types of cyanobacteria identifi ed on the basis of traditional
taxonomic keys have been presented in Figs. 1 to 8.
A classifi cation of cyanobacteria under ICBN has been proposed (Anagnostidis and Komárek,
1985) and revised (Komárek and Anagnostidis, 1989) that took into account the Bacteriological code
as well. In this classifi cation they recognized four orders- Chroococcales, Oscillatoriales, Nostocales
and Stigonematales, which are further subdivided into families, subfamilies, genera and species.
They emphasized that the taxonomic keys developed by them would be greatly helpful for ecologists
to study the diversity of cyanobacteria from their natural habitats. Many objective features and
characteristics for distinguishing various genera and species have been defi ned by them. Due to
the application of both botanical and bacteriological criteria, an attempt has been made to reconcile
the differences between the botanical and bacteriological codes. The taxonomic criteria based on
botanical code included cell morphology, cell division, polarity, methods of false-branching, tapering
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