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Morphology of blue-green algae or cyanobacteria may be influenced by environmental factors making identification very difficult. Drouet and Daily (1956) recognized only six genera of coccoid blue-green algae from about four and one-half of a dozen of genera. Drouet (1981), after examining a large number of herbarium specimens including the types of many taxa of Stigonemataceae, considered all the genera as synonyms of Stigonema. Aziz (1989) documented Hapalosiphon, Fischerella and Stigonema-like habits of a non-thermophilic blue-green algal strain D612 in axenic culture and considered all these forms/habits of organisms as Stigonema. Based on culture studies of strains designated as Hapalosiphon, Mastigocladus, Westiellopsis, Fischerella and Stigonema, Begum et al. (1994) suggested to consider these truly branched forms as Stigonema-complex. On 26 November 1995, a brownish-pink filamentous algal sample was collected along with dry coconut palm-bark from Panchagar district, Bangladesh. It was found to contain brownishpink filaments with thick sheath that is frayed at the tips and was identified as Porphyrosiphon notarisii (Menegh.) Kutz. 1850 (Fig. 1) (Desikachary 1959, 248, Pl. 47, Fig. 9; Starmach 1966, 425, Fig. 630). The genus Porphyrosiphon Kutz. 1849 is separated from Lyngbya Ag. 1824 by having a multilayered sheath with a red or brown pigment (Bourrelly, 1985). Lyngbya aestuarii Liebn. ex Gomt., L. maior Menegh. ex Gomt. 1892 and L. majuscula Harv. ex Gomt. 1892 produce multilayered sheath (Starmach, 1966, Figs 338, 343 and 345, respectively). The coloured filament is due to the production of scytonemin pigment to protect the cyanobacterium from near UV radiation (Muehlstein and Castenholz, 1983). Many of the features mentioned in the literature of Porphyrosiphon have been highly variable and resemble Lyngbya majuscula (Desikachary 1959). In L. ceylanica Wille, Aziz and Tanbir (1999) observed variation in colour, being reddish-brownish in upper filaments that received more light when grew on moist rice-field soil (Fig. 2), with dimensions very similar to Porphyrosiphon. Drouet (1937) included Lyngbya arboricola Bruhl et Biswas and L. dendrobia Bruhl et Biswas under Porphyrosiphon notarisii. To ascertain the identity and also the variability in regards to nature of filaments, a portion of the bark sample with brownish-pink layer was placed in a Petri dish containing Chu 10D medium, incubated at a continuous light quantity of about 50 μE m s and a temperature of about 25o C. Many hormogonia of 4 to 9 (−20) cells were found to be released, develop into blue-green filaments. After about 15 days, many filaments attaining about 3 cm long developed centering the bark (Fig. 3). Initially filaments were blue-green with relatively thin non-stratified sheath like Lyngbya (Fig. 4). But later on some filaments produced false-branches like Plectonema Thur. (Fig. 5), while many filaments became purplish to reddish-brown with 10−12-layered sheath like Porphyrosiphon (Figs 6, 7), even more than that found in nature (Fig. 1). Further growth resulted much wider filaments with 2 or 3 trichomes within highly thickened rigid sheath (Figs 8, 9) that

Polymorphism in <i>Lyngbya notarisii</i> (Meneghini) Wille in culture