(i) Heterocysts are present in some cyanobacteria. It is
a large-sized, pale coloured, thick-walled cell which
occurs in terminal, intercalary or lateral position in
filamentous cyanobacteria, e.g., Nostoc.
(ii) Features of cyanobacteria are
(a) Mucilage sheath is present.
(b) Thylakoids lack phycobilins. Therefore,
photosynthesis is absent, but cyclic
photophosphorylation occurs.
(c) Heterocyst is dependent for its nourishment on
adjacent vegetative cells.
The thick wall is impermeable to oxygen, but
permeable to nitrogen. It has enzyme
nitrogenase which is specialised to perform
nitrogen-fixation.