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