Heterocyst of Nostoc are

The correct option/answer is: both terminal and intercalary.

Let's break down the explanation of heterocysts in Nostoc step-by-step:
1. What are Heterocysts?
Heterocysts are specialized, pale-yellow, thick-walled cells found in certain filamentous cyanobacteria (blue-green algae), including the genus Nostoc. Their primary function is nitrogen fixation, converting atmospheric nitrogen into a usable form (ammonia) using the nitrogenase enzyme.
2. Position of Heterocysts:
Depending on their location within the filament (trichome), heterocysts can be classified as:
• Terminal: Located at the ends of the filament.
• Intercalary: Positioned along the length of the filament, sandwiched between ordinary vegetative cells.
3. In Nostoc:
In Nostoc, heterocysts are typically intercalary (meaning they occur at intervals within the chain of vegetative cells). However, during fragmentation of the filament or in young trichomes, they can also occur at the ends, functioning as terminal heterocysts. Therefore, heterocysts in Nostoc can be both terminal and intercalary.