The Botanical Survey of India (BSI) is headquartered in Kolkata, West Bengal. Let's explore the reasons behind this answer:

1. Origin and Establishment:
- The BSI was established on 13 February 1890 with the goal of exploring and documenting the plant resources of the Indian subcontinent.
- It was originally established as the 'Imperial Forest Research Institute' and was later renamed the 'Botanical Survey of India' in 1916.
- Given its historical significance, the BSI was established in Kolkata, which was the capital of British India at that time.

2. Importance of Kolkata for Botanical Research:
- Kolkata has a long history of botanical research and education. It is home to several renowned educational institutions and research centers that focus on botanical studies.
- The Indian Botanic Garden in Kolkata, also known as the Acharya Jagadish Chandra Bose Indian Botanic Garden, is one of the oldest and largest botanical gardens in India. It serves as an important center for plant taxonomy and conservation.
- Kolkata's favorable climate and geographical location make it an ideal hub for botanical research. The region has diverse ecosystems ranging from mangroves to wetlands, providing a rich variety of plant species to study and document.

3. Administrative Convenience:
- Kolkata's central location within the Indian subcontinent makes it easily accessible for researchers and scientists from different parts of the country.
- The city is well-connected by air, rail, and road networks, facilitating the movement of experts, researchers, and plant specimens to and from the BSI headquarters.
- Kolkata also offers a supportive infrastructure with various amenities required for running an organization of such national importance.

In conclusion, the Botanical Survey of India is headquartered in Kolkata due to its historical significance, the city's strong tradition of botanical research, and its convenient location for administrative purposes.