Hi

Yes after completing your Bsc(hons) you will be eligible for Msc degree. In Msc degree you can choose to continue the same subject i.e Botany or you can even change subject as you wish and as per the rules of the university you would be applying to.
For Msc degree some entrance exams are available like IIT JAM is for getting into IITs to study Msc, IPU CET for IP university etc.

For any more doubt feel free to revert back

Thanks!