Hey,

Well changing stream will cause you some disadvantage to ypu and even confuse you further. It might be difficult for you to do and rather would be better to stick to one filed. 

Botany (being a mother subjet like Zoology), has many things incorporated within itself. A student of Botany/Zoology has to read Microbiology also (right from structure of a virus, viz TMV or classification of viruses, or processes like Trasncription, Conjugation and Translation and many more topics).

My suggestion to you would be, opt for Chemistry as a pass subject along with botany honours. This will help you in the future as well.

Hope this helps.