Crossword Genius

Origins of botany? (5)

Ross

I believe the answer is:

roots

I'm a little stuck... Click here to teach me more about this clue!

'origins' is the definition.
(I know this)

'of botany?' is the wordplay.
I cannot quite understand how this works, but
'of' could be 'o' and 'o' is present in the answer.
The remaining letters 'rots' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for roots that I've seen before include "Radicals" , "Shows enthusiastic support for" , "Underground growths" , "Underground parts of a plant" , "ethnic music in > TV series" .)

I've seen this clue in The Mirror.
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