Square part of a plant? (4)

I believe the answer is:

root

I believe this clue is a double definition.

'square' is the first definition.
(square root is a type of mathematical root)

'part of a plant?' is the second definition.
(I know that root is part of plant)

(Other definitions for root that I've seen before include "Underground growth" , "Lowest part of plant" , "Derivation" , "Basic cause or origin" , "Reptile" .)

I've seen this clue in the Evening Standard.

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