It is commonly believed that only roots develop below the ground. how would prove that potato tuber is a stem and not root?

Potatoes  are from the grounds but they are considered as modified stem because they are capable of photosynthesis as what other stem plants do and they also  develop as thickened portions of stolons. these stolons are stem tissue, as they arise from stem tissue and they contain the vascular architecture of stem tissue buds, which are a characteristic of stem tissue and not the roots. moreover, the roots are incapable of photosynthesis. therefore, potato tuber is indeed a stem and not a root.

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