Question

Why potato is called underground stem whereas a sweet potato is called an underground root?

Answer 1

Bcoz potato has presence of nodes and internodes from where buds arrise which is property of stem

Whereas there is no nodes and internodes in sweet potato so it is called root

Answer 2

Potato (Solanum tuberosum) shows the presence of nodes and internodes; thus, is classified as a stem, whereas sweet potato (Ipomoea batatas) is a modified adventitious root having an aerial shoots.  

The most important distinguishing characteristic between the two is the presence of ‘eyes’ on the potato tuber. These ‘eyes’ are nothing but the nodes which would be found on the stem of the plant, which bear buds that can grow to form aerial shoots during favorable conditions.

This trait is absent in the sweet potato plant and is thus categorized as a root.