Question

How can we say that potato is a modified stem while sweet potato is a modified root?

Answer 1

Potato has a modified stem whereas sweet potato is a modified root.

Explanation:

• The modified stem is potato itself. This is because the bottom portion of the stem which is actually underground that stores food is called the tuber. The roots also develop from this bottom swollen part. Then there are potato's eyes from which the branches arise which eventually form the leaves.  
• Sweet potato on the other hand is a modified root . It is basically a modified adventitious root but it is sometimes also referred under tuberous root. It is present to store water and food , in the form of starch. It is also called root tuber.

Answer 2

Answer: Physical characteristics

Explanation:

Sweet potato is a modified root, whereas potato is modified stem

Sweet potato is actually a modified food storage root. Potato is, infact, the swollen tip of an underground stem branch which is called stolon. It also has nodes (eyes). These nodes have one or more buds subtended by a leaf scar.

Sweet potato is a swollen adventitious root. It is a tuberous root. It does not have any nodes, internodes and buds like a stem.  

Potato stem modification is referred to as the rhizome whereas in sweet potato the roots store food are known as tuberous roots.

Hope it helps!