Which underground stem modification does not posses roots?

@ deepika 
tuber is the underground stem modification which donot posses roots . 
tuber is a swallon end of underground branch which arises from the axil of a flower leaf . 
these underground branch grow horizontally outward in the soil , each tuber is irregular in shape , due to deposition of food material . on the surface of each tubers many leaf scars are seen . 
HOPE IT CLEARS YOUR DOUBT 
ALL THE BEST ..

Tuber arises from the axis of the lower leaf.  

The underground branches grow horizontal outward in the soil.On the surface of each Tuber many leaf scar are seen.

A leaf scar with an axillary bud is called its EYE.

These eye of potato are capable of producing new plant by Vegetative propagation.

Hope it’s clear all your doubt.