June Hughes writes

It is very strange that a few stray potato plants have appeared in the
earth next to my pots of potatoes and they are totally unscathed.

Slugs haven't got that strong mouth parts and softness of food is
important to them that's why so many species of slug only eat rotting
vegetation. The potatoes in the ground will be harder grown than the
ones in the tubs - less fertile soil, possibly less fertilizer - so the
leaves and stems will be that bit tougher and less attractive to slugs.
--
Kay