As I was dividing overgrow plants it reminded of some of the unusual parts of plants you find underground.
Pachysandra and stolons.
Stolons are stems which grow at the soil surface or just below ground. Plants use these to grow into large colonies.
Root-tubers of daylilies.
A tuberous root or storage root, is a modified lateral root, enlarged to function as a storage organ.
Cleistogamous flowers of violets
Cleistogamous flowers are self-pollinated flowers that do not open and lack petals. They never open and go straight to seed production. This clump of common violet has about 20 or so at ground level. This must explain why they multiply by seed so fast?