Vegetative propagation in Pistia occurs by

An offset is a lateral branch with short internodes, each node bearing a rosette of leaves and a tuft of roots.
It is found in aquatic plants like $Pistia$ and $Eichhornia$; the offsets break off and grow into new plants.
Runners are found in grass and strawberry, stolons in mint and jasmine, and suckers in banana, pineapple and $Chrysanthemum$.
Hence vegetative propagation in $Pistia$ occurs by offset.