Sometimes the roots of the scion will eventually grow right through the dead stocks vascular bundles. Often when you unpot some of the Melocactus and other plants sold in the UK from the Continent you will find they were originally stump grafted and later the stock buried. The green part of the stock then gradually degenerates since it can no longer photosynthesize below soil level and the scion is either still supported through the vascular bundles, or as MagiCarpus says either roots through the old dead stock, or from above it.
It's surprising though how the soft photosynthesizing stem tissue can degenerate and the vascular bundles still keep working and a plant alive. Copiapoa decorticans, though not grafted, gets it's name from this habit (decorticans = off + skin, therefore, peeling skin) with the heads still alive and growing even though the tissue of the stem below has degenerated and only the vascular bundles are still functioning.
Note the long withered stem on the bottom right of this link with the head still growing.
https://www.flickr.com/photos/damiano_s ... 075889596/
