callus stem

What does it mean when the stem of a potted plant is callusing?

Callus: a mass of unorganized cells

All I can really tell you is its not a bad thing, callusing can be caused by physical agitation or scaring.
Have you ever bent or broke shoot and come back the next day see a huge "knuckle" or callus on the spot where it was bent?

Hope that helps?..post a pic if you need more specifics