A "Karusu" is a Japanese word that means callus in English. It refers to the callous that forms around the edge of wounds on trees, such as when branches are cut or air layered. Karusu helps heal the wounds and prevent infections or rotting. Karusu can also be used to create interesting shapes or features on the bonsai trunk or branches, such as knobs or scars. Karusu is considered a sign of good care and technique in bonsai art.