[cool2011]

A stem can be defined as a root to which an affix can be added. Thus, the root dog is also a stem, because, even though it contains no affix(es), an affixx could be added to it - to form, e.g., dog-s.
This notion is necessary because not all roots are such that affixes can be added to them - e.g., of, or, I, etc

بليزالشرح فلم يتبقى لي الا هذه النقطه من ماده الصرف