Quite often, the speed of the roots exceeds that of top growth and
that is true in seedlings as well as cuttings. The reason for the
sudden appearance or roots after a period of apparent non-activity is
because the cut end is forming a callus from which the roots can
develop. The callus seals the stem ends preventing bacterial and/or
fungal infection and in the very first instance, is merely a mass of
rapidly dividing, undifferentiated cells.

Auxins (hormones) produced by the cutting trigger these cells into
forming roots when sufficient callousing has taken place. The
cutting's survival depends upon root formation and once the 'root
buds' have formed within the callous, rapid root growth ensues. There
are exceptions and callousing with subsequent root formation can be a
very protracted affair taking several to many months, especially with
certain evergreens. Hardwood cuttings taken in early winter, will also
develop a basal callous relatively slowly and roots may not emerge
until after spring growth has commenced.

Some plants develop adventitious roots (roots along the stems or at
the leaf nodes) and the 'root buds' for these are already present. In
such plants, stem-end callousing may or may not take place and serves
mainly to prevent infection at the cut stem ends. Roots develop along
the stem length or at the nodes if the cutting is given the correct
treatment. Roots may also form subsequently at the calloused end, but
adventitious roots commence growth almost immediately and are not
reliant upon stem end callousing.