  The original problem of phyllotaxis was focused on the regular arrangements
of leaves on mature stems represented by common fractions such as 1/2, 1/3,
2/5, 3/8, 5/13, etc. The phyllotaxis fraction is not fixed for each plant but
it may undergo stepwise transitions during ontogeny, despite contrasting
observation that the arrangement of leaf primordia at shoot apical meristems
changes continuously. No explanation has been given so far for the mechanism of
the phyllotaxis transition, excepting suggestion resorting to genetic programs
operating at some specific stages. Here it is pointed out that varying length
of the leaf trace acts as an important factor to control the transition by
analyzing Larson's diagram of the procambial system of young cottonwood plants.
The transition is interpreted as a necessary consequence of geometric
constraints that the leaf traces cannot be fitted into a fractional pattern
unless their length is shorter than the denominator times the internode.
