5.22∗ Ridout et al. (1999) develop the following model for the strawberry branching structure of Exercise 2.14. Let X denote the number of flowers at rank r, given the number of flowers, m, at the previous rank. The branches at the previous rank are called parent branches. If Yi denotes the number of offspring branches resulting from the ith parent branch, then we can write:

X = Y1 + Y2 + ··· + Ym.

Assuming the parent branches produce no more than two offspring branches, then we may model the distribution of Y by the probability function: