A continuing and future challenge in plant science is the "genetics of geometry" : the recovery of information about the dynamics of the genetic mechanisms by which plants control the development of various features of their geometry. Some representative publications dealing with such issues include: (i) the modelling of plant architecture using L- systems and rewriting , (ii) the genetic control of floral development [10, 4], and (iii) the positioning of the trichomes (hairs) on the leaves of plants such as Arabidopsis thaliana [25, 23]. It is the positioning of trichomes which is examined in this chapter. The use of reaction- diffusion models is compared with cellular signalling and switching models. It is concluded that, in performing simulations to understand the dynamics of the mechanisms that control pattern formation in plants, it is necessary to work with a cellular model of the plant organ being studied in order to improve on current understanding about how the genetics controls the signalling and switching between cells to produce the observed patterns.