mycorrhizal colonization, in order to pinpoint differences in the interactions that occur in different plant–fungus combinations. Currently, descriptions are based almost entirely on morphological changes in the development of plant and fungus because there is little supplementary physiological or molecular information As outlined in Chapter 2, the main features of the sequence are spore germination, growth of the external mycelium, formation of appressoria and colonization of the root cortex by an intraradical mycelium, development of arbuscules and growth of the extraradical mycelium However, some details are not included in this list and Table 3.1 provides an expanded list of stages at which there may be important developmental changes, together with abbreviations which will be used later in the chapter (Smith, 1995). Schemes of this type are useful if they provide a means of accurate description and comparison of different interactions and consequently help to predict steps which may be under genetic control. However, they must remain flexible and be updated as new information becomes available.