THE THEORY AND PRACTICE OF BRANCH AUTONOMY

The past 15 years have seen a surge of interest in modularity in plants; that is, in the implications of the fact that plants are composed of repetitive modules that may in some ways behave as in independent units (32). Much has been written about the importance of modularity in plant population biology (e.g. 33, 34, 113, 114), and also about the advantages and disadvantages of independence or interdependence among separate but connected modules (7, 31, 69). The interest in modularity is mainly at two scales. At the smallest scale, interest has focused on the "nutritional unit" (1) or "physiologically independent subunit" (111, 112), comprising a unit of foliage, the section of stem to which it is attached, and the subtending axillary bud. At the opposite end of the spectrum, research has focused on clonal herbs (3, 81, 82, 113), in which each module (ramet) contains all of the structural parts and physiological processes necessary for independent existence. However, this upsurge of interest in modularity has also led to renewed speculation about other, intermediately scaled, functional units that may also behave semiautonomously (112). For woody plants, considerable interest has focused on the degree of autonomy of individual branches' on a tree or shrub (107, 112, 117).