Roots Associated with Micro-Organisms
Many vascular plants form symbiotic relationships with soil microorganisms. In legumes, nitrogen-fixing bacteria invade the root cortex through root hairs, and stimulate meristematic activity in the cortex (and sometimes also in the pericycle) to form a root nodule, which often becomes elongated to resemble a short lateral root. Other soil micro-organisms may induce the formation of modified lateral roots. For example, in many woody angio-sperms, invading filamentous bacteria promote the development of short, swollen lateral roots, and in some temperate woody forest species, especially in the families Fagaceae and Betulaceae, ectomycorrhizal fungi form a mantle over stunted lateral roots. By contrast, the more common endomycorrhizal fungi, which invade the cells of the host root, often have little influence on root morphology (Fig. 3.6).