Figure 2. The vegetative growth cycle of A. nidulans. When an A. nidulans conidiospore germinates, it grows isotropically for a period of time, after which a polarity axis is selected (“polarity establishment”). This emerging polarity axis gives rise to a germ-tube, into which several nuclei migrate before the first septum is formed. This polarity axis can be maintained indefinitely, and subsequent septation events will take place as apical extension proceeds. Subapical cells may also establish new polarity axes (“branching”), which are also colonized by nuclei, giving rise to further hyphal tip cells. In the presence of appropriate stimuli, vegetative growth arrests and the developmental program starts with the emergence of a thick-walled “foot cell.” The tip of the foot cell undergoes a characteristic swelling, giving rise to a vesicle. Nuclei align in the periphery of the vesicle and migrate into cells that bud from the vesicle, denoted metulae. Metulae give rise to phialides (secondary sterigmata) and the latter produce conidiospores.14,97 Green arrows indicate the direction of polarized growth.