fungal structure sterigma A small hyphal branch or structure which supports a sporangium, a conidium, or a basidiospore. In species with biseriate (two-layered) sterigmata such as A. nidulans, the first layer comprises primary sterigmata (metulae) and the second layer secondary sterigmata (phialides). Some species, e.g. A. fumigatus, A. oryzae, and others, have only phialides.